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S e lec tive (B rush ) P la t ing

S E L E C T I V E P L A T I N G , a l so k n o w n a s b r u s h

pla t ing, dif fe rs f rom tradi t ional tank or ba th pla t-

ing in tha t the w orkpie c e i s no t imme rse d in a

pla t ing so lut ion (e lec trolyte) . Ins tead, the e lec tro-

ly te i s b roug ht to the pa r t a nd a ppl i e d by a ha nd-

he ld a node or s ty lus , w hic h inc orpora te s a n a b-

sorbe nt w ra pping for a pply ing the so lu t ion to the

w orkpie c e ( c a thode ) . A d i r e c t c urr e n t pow e r pa c k

dr ive s the e l e c t roc he mic a l r e a c t ion , de pos i t ing

the des ired meta l on the substra te . In prac t ice ,

move me nt be tw e e n the a node a nd c a thode i s r e -

qu i r e d for op t imum re su l t s w he n p la t ing , s t r ip -

p ing , a c t iva t ing , a nd so on . A sc he ma t ic of the

se lec t ive pla t ing process appears in Fig. 1.

Cur re n t ly , a b roa d r a nge of e l e me nts a nd a l -

loys c a n be brush p la te d ; the ma jor i ty a r e li s t e d

in Ta ble 1 . Eve n though s e le c t ive p la t ing c a n

b e d o n e i n a ta n k , l a b o r - i n t e n s iv e m a s k i n g a n d

T a b le I E n e r g y a c t o r s f o r s e l e c t iv e p l a t in g

Element

o r a l l oy E n e r g y f a c t o r

Cadmium 0.006

Chromium 0.200

Cobalt 0.014

Copper 0.013

Gallium 0.014

Gold 0.006

Indium 0.009

Iridium 0.069

Iron 0.025

Lead 0.006

Mercury 0.008

Nickel acid 0.047

Nickel alkaline 0.017

Nickel, black 0.025

Nickel, natural 0.025

Palladium 0.019

Platinum 0.015

Rhodium 0.030

Silver, noncyanide 0.750

Silver, pure 0.004

Babbitt 0.006

Brass 0.017

Bronze

0.017

Cobalt-nickel 0.019

Cobalt-tungsten 0.015

Nickel-cobalt 0.020

Nickel-tungsten 0.020

Tin-cadmium 0.007

Tin-indium 0.008

Tin-lead (90/10) 0.006

Tin-lead (604 0) 0.007

Tin-nickel 0.010

Note: The energy factor is the ampere-hours required to produce a

deposit thickness of 0.003 mm (0.0001 in.) on a square inch of

a r e a .

s u b s e q u e n t r e m o v a l o f t e n m a k e t h is o p t i o n im -

prac t ica l .

A d v a n t a g e s a n d L i m i t a t i o n s

The ke y a dva nta ge o f s e le c tive p la t ing i s por t-

a b i l i ty . Ma ny sys te ms c a n be move d t o va r ious

loca t ions in a production fac i l i ty or be t rans-

ported to the job s i te . Se lec t ive pla t ing is a lso

versa t i le ; i t permits most e lec tropla te types to be

de pos i t e d on to a ny c ondu c t ive subs t r a t e tha t c a n

be touc he d w i th a n e le c t rode . Ca s t i ron , c oppe r ,

s tee l , s ta inless s tee l , high- tempera ture nickel-

ba se a l loys , a luminum, a nd z inc a r e c ommonly

pla te d by th i s me th od a nd e xhib i t good a dhe s ion .

L imi te d a dh e s ion c a n be ob ta ine d w i th o the r ma -

ter ia ls , such as t i tanium, tungsten, and tanta lum.

Se le c t ive p la t ing a l low s h ighe r c ur r e n t de ns i -

t i es tha n t a nk p la t ing , w h ic h t r a ns la te in to h ighe r

de pos i t ion r a t e s , up to 0 .010 mrn/min (0 .0004

in . /min) . In a ddi t ion , inhe re n t ly pre c i s e th ic kne s s

c ont ro l pe rmi t s p la t e bu i ldup or r e pa i r w i thout

the ne e d for subse que nt ma c hin ing . In r e ga rd to

de pos i t ha rdne s s , a 70 H RC t r iva le n t c hrome i s

a va i l a b le for s e le c t ive p la t ing of th ic k de p os i t s ,

w hic h p la c e s s e le c t ive p la t ing on a pa r w i th

he xa va le n t t a nk c hrome . The e xc e pt ions a r e the

ha rde r de pos i t s o f c oba l t a nd go ld (Ta ble 2). Ta -

b le 3 provide s a po in t -by-poin t c ompa r i son of

se le c tive p la t ing w i th c om pe t i t ive proc e s se s , in -

c lud ing t a nk p la t ing .

Be s ide s e l e c t rop la t ing , s e le c t ive p la t ing sys -

t e m s c a n p e r f o r m s e v e r al o t h e r a n c i l l a ry o p e r a -

t ions :

T a b l e 2 D e p o s i t h a r d n e s s a t t a i n a b le w i

s e l e c t iv e p l a t i n g v e r s u s b a t h p l a t in g

M i c r o h a r d n e s s , DPH

M e t a l B a t h S e l e c t i v

type plating plating

Cadmium 30-50 20-27

Chromium 750-1100(a) 850-110

Cobalt 180-440 510

Copper 53-350 140-21

Gold 40-100 140-15

Lead 4-20 7

Nickel 150-760 280-58

Palladium 85-450 375

Rhodium 550-1000 800

Silver 42-190 70-1 40

T m 4-15 7

Zinc 35-125 40-54

(a) Usual range, but hardnesses of 280-1200 DPH are p

Source: Ref 2

E l e c t r o s t r i p p i n g

for de p la t ing of ma ny m

a nd a l loys

A n o d i z i n g

for p ro te c t ing a luminum a nd

E l e c t r o m i l l i n g for r e moving ba se me ta l ,

c he mic a l mi l l ing

E l e c t r o e t c h i n g for pe rma ne nt ly ide n t

par ts

E l e c t r o p o l i s h i n g

for r e f in ing a sur f a c e c

ca l ly

D e pe nding on pa r t s i z e , d ime ns iona l c ons

t ions , and required surface charac ter is t ics ,

t h e s e o p e r a t i o n s c a n b e d o n e w i t h t h e

e quipm e nt a nd s imi la r e l ec t rode s . O nly the

t ions a re dif fe rent .

