Solder coating process for printed copper circuit protectio

Wittmann, A. E.

English

NASA Technical Reports Server

REPLY TO 
ATTN OF: GP 
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
WASHINGTON. D.C. 20546 
March 27, 1971 
TO : USI/Scientific & Technical Information Division 
FROM r GP/Office of Assistant General 
Attention: Miss Winnie M. Morgan 
Counsel for Patent Matters 
SUBJECT: Announcement of MASA-Qwned 
U.S. Patents in STAR 
In accordance with the procedures contained in the Code GP 
to Code US1 memorandum on this subject, dated June 8 ,  1970, 
the attached NASA-owned U.S. patent is being forwarded for 
abstracting and announcement in NASA STAR. 
The following information is provided: 
U,S. Patent No. : 3,359,132 
Corporate Source : Marshall Space Flight Center 
Supplementary 
Corporate Source 
NASA Patent Case No, : XMF-03.599 
Gayle Parker 
Enclosure: 
Copy of Patent 
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https://ntrs.nasa.gov/search.jsp?R=19710011230 2020-03-17T02:53:52+00:00Z
393 A. E. WlTTMAN 
METHOT, OF COATING CIRCUIT PATHS ON PRINTED 
Dec. 19, 1967 
CIRCUIT BOARDS WITH SOLDER - 
Filed July 10, 1964 
CONTROL AND 
SOURCE OF 
48 46 44 
FIG. I 
F I G . 2  
lNVENTOR 
ALBIN E WITTMANN 
1 
satisfactory results in that the resulting coating generally 
is rough, with a wavy surface, and is not of uniform 
thickness. 
The above enumerated disadvantages of both the gc'd 
and solder coatings have been overcome by the vastly 
superior method of coating that is the subject of this ap- 
plication and which is accomplished in the following 
manner. A printed circuit board is held above a bath 
I 
3,359,132 
METHOD OF COATLR'G C I R C m  PATHS ON 
PRPNTED CIRCUIT BOARDS WITH SOLDER 
Albin E. Wittmann, Huntsville, AI&, assignor to the 
United States of America as represented by the Na- 
tional Aeronautics and Space Administration 
Filed July 10,1964, Ser. NO. 381,940 
10 Claims. (CI. 117-212) 
of molten solder having a tliin layer of oil floating there- 
on. Flux is applied to the board which is then pre-heated 
ABSTRACT OF TNE DISCLOSURE lo by holding it close to the surface of the hot oil. The board 
A method of coating copper circuit paths On printed is then submerged in the molten solder, withdrawn, and 
circuit boards to prevent oxidation. A printed circuit spun. The spinning action clears the solder from the com- 
board is held above a bath of molten solder and flux is ponent securing holes on the circuit board and removes 
applied. The board is then submerged in the bath for a 16 excess solder from the circuit paths leaving a complete, 
short period of time, withdrawn and spun rapidly to re- uniform, and smooth finish- 
move excess solder. A smooth, uniform coating is ob- Several advantages result from the use of the dip and 
taincd, and an intermetallic compound is formed at the spin method in addition to the uniform smooth finish. 
interface, ensuring maximum adhesion of the coating. For one, the thickness of the coating can be controlled 
20 by varying the spinning speed and hence the amount of 
solder removed. Also, the spinning action promotes a 
The invention described herein may be manufactured rapid cooling of the circuit board which reduces the sur- 
and used by or for the Government of the United States face tension d the solder coating which, as is well known 
of America for government purposes without the pay- in the art is very desirable. Furthermore, a new inter- 
ment of any royalties thereon or therefor. 26 metallic compound of uniform thickness is produced at 
This invention relates to a method for the surface coat- the interface consisting of the copper of the circuit path 
ing of articles with solder of a controlled thickness and and the solder alloy. This alloy is highly desirable in that 
uniformity. More particularly this invention relates to a it insures n w h u m  adhesion between the solder and the 
method for coating circuit paths of a printed circuit board copper circuit Path* 
by dipping the board into a bath of molten solder, re- 30 Perhaps one of the most important advantages result- 
moving it from the ba%, and subjecting it to a contro&d ing from the dip and spin method of solder coating is 
spinning motion to remove excess solder. the quality control factor which is introduced. If there 
A persistent and long standing problem in the art of is a corroded spot on the circuit path before coating, the 
manufacturing printed or etched circuit boards results cast from that point by the spinning action 
from the fact that oxidation of the copper circuit paths 35 since there is no adhesion between the products of cor- 