Anode wrap

Bui ldup -~

/ ~ Anode Stylus handle

Deposi t

Power pack

Q

F i g . 1

Man ual op erat ion of select ive plat ing process. Source: Ref I

ASM Handbook, Volume 5: Surface Engineering

C.M. Cotell, J.A. Sprague, and F.A. Smidt, Jr., editors, p 277-281DOI: 10.1361/asmhba0001261

Copyright 1994ASM InternationalAll rights reserve

www.asminternational.o

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278 / Plat ing and Electroplat ing

T a b l e 3 S e l e c t i v e p l a t in g v e r s u s o t h e r p r o c e s s e s

Selective

C ha r a c te r i s t i c p l a t i ng

F l a m e s p r a y

o r p l a s m a

W e l d i n g m e t a l l i z i n g E l e c t r o p l a t in g

Precision buildup capability Excellent

Qualityof bond Excellent

Heat distortionor stresses None

Heat cracking None

Speed of deposit Fast

Density of deposit (porosity) Very dense(a)

Portability Yes

Requirement or post- Not required on Always required

machining thicknesses up to 0.254

mm (0.010 n.) on

smooth surface

Hydrogenembrittlement No(b) No

Poor Poor Fai r to good

Excel lent Fai r to good Good

Frequent ly Somet imes None

Frequent ly Somet imes None

Very fast Very fast Slow

Very dense, but 70-90% of theoret ical Moderately dense

wi th blowholes densi ty

Yes Somet imes, but over- No

spray precludes i t s use

Almost always requi red Usual ly requi red

No Yes

(a) Generally 25% less porous than electroplating and 70% less porous than f lame spray or plasma metaUizing. (b) Specif ic cadmium,

nickel , and nickel- tungsten deposits have been tes ted on high-s trength s teel and were found to be nonembr it t l ing. Other dep osits may not

c a u s e

embr it t lement. Source: Ref 1

Se l e c t i ve p l a t i ng o f sma l l pa r t s i s more t he

e xc e p t i on t ha n t he ru l e , a nd l a rge vo l ume s o f

s m a l l p a r t s a r e m o r e e c o n o m i c a l l y p l a t e d b y

h i gh-produc t i on- ra t e p roc e s se s , suc h a s ba r re l

p l a t i ng . P l a t i ng o f e n t i r e c om pon e nt s w i t h c om-

pl e x ge ome t r i e s i s be t t e r l e f t t o p roc e s se s suc h a s

t a n k p l a t in g , w h i c h i s m o r e e c o n o m i c a l b e c a u s e

so l u t i ons a re le s s c os t l y a nd t h rough put i s h i ghe r .

Anot he r l i m i t a t i on i s de pos i t r a t e ; bo t h f l a me

spra y i ng a nd we l d i ng d e pos i t me t a l a t a c ons i de r -

ably fas te r ra te .

Appl ica t ions

In ma ny c a se s , a n ope ra t o r b rush p l a t e s on l y

one pa r t a t a ti me . Ne ve r t he l e s s , s e l e c t i ve p l a t i ng

i s a n e f fe c t i ve a nd e c onomi c a l e l e c t rop l a t i ng

proc e s s whe n use d i n a pp l i c a t i on fo r whi c h i t i s

de s i gne d . T he se i nc l ude p l a t i ng pa r t s t ha t a re t oo

l a rge t o i mme rse i n so l u t i on , p l a t i ng a sma l l a re a

o f a l a rg e r c o m p o n e n t , a n d t o u c h u p a n d r e p a i r o f

c ompone nt s , l a rge o r sma l l , t ha t woul d c os t t oo

muc h t o s t r i p a nd re ma nufa c t ure . T he l a rge s t

p a r ts e v e r p l a t e d a r e b u i l d in g d o m e s a n d c h u r c h

s t e e p l e s , up t o a b out 1390 m 2 (15 ,000 f t2 ) , i n

c oppe r , n i c ke l , a nd go l d . T he sma l l e s t a re c e ll s on

r o t o g r a v u r e r o il s , 0 .0 7 6 b y 0 . 0 3 8 m m ( 0 . 0 0 3 b y

0 .0015 i n . ). T he p roc e s s c a n a l so pe r form a t h i gh

p r o d u c t i o n v o l u m e s , s u c h a s 4 , 0 0 0 , 0 0 0 p a r t s / d a y

for n i c ke l -p l a t e d b ra s s e l e c t r i c a l c on t a c t s on a l ka -

l ine ba t te r ies .

Cosm et i c or decorat ive app li ca tions invo lve

part s tha t range in s ize from jewe l ry to la rge exter ior

s t ruc tura l domes and lobby inter iors .

C o r r o s i o n p r o t e c t i o n a p p l i c a t i o n s

a l l o w

t ouc hup o f sma l l rus te d o r c or rode d a re as wi t hou t

d i s a s se mbl y . E xa mpl e s i nc l ude a i r c ra f t l a nd i ng

gears , vi r tua l ly a ll kinds of mach inery, mix ing va ts ,

and tanks . Part s too la rge to f i t into pla t ing tanks a re

a l mos t a l wa ys good c a nd i dat e s .

Repair o f e l ec t ron ic part s i s u s u a l l y d o n e

wi th prec ious meta l s , such as gold, s i lver , and rho-

dium. Typica l appl ica t ions a re repa i r of brok en

ci rcui t s and sm al l c ircui t board contac t s , and appl i -

c a t i on o f c onduc t i ve c oa t i ngs on h i gh-vo l t a ge

sources . Se lec t ive pla t ing minimizes scrap, a l lows

i mme di a t e r e pa i r on s it e , a nd doe s no t r e qu i re i m-

mers ion of the ent i re part in solut ion.

Rest ora t ion o f l arge part s

i s a na tura l appl i -

ca t ion for se lec t ive pla t ing i f the part s a re perma nent

f i x t u re s t ha t c a nnot be r e move d or d i s a s se mbl e d .

T he proc e s s ma ke s i t pos s i b l e t o r e pa i r a nd re s t o re

d i me ns i ons o r a pp l y a we a r - re s i st a n t o r c or ros i on-

res i s tant surface to jus t about any part . Portable

systems a l low access to part s a t grea t e leva t ions and

in remote a reas , such as protec t ing the ins ide surface

of a l a rge mixing v esse l and rep a i r ing cr i t ica l equip-

ment on ships a t sea .

S p e c i a l i z e d a p p l i c a t i o n s of se lec t ive pla t ing

i nvol ve unusua l e nv i ronm e nt s o r t he ne e d fo r c e r -

ta in propert ies , such as con duct ivi ty or imp act res is-

t a nc e . E xa mpl e s i nc l ude b l oc k i ng ra d i o f r e que nc y

interference wi th a soft , radiowave-res i s tant mate-

r ia l such as t in; pla t ing a lum inum to fac i li t a te solder-

i ng ; p l at i ng ma c hi ne ry c ompo ne nt s t o e nsure i n i ti a l

lubrica t ion; and even pla t ing tes t probes to res i s t

e x t re me e nv i ronm e nt a l c ondi t ions .

Fo l l owi ng a re e xa mpl e s o f spe c i f i c a pp l i c a -

t i ons o f s e l e c t i ve p l a t i ng a nd a nodi z i ng :

A d h e s i v e - b o n d e d a i r cr a ft p a r t s : a n o d i z i n g

( p h o s p h o r i c a c i d ) u s e d t o p r e p a r e a l u m i n u m

ski ns a nd f r a me s fo r a dhe s i ve r e p a i r o f da m-

a ge d sk i ns

A i r c r a ft en g i n e c o m p o n e n t s : su l fa ma t e n i c ke l

on h i gh- t e mpe ra t u re n i c ke l a l l oys use d a s a

pre bra z i ng ope ra t i on fo r s e c ond-s t a ge t u rb i ne

va ne s , b l a de s , a nd s t a t o r s e gme nt s ; a l so use d

for r e s t o ra t i on o f be a r i ng a re a on t u rb i ne e x-

ha us t c a se , be a r i ng hous i ng a nd suppor t , a nd

c ool i ng a i r duc t

A i r c r a f t la n d i n g g e a r s : c a d m i u m t o u c h u p ; o n

2 0 1 4 a l u m i n u m , h a r d c o a t o n i n n e r d i a m e t e r o f

o i l a i r s t ru t use d t o a c c ommoda t e p re s s f i t o f

b r o n z e b e a r i n g

A i r c r a f t s k i n s : a nodi z i ng (c hromi c a c i d o r su l -

fu r i c a c i d ) use d t o r e pa i r da ma ge d c hromi c -

a c i d a nd su l fu r i c -a c i d c oa t i ngs , r e spe c t i ve l y

A i r c r a f t w h e e l : a nodi z i ng ( su l fu r i c a c i d ) use d

t o c o r re c t a m i s m a c h i n e d b e a r i n g b o r e

B u s b a r s : s i l v e r o n c o p p e r o r a l u m i n u m

for e l e c t r i c a l c on t a c t s

E l e c t r i c m o t o r s : n i c k e l a n d c o p p e r u s e

d i m e n s i o n a l r e s t o r at i o n o f m o t o r s h a ft

b e a r i n g e n d c a p s ; t i n u s e d f o r l o w - p o w e

t or s t o r e duc e v i b ra t i on ; a nd p l a t i num, go

r h o d i u m u s e d t o i n c r e a s e w e a r l i fe a n d r

a r c i n g o f c o m m u t a t o r s a n d s l i p r in g s

H y d r a u l i c s :

c oppe r , n i c ke l , c oba l t , o r c

use d t o r e pa i r s t e e l - o r c hrome -p l a t e d h

l ic c o m p o n e n t s , o f t e n w i t h o u t d i s a s s e m b

M e t a l r e c o v e r y s y s t e m s : p l a t i n u m c o a t i n

o n t i t a n i u m p e r m a n e n t a n o d e s

M i s s i l e l a u n c h r a il : ha rd c oa t use d on a

T 6 sur fa c e t o b r i ng d i me ns i ons i n t o t o l e

a nd prov i de we a r r e s i s t a nc e a nd c or ros i

s i s t a nc e

M o l d r e p a i r : gol d p l a t i ng use d on p

m o l d s w h e r e c o r r o s i v e g a s e s a t t a c k e x

c hrom e ; f i na l p l a t ing o f c hrom e use d t o

rubbe r mol ds ; c oppe r , n i c ke l , a nd c oba l

t o r e pa i r o t he r t ype s o f mol ds

P r i n t e d c i r c u i t b o a r d s : g o l d o v e r c o p p e

on c o n t a c t t a bs , f i nge r s , e t c .

P r i n t i n g p r e s s e s :

c oppe r , s i l ver , n i c ke l , c

or c hrome p l a t i ng use d t o r e pa i r c y l i

i n - p r e ss o v e r h a u l p l a t i n g o f s i d e f r a m e u s

b e a r i n g r e t e n t io n a n d t o c o r r e c t o u t - o f - c o

t r i c i t y c ondi t i on

R a i l r o a d a x l e s : n i c k e l u s e d f o r d i m e n

r e s t o r a t i o n o n b e a r i n g j o u r n a l s a n d s e

w e a r r i n g g r o o v e s

S p a c e s h u t t l e m a n i f o l d s : u p t o 0 . 5 m m

i n . ) o f c o p p e r u s e d t o i n c r e a s e w a l l s t r

o f a n i c k e l - e l e c t r o f o r m e d h y d r o g e n - o

m a n i f o l d fo r t h e p r o t o t y p e C o l u m b i a

g i n e

S t e e l m i l l c o a t i n g l i n e : c a d m i u m , z i n

n i c ke l -z i nc use d t o i nc re a se t he t h i c kn

t h e p r o t e c ti v e c o a t i n g o n s t ee l s h e e t m o v

137 rn / mi n (45 0 f t / mi n)

S u b m a r i n e s : r h o d i u m a n d g o l d u s e d o n c

s l i p t i ngs

T u r b i n e s : c oba l t , n i c ke l , n i c ke l / c oba

c h r o m e u s e d t o r e s t o re d i m e n s i o n s o n b

a nd se a l su r fa c e s ; s i l ve r use d t o r e pa i r

c u t s on hor i z on t a l s e a l su r fa c e s

Equ ip m e n t

T ypi c a l s e l e c t i ve p l a t i ng sys t e ms i nc l

powe r pa c k , p l a t i ng t oo l s ( c a l l e d s t y l i o r a n

a n o d e c o v e r s , s p e c i a l ly f o r m u l a t e d p l a t i n g

t i o n s , a n d a n y a u x i l i a r y e q u i p m e n t r e q u i r

t h e p a r t i c u l a r a p p l i c a t i o n . T o a c h i e v e o p

d e p o s i t s , e q u i p m e n t s h o u l d b e d e s i g n e

pre s s l y fo r s e l e c t i ve p l a t i ng . Al t hough t a nk

i ng so l u t i ons a nd re c t i f i e r s a re oc c a s i ona l l y

t h e y a r e n o t u s u a l l y r e c o m m e n d e d b e c a u

re su l t a n t de po s i t s a re t h i n a nd qua l i t y i s t y

be l ow pa r .