commences the moment thcy are exposed to the atmos- rosion and the solder. Such bare spots can be easily de- 
phere. Obviously such is highly undesirable in that it tected and corrective action taken. 
leads to very difficult manufacturing problems such as AccordiWlY, it is an object af this invention to provide 
short siorage life and poor, weak, unreliable connections a method for coating circuit paths of printed circuit 
between the circuit path and the mounted electronic com- 40 boards with 
ponents. Short storage life results in  high inventory Costs It is another object of the invention to provide a meth- 
while tlie lack of reliable connections gives rise to ob- od for coating the Circuit paths of printed circuits in such 
vious disadvantages. a manner as to provide a uniform. smooth protective 
A common method for preventing oxidation of copper layer* 
CiEUit paths on printed circuit boards is electro-plating 45 Yet another object of the invention is the utilization of 
with gold. Whi!e such an expedient does offer Some pro- a method for solder coating circuit paths on printed cir- 
on against corrosion, it is subject to severd very cuit boards in which the solder is removed from soiled or 
serious drawbacks in addition to the evident high cost. corroded 
For example, the quality of the plating is directly related another object of the invention is to provide a 
to the dependability of the operator. and is dependent on 60 method for coating the circuit paths on printed circuit 
such factors as cleanliness, current, temperature, time, boards in which rapid cooling of the board is effected. 
etc. Furthermore, no accurate prediction can be made of Another object of the invention is to provide a method 
the existence of oxidation and impurities between the for coating circuit paths on printed circuit boards in 
plating and base material. Obviously, any point where which an alloy of the material of the path and the solder 
oxidation OccLIrL is a "weak link" subject to physical or 55 is produced at the interface between them. 
electrical breakdovr n which will incapacitate the circuit. These and other objects and advantages of this inven- 
Another tlndesirable rcsult of gold plating arises from tion be more apparent Upon reference to the follow- 
the bch'lvior of tho gold (with a melting point of 1940° ing specification, appended claims, and drawings wherein: 
F.) going into solution with molten solder. This phenom- FIGURE 1 is a side elevational view, partlyin section, 
en011 leads to two very undesirabie conditions. Namely; 60 Of utilized in carving OUt the dip and spin 
( 1 ) the gold will be absorbed by the tin of the molten coating which is the subject matter Of 
solder compound leaving the oxidation problem unre- FiGURE 2 illustrates a broken out portion, in section solved, and (2 )  oxidation and/or the alloy formed by the of a circuit board having a copper circuit path thereon, solder-gold combination results in under-strength solder 65 a solder coating on the circuit path, and a solder and 
copper alloy interface. joints. 
In order to alleviate Some of the disadvantages of gold 
plating, a solder coating has been utilized. To obtain such coating it be described in relation to the depositing 
a coating, a circuit board is dipped into a bath Of molten of a layer of solder on the circuit paths of a printed 
solder, withdrawn and allowed to Cool. Oftentimes the 70 circuit board, It is to be understood, however, that vari- 
board is mechanically or hand slung as it is withdrawn ous other uses may be found for this novel method. F~~ 
from the bath. This method, however, also produces un- example, it could be used to deposit a uniform layer of 
Will 
method Of 
the present invention; and 
order to better understand this novel method 
3,359,132 
3 
paint or a thermo-setting plastic on articles where such 
is deemed desirable. Other uses will be readily apparent 
to those skilled in the art. 
With continued reference to the accompanying figures 
wherein likc numerals designate similar parts through- 
out the various views, and with initial attention directed 
to FIGURE 1, reference 10 designates an upright standard 
tncluding a base I2 at its lower end and a plurality of 
gear teeth 14 formed along one of its sides. A horizontal 
mounting arm 16 is slidnbly secured to the standard and 
includes a motor mounting shelf 18 and mounting ears 
20 disposed on either side of the standard 12 to which 
a shaft 22 is rotatably secured. A motor 24 is secured 
to the shelf 18 and drives a gear (rlot shown) mounted 
on the shaft 22 through the gear box 26. The