P o w e r p a c k s r e c t if i er s ) s u p p l y t h e

current and are spec ia l ly designed wi th the f

a nd / or c on t ro l s r e qu i re d by t he p roc e s s . Out pu

a ge c a n t yp i c a l ly be va r i e d f rom 0 t o be t we e n

30 V, com pare d to the usual 6 to 12 V for t an

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S e l ec ti ve P l a t i n g

T a b le 4 C o m m e r c i a l l y a v a i la b l e p o w e r

p a c k s

A l t e r n a t in g c u r r e n t M a x i m u m o u t p u t

input , V P h a s e c u r r e n t , A(a)

115 or 230 1 30

115 or 230 1or 3 60

230 or 460 1or 3 100

230 or 460 1or 3 150

230 or 460 1or 3 200

230 or 460 3 500

(a) At 25 V dc. So urce:Ref2

i ng powe r pa c ks . Powe r pa c ks a re a va i l a b l e i n a

variety of ratings to su it specific applications (Table 4).

Vol tage control i s e x t r e m e l y i m p o r t a n t b e c a u s e

i t r e gu l a t e s t he c ur re n t sup p l i e d t o t he p roc e s s . In

t u r n, t h e a m o u n t o f c u r r e n t c o n s u m e d o v e r t im e ,

m e a s u r e d i n a m p e r e - h o u r s ( A . h ) , d e t e r m i n e s t h e

d e p o s i t t h i c k n e s s . A m p e r e - h o u r r e q u i r e m e n t s

va ry w i de l y fo r d i f f e re n t t ype s o f e l e c t rop l a t e s , a s

i nd i c a t e d by t he e ne rgy fa c t o r s i n T a b l e 1 .

S t e p l e s s vo l t a ge c on t ro l i s a t yp i c a l f e a t u re o f

s e l e c t i v e p l a t i n g p o w e r p a c k s ; m o s t c a n b e a d -

j u s t e d f r o m 0 to 1 0 0 % . T h e m a g n i t u d e o f p o w e r

re qu i re d de pe nds on t he pa r t s i z e , t he de pos i t

t h i c kne s s r e qu i re d , a nd t he p l a t i ng t ype . For e x-

a mp l e , fo r de l i c a t e e l e c t ron i c s pa r t s , a n ou t pu t o f

5 A a n d 1 2 V s h o u l d b e a m p l e , w h e r e a s p l a t in g o f

l a r g e a r ea s w i t h t h i c k b u i l d u p s r e q u i r e s a m u c h

l a rge r sourc e .

A v o l t m e t e r a n d a n a m m e t e r s h o u l d be a va i l -

a b l e a n d s h o u l d s h o w 0 t o 1 0 0 % r a n g e s . T h e

v o l t m e t e r p e r m i t s m o n i t o r i n g o f t h e v o l t a g e r e -

q u i r e d t o p l a t e a g i v e n p a r t. T h e a m m e t e r d i s p l a y s

t h e a m o u n t o f c u r r e n t f l o w i n g b e t w e e n t h e a n o d e

a nd t he c a t hode , a l l owi ng c a l c u l a t i on o f t he c ur -

re n t d e ns i t y .

A p o l a r i t y - r e ve r s i n g s w i t c h

a l l o w s t h e o p e r a t o r

t o a u t oma t i c a l l y c ha nge c ur re n t f l ow d i re c t i on ,

w h i c h i s n e c e s s a r y i n p r e p a r a t o r y o p e r a t i o n s

(e .g . , e t c h i ng a nd de smut t i ng) a nd i n s t r i pp i ng ,

w h e n t h e c u r r e n t m u s t r u n o p p o s i t e t o t h e d i r ec -

t i on use d i n e l e c t rop l a t i ng . Ot he rwi se , t he c on-

n e c t i o n s t o t h e a n o d e a n d c a t h o d e m u s t b e

c h a n g e d m a n u a l l y , s l o w i n g d o w n t h e e n t ir e p r o c -

e s s . Ind i c a t o r l i gh t s t o show po l a r i t y d i r e c t i on

m a y a l s o b e i n c o r p o r a te d .

Safe ty c i rcui t breakers i ns t a n t l y shu t o f f t he

c u r r e n t s h o u l d a s h o r t c ir c u i t o c c u r b e t w e e n t h e

a n o d e a n d t h e w o r k p i e c e . T y p i c a ll y , s h u t d o w n i s

w i t h i n o n e - h a l f c y c l e ( 1 / 1 2 0 s ) , p r e v e n t i n g w o r k -

p i e c e da ma ge a nd i n j u ry t o t he ope ra t o r .

E n e r g y c o u n t e r s ( a m p e r e - h o u r m e t e r s ) a r e r e -

q u i r e d b y m a n y i n d u s t r ia l a n d g o v e r n m e n t s p e c i-

f i c a t i ons . B e s i de s ke e p i ng t r a c k o f t he e ne rgy

b e i n g u s e d , t h e s e m e t e r s m a k e i t p o s s i b l e t o c o n -

t ro l t he t h i c kne s s o f t he de pos i t . C ount e r s ma y

i nc orpora t e s e t po i n t s wi t h v i sua l a nd / or a ud i o

w a r n i n g s t o i n d ic a t e w h e n t h e d e s i r ed t h i c k n e s s

h a s b e e n r e a c h ed .

Opt i onal mi c roproc e s sor -c on t ro l l e d sy s t e ms

r e d u c e t h e c h a n c e o f m i s c a l c u l a t i o n a n d a l l o w

c o n t i n u o u s m o n i t o r i n g o f t h e p r o c e s s v i a a n

a l p h a n u m e r i c r e a d o u t . T h e a m o u n t o f e n e r g y r e -

qu i re d fo r a n a re a t o be p l a t e d t o t he de s i r e d

t h i c kne s s a nd t he a pp l i c a b l e pa ra me t e r s a re de t e r -

m i n e d b y a m i c r o p r o c e s s o r , n o t t h e o p e r a t o r .

S u c h e q u i p m e n t i s a l s o o f m e r i t f o r p l a ti n g m u l -

t i p l e pa r t s t o t he s a me spe c i f i c a t i ons . In some

s y s t e m s , t h e s o f t w a r e p r o m p t s t h e o p e r a t o r f o r

da t a e n t ry fo r a pa r t i c u l a r ope ra t i on , t he n d i sp l a y s

t h e c o r r e c t p r o c e s s p a r a m e t e r s .

The p la t ing t oo l s t y lus) mus t ha ve a n i nsu-

la t ing handle and an anode materia l tha t i s inert ,

insolub le in pla t ing solut ions , and ab le to carry high

current . Graph i te i s by far the mo st prac t ica l choice

for anode materia l . High puri ty i s preferred; any

a ddi t i ve use d t o ha rde n t he g ra ph i t e o r g i ve i t a

di ffe rent physica l charac ter i s t i c inte rferes wi th i t s

be ne f i c i a l qua l i t i e s . Gra phi t e c a n be ma c hi ne d or

sha pe d t o f i t t he c on t our o f t he pa r t be i ng p roc e s se d .

S t a i n le s s s t e e l is mu c h more dura b l e , bu t i t d i s so l ve s

in some pla t ing solut ions , and af te r a short t ime i t

changes the charac ter i s t i cs of the e lec t rolyte be ing

use d . P l a t i num- i r i d i um, p l a t i num-c l a d n i ob i um,

and pla t inum-clad t i t anium are inert to a l l of the

e lec t rolytes used in se lec t ive pla t ing opera t ions , and

t he y a re use d fo r sma l l e r -d i a me t e r a node s . T he i r

d i s a dva n t a ge s a re i nc re a se d c os t a nd h i gh ha rdne s s ,

whi c h ma ke s sha p i ng d i f f ic u l t.

T h e a n o d e c o v e r s (w r a p p i n g m a t e r i a l s )

se rve a s a n i nsu la t o r be t we e n t he a node a nd c a t hod e

a nd he l p e nsure smoot h de pos i t s a t h i gh c ur re n t

densi t i es . Because they hold the e lec t rolyte , they

mus t b e a bso l u t e l y f re e o f o i l a nd fo re i gn ma t e r ia l s .

Any subs t a nc e t ha t c on t a mi na t e s t he e l e c t ro ly t e ha s

a de t r imenta l e ffec t on the pla t ing. For exam ple , any

oi l c on t a mi na t i on wha t soe v e r r e su l ts i n poor a dhe -

s i on . C onse que n t l y , i n a ma c hi ne shop or o t he r

e nv i ronm e nt whe re o i l is wi de l y use d , t he work a re a

shoul d be c a re fu ll y s e l e c t e d . Whe n ne c e s sa ry , a pp l i -

c a b l e pa r t su r fa c e s c a n be so l ve n t o r va por de -

gre a se d or go t h rough a s e pa ra t e c le a n i ng c yc l e .

Va r i ous f i be r l i ke ma t e r i a l s ma ke su i t a b l e c ov-

e r s . C ot t on works ve ry we l l i f i t i s s t e r i l e , l ong

f i ber . Syn t he t i c f i be r s , suc h a s po l y e s t e r a nd ny-

l o n , d o n o t w e t o r h o l d e l e c t r o l y t e s a s w e l l a s

c o t t on , bu t t h i s doe s no t p re c l ude t he i r use . Po l y-

e s t e r f e l t i s t yp i c a l l y s e l e c t e d whe n t he s a me

a n o d e w i l l b e u s e d f o r n u m e r o u s p a r t s o r f o r

h e a v y d e p o s i t s . M o s t o f t h e s e m a t e r i a l s w o r k

w e l l i n t h e f o r m o f t u b e g a u z e ( b a n d a g e c o v e r s )

a s c o v e r s o v e r c o t t o n . S c o t c h - B r i te h a s b e e n u s e d

w h e n h e a v y / h a r d d e p o s i t s a r e r eq u i r e d , a n d i t ca n

1

>

0

E

I , .

( .1

P o o r r a n g e :

B u r n e d o r p o r o u s d e p o s i ts

j ~ P o o r a n g e :

/ ~ S low deposi t ion rates a

poor depos i t qual i ty for

s o m e s o l u ti o n s

A n o d e - t o -c a t h o d e s p e e d -

F i g

2

Relationship between current density and

to-catho de speed. Source: Ref 3

a l s o f u n c t i o n a s a b u r n i s h i n g t o o l , i m p r

t h e s u r f a c e a s p l a t i n g c o n t i n u e s . I f a s u r f a

s o f t a n d e a s i l y s c r a t c h e d , a d i f f e r e n t

s h o u l d b e c h o s e n .

K e y P r o c e s s E le m e n t s

A n o d e - C a t h o d e M o t i o n . C o n t r o l l i n g

t in u o u s m o v e m e n t b e t w e e n t h e a n o d e a n d t h e

p i e c e , o r c a t hode , i s a ke y e l e me nt i n ob t

h i gh-qua l i t y b rush-p l a t e d de pos i t s. Ho we ve r ,

i ty a l so depends on pla t ing wi thin a spec i f ic c

densi ty range , so both variables a ffec t ul t ima

posi t qua l i ty. This re la t ionship i s i l lus t ra ted i

2 . So l u t i on suppl i e r s rou t i ne l y r e c omme nd r

of a node -c a t hod e spe e ds a nd c ur re n t de ns i t y

for each solut ion; a representa t ive l i s t i s giv

Table 5 .

T h e v i s u a l a p p e a r a n c e o f t h e e l e c t r o p l

a l so a n i nd i c a t o r o f qua l i t y . A da rk g ra y o r

c o l o r u s u a l l y c o r r e s p o n d s t o a b u r n t d e

w h i c h r e s u l ts f r o m t o o h i g h a c u r r e n t d e n s

i n s u f f i c i e n t m o v e m e n t . I n c o n t r a s t , i n a d e

c u r r en t d e n s it y o r t o o m u c h m o v e m e n t p r o

a ge ne ra l l y sh i ny sur fa c e .

A n o d e - t o - c a t h o d e m o v e m e n t m a y b e a c h

m a n u a l l y o r m e c h a n i c a l l y , s u c h a s b y u s i n g

i n g e q u i p m e n t t o p r o v i d e a c o n s t a n t r o t a

T a b l e 5 A n o d e - c a t h o d e m o t i o n a n d c u r r e n t d e n s i ty f o r s e l e c t i v e p la t i n g s o l u ti o n s

A n o d e - c a t h o d e m o t i o n C u r r e n t d e n s i t y

S e l ec t i ve p l a t i n g s o l u t i on m /s f tY m i n A / d m 2

A/ft2

Cadmium acid) 0.26-0.561 50-110 86.4

Cadmium LHE (lowhydrogenem- 0.20-0.41 40-80 86.4

brittlement ormula)

Chromium 0.02-0.03 4-6 86.4

Cobalt machinable) 0.13-0.26 25-50 115.2

Copper (high speed,acid) 0.20-0.51 40-100 144.0

Gold 0.15-0.31 30-60 28.8

Lead 0.15-0.26 30-50 86.4

Nickel (acid) 0.10-0.26 20-50 86.4

Nickel (high speed) 0.20-0.41 40 '80 144.0

Nickel-tungstenalloy 0.10-0.15 20-30 72.0

Rhodium 0.03-0.05 5-10 43.2

Silver heavybuild) 0.10-0.31 20-60 72.0

Tin (alkaline) 0.10-0.41 20-80 86.4

Zinc (alkaline) 0.15-0.612 30-120 115.2

Data providedcourtesyof SIFCO SelectivePlating

864

864

864

1152

1440

288

864

864

1440

720

432

720

864

1152

8/12/2019 Asmhba0001261 Ouro Escova Otimo

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2 8 0 / P l a t i n g a n d E l e c t r o p l a t i n g

, i l i i i i i i i ) i i i i i i i i i i i i ii i i i i i i i i i i i i i i i i i i i i i

. . . . . . . . . . . . . . . . . . . . . . . . . : : : : : : - . . . . .. . . . .. . . . .. . . . .. . . .. ~ . ~ . ~ . ~ . ~ . ~ : ~ : : : : , : , :, : : : ~ : : : : ~ : ~ : : , : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :: : : : : :: : : : : : :: : : : : :: : : : : : ~ : : ~ . ~

. . . . . . . . . . . . . . . . . :~:~:~'~ :~ '~:~'~ : : : ~ . . .. . .. .:~:~:~,~:~'~'~i~ :~ : :~ :: :: :: :: :~ %i : } ~ i

. : . . : i . . . . . . . . . . . . . . . : : : : i : : : : : : : :: : ~. . . . : . : i i l Z i : i i i : :

~ : : : . ~ : : . . . ~ : . ~ : : : ~

i~i~i~;..... "::::::::::::::::::::::::: ~::~.~:~............~ : - ~:~:~: ~.::~

. . . . .. . . . : . : .

~ i ~ i~ i ~ i~ i y ~i ~ i i ~ i ~ i ~ ~ i ~ }i ~ i~ ;~........

Fig. 3 T u r n i n g h e a d . C o u r t e s y o f S I F C O S e l e c t i v e P l a t in g

R O T O S T

YLUS

F i g . 4 R o t o s t y l u s . C o u r t e s y o f S I F C O S e l e c t i v e P la t i n g

speed f o r cy l i nd r i ca l pa r t s ( F ig . 3 ) o r by u s ing

spec i a l l y des igned t i l t i ng t u r n t ab l es t o r o t a t e

l a r ge pa r t s a t con t r o l l ed speeds . A no the r op t i on i s

t he r o to s ty lu s ( F ig . 4 ) , wh ich r o t a t e s t he anode

ins t ead o f t he wor kp i ece .

A n o d e s a n d F l o w t h r o u g h . F o r t h e p l a t i ng

process to be ef f ic ient , the plat ing solut ion must

f low between the anode and the area being plated.

Solut ion can be supp l ied by per iodical ly dipping the

plat ing tool in to the elect rolyte . However , the m ost

ef f ic ient meth od is to pum p the s olut ion through the

block anode and into the in ter face between the an-

ode and the work piece (Fig . 5). P lat ing of large areas

at h igh cur rents requires the use of a pum p to reci r -

culate the solut ion. This keeps the solut ion f rom

overheat ing, resul ts in th icker bui ldups on large

areas, and al lows the use of h igh er cur rent densi ties.

In addition, the entir e process is faster. Various type s

o f pum ps can be u sed , depend ing on t he am per age

and on wheth er preheat ing and/or f i l ter ing is neces-

sary for the solut ion used.

S o lu t i ons . Th r ee bas i c t ypes o f so lu t i ons a r e

used i n se l ec t i ve p l a t ing : p r epa r a to r y , p l a t i ng , and

spec i a l - pu r pose . Tab l e 6 shows t he m os t com m on

so lu t i ons and t he i r u ses . The base m e ta l and t he

type o f p l a t i ng gene r a l l y d i c t a t e wh ich so lu t i ons

are appropr iate .

Preparatory solut ions

clean the subst rate sur -

face so that i t can ef fect a bet ter bond wi th the

e l ec t r op l a t e . P r epa r a t i on t yp i ca l l y i nvo lves p r e -

c l ean ing , e l ec t r oc l ean ing , and e l ec t r oe t ch ing ;

som e base m a te r i a l s a l so r equ i r e desm ut t i ng , ac -

t i va t i on , and p r ep l a t e ope r a t i ons . P a r t s w i th

heavy co r r os ion , l ub r i can t s , o i l , and so on o r d i -

na r i l y r equ i r e m or e agg r ess ive c l ean ing , such a s

vapor / so lven t deg r e as ing o r g r i t b l a s ti ng , p r i o r t o

p r ec l ean ing .

Plat ing solut ions

used f o r se l ec t i ve p l a t i ng

have a m uch h ighe r concen t r a t i on o f m e ta l , u su -

a l l y a s o r ganom eta l l i c sa l t s , t han do so lu t i ons

used f o r t ank p l a t i ng . Th i s h ighe r m e ta l con t en t

pe r m i t s t he u se o f h ighe r cu r r en t dens i t ie s , wh ich

r esu l t s i n f a s t e r depos i t i on , be t t e r bond s t r eng th ,

and l e ss po r os i t y t han i n t ank p l a t i ng .

Two bas i c k inds o f p l a t i ng so lu t i ons a r e u sed .

One depo s i t s a t h in p r ep l a t e ( s t r i ke ) t ha t boos t s

adhes ion on ce r t a in m e ta l s and a l l oys , and t he

o the r bu i l d s up t he coa t i ng t o i t s f unc t i ona l t h i ck -

ness . S upp l i e r s u su a l l y o f f e r a cho i ce o f so lu t i on

f o r each t ype o f e l ec t r op l a t e , because d i f f e r en t

p r ope r t i e s a r e r equ i r ed f o r d i f f e r en t app l i ca t i ons

( e .g . , h igh ha r dness and wear r e s i s t ance f o

duct i l i ty for another ) .

Special-purpose solut ions i nc lude t hose

f o r pos t - t r ea tm en t , anod i z ing , and e l ec t r op

ing.

Thickness C o n t r o l . The t h i ckness o f a d

can be con t r o l l ed by m on i to r i ng t he am per

meter . Each solut ion has a prescr ibed e nergy

which i nd i ca t e s how m any am per e - hou r s a

S o [ u t ~ o n

i m f i o w

c o o : l ~ r

/ 2 " . . . .

[ "

St1us handle

F : l o w ~ : t h r o u g h

anode { S c o t : c h ~ b r i t e wrap}

R o t a t i o n M e t a l d e p o s i t

d i r e c t i o n

oiuiion sump

F i t e r S o k ~ t i o n

cartridge pump

{ p e r i s t a l t i c }

F i g . S F l o w - t h r o u g h d e p o s i t io n . C o u r t e s y o f V a n g u a r d P a c i f i c , I n c .

8/12/2019 Asmhba0001261 Ouro Escova Otimo

5/5

S e l e c t i v e P l a t i n g

quired to deposi t a given meta l thickness on a given

area:

A . h= F x A x T

Table 6 Typica l se lec t ive p la t ing solut ions

P r e p a r a t o r y s o l u t i o n s

Activating (for industrial hard chrom ium)

Cadmium activator

Chromium activator (for dec orative applications)

Cleaning (for most materials)

Desmutting (for cast iron, carbon and alloy steels, copper alloys)

Etching (for aluminum alloys, steels, cast iron)

Etching and activating (for high-temperature nickel-base alloys

and stainless)

P l a t i n g s o l u t i o n s f o r f e r r o u s a n d n o n f e r r o u s

met a l s

Nickel (acid strike)

Nickel (alkaline)

Nickel (dense)

Nickel (ductile, for corrosion protection)

Nickel (neutral, for heavy buildup)

Nickel (sulfamate, hard, low

stress)

Nickel (sulfamate, moderate hardness)

Nickel (sulfamate, soft, low stress)

Antimony

Bismuth

Cadmium (acid)

Cadmium (alkaline)

Cadmium (no-bake)

Chromium (hexavalent)

Chromium (trivalent)

Cobalt (for heavy buildup)

Copper (acid)

Copper (alkaline)

Copper (high-speed acid)

Copper (high-speed alkaline for heavy buildup)

Copper(neutral)

Iron

Lead(alkal ine)

Lead (for alloying)

Tin (alkaline)

Zinc (acid)

Zinc (alkaline)

Zinc (bright)

Zinc (neutral)

P l a t i n g s o l u t i o n s f o r p r e c i o u s

met a l s

Gal l ium

Gold (acid)

Gold (alkaline)

Gold (neutral)

Indium

Palladium

Platinum

Rhenium

Rhodium

Silver (hard)

Silver (noncyanide)

Silver (soft)

P l a t i n g s o l u t i o n s f o r al loys

Babbitt Navy Grade 2

Babbitt SAE 11

Brass

Cadmium-fin

Cobalt-tungsten

Nickel-cobalt

Nickel-tungsten

Tin-indium

Tin-lea_d-nickel

S p e c i a l - p u r p o s e s o l u t i o n s

Anodizing (chromic)

Anodizing (hard coat)

Anodizing (sulfuric)

Chromate treatment

Electropolishing solution

Source: Re f 2

where A. h is the ampere-hours required, F is the

energy fac tor , A is the a rea to b e pla ted ( in square

centimeters) , and T is the thickness of the deposi t ( in

microns) . The ca lcula t ion yie lds a fixed va lue tha t

can be mo nitored on the meter (or se t on the amp ere-

hour counter , i f the power pac k is so equipped) . I f

needed, s imple addit ional ca lcula t ions can a lso be

performed to de termine the opt imum current , pla t-

ing t ime, volum e of pla t ing solut ion, and even rota-

t ional speed (Ref 4) .

S p e c i f i c a t i o n s

Sinc e the mid 195 0s , w he n the f i r s t c om me rc ia l

s e le c t ive p la t ing spe c i f i c a t ion w a s in t roduc e d ,

the numbe r of spe c i f i c a t ions ha s surpa s se d 100 .

The fo l low ing l i s t i nc lude s some of the more

impor ta n t indus t r i a l , gove rnme nt , a nd mi l i t a ry

specif ica t ions:

M I L - S T D - 8 6 5 C ( U . S. A i r F o r c e )

M I L - S T D - 2 1 9 7 S H ( U .S . N a v y )

B A C 5 8 4 9 , 5 8 5 4 ( B o e in g )

D PS 9 .8 9 (D ougla s A i rcra f t )

MPS 1118 A (Loc kh e e d A i rc ra ft )

SS 8 413 , 8 494 (S ikorksy A i rc raf t )

B P S F W 4 3 1 2 ( B e l l H e l i c o p te r )

FPS 1046 (G e ne ra l D yn a mic s )

H A 0 1 0 9 - 0 1 8 ( N o rt h A m e r i c an R o c k w e l l )

G S S P O 6 0 B ( G r u m m a n )

S ta nda rd Pra c t ic e Ma n ua l 70-45 -03 a nd T .M.

N o. 72-191 (G e ne ra l E le c t ri c )

PS 137 , Is sue 1 (D ow ty Roto l L td . , Engla nd )

ITF 40-8 39-01 (Me ss ie r -H ispa no , Fra nc e )

TCM K -5 (Sa a b-Sc a nia , Sw e de n )

R e p o r t N A E C - A M L 1 6 1 7 ( N av a l A i r E n g i-

ne e r ing Ce nte r , Ph i l a de lph ia )

M - 9 6 7 - 8 0 ( A s s o c i a t i o n o f A m e r i c a n R a i l -

roa ds )

A C 43 .13-1A , Chg. 1 (Fe de ra l A via t ion A d-

mini s t r a t ion)

A M S 2 4 3 9 , 24 4 1 , 2 4 2 4 C

SPO P 321 (Prat t & Whi tne y)

In a ddi t ion to the a bove spe c i f i c a t ions , ma ny

U .S . N a vy a nd U .S . A i r Forc e a c t iv i t i e s ha ve

ge ne ra te d ins t ruc t ions for s e le c t ive p la t ing of

spe c i f i c ha rdw a re o r c ompo ne nt s a t spe ci f i c mi l i -

ta ry fac i l i t ies .

In the e l e c t ron ic s f i e ld , the r e pa i r o f p r in te d

c i r c u i t boa rds for m i l i t a ry a ppl i c a t ions i s a u thor -

i z ed b y M I L - S T D - 8 6 5 C . F o r c o m m e r c i a l a p p l i-

c a t ions , the Ins t i tu te for In te rc onne c t ing a nd

Pa c ka ging E le c t ron ic C i r c u i t s a pprove s the use

of s e le c t ive p la t ing for r e pa i r o f bo th ba re a nd

a s se mble d pr in te d c i r c u i t boa rds (Re f 5 ) . I n a ddi -

t ion , ma ny e le c t ron ic s c ompa nie s ha ve i s sue d

the i r ow n spe c i f i c a t ions for in -house r e pa i r o f

c i r c u i t boa rds a nd o the r e l e c t ron ic c omp one nt s .

H e a l t h a n d S a f et y C o n s id e r a t io n s

The re a r e tw o sa fe ty i s sue s in s e le c t ive p la t ing :

the c he mic a l s a nd the e quipme nt use d to a pply

the c he mic a l s . O ve ra l l , t he proc e s s me e t s the

sa fe ty r e qui r e me nts s e t by the O c c upa t iona l

Sa fe ty a nd H e a l th A dmini s t r a t ion (O SH A

in-p la n t use . E le c t r i c a l p ro te c t ion of the

p a c k , w o r k p i e c e , a n d o p e r a t o r a r e p r o v i d

a d i r e c t c u r r e n t c i r c u i t b r e a k e r w i t h i

p o w e r p a c k . T h i s c i r c u i t b r e a k e r t r ip s w h

o v e r l o a d o c c u r s . I f t h e a n o d e s h o r t s o

c onta c t ing the p~a rt , o r i f t he po w e r pa c

c e e d s i t s a m p e r a g e r a t i n g , t h e b r e a k e r a

i n s t a n t l y s t o p s p o w e r f l o w .

Toda y , ma nufa c ture r s p rovide ma te r i a l

da ta she e t s (MSD S) w i th so lu t ions , in c

m a n c e w i t h O S H A r e q u i r e m e n t s . T h e s o l

a re indus t ri a l c he mic a l s a nd sho uld be ha nd

suc h . V e nt i l a t ion to r e move fume s i s r

me nde d , a nd c onve nt iona l s a fe ty gu ide l in

c lud ing the use of s a fe ty g la s se s a nd

g love s , should be rou t ine ly fo l low e d. Re vi

the MSD S w i l l a l e r t pe r sonne l to a ny s

ha ndl ing o r o the r s a fe ty pre c a ut ions r e qui r

A C K N O W L E D G M E N T S

The fo l low ing ind iv idua l s c ont r ibu te d

r e v i s ion of th i s a r ti c l e: Robe r t R . Broo kshi

L e s t e r S i m m o n s , B r u s h t r o n i c s E n g i n e

Fra nc ine H a mme r , Jose ph C . N or r i s , a nd

V a ne k , SIFCO Se le c t ive P la t ing ; a nd B

H u t c h i n s o n , L i q u i d D e v e l o p m e n t C o .

REFERENCES

1. M. Rub ins te in , Electrochemical Metal

Vinm ar Press , 1987

2. J.C. Norris, Brush Plating: Part I, Metal F

ing, July 1988, p 44-48

3. DA LIC Process Ins truc t ion M anual , 5

SIFCO Selec t ive Pla t ing, 1990

4. J .C. No rr is , Brush Pla t ing: Par t I I , Metal F

ing, Aug 1988, p 45-47

5. Modif ication and Repair or Printed Boar

Assemblies,

Man ual IPC-R 700B , Ins t i tute

te rconnect ing and Packag ing Elec tronic C

SELECTED REFERENCES

Brush P la ting Course Manua l ,

L i q u i d D

opm e nt Co. , 1993 , p 2-7

J . La nga n a nd M. Ca pora so , Brush P

A ppl ic a t ions U t i l i z ing A e roN ikl Sul

N i c k e l o n A i r c ra f t C o m p o n e n t s , p a p e

s e n t ed a t S A E 2 7 t h A n n u a l A e r o s p a c e / A

P l a t in g a n d M e t a l s F in i s h i n g F o r u m a n d

s i t ion (Sa n A ntonio , TX ) , Ma rc h 1991

D .W. Ma i t l a nd a nd M. J . D e i t s c h ,

S

Cleaning, Finishing, and Coating,

9th e

5 , Me ta ls Handbook , A SM In te rna

198 7 , p 292-299

J .C . N or r i s , B rush A nodiz ing in the

c ra f t /A e rospa c e Indus t r i e s , Pla t ing an

face F in ish ing , A ug 1991

J .C . N or r i s , B rush H a rd Coa t ing A lum

a n d A l u m i n u m A l l o y s , p a p e r p r e s e n

S A E 2 6 t h A n n u a l A e r o s p a c e / A i r l i n e P

a n d M e t a l F i n i s h i n g F o r u m a n d E x p o

(Tulsa , O K ) , A pr i l 1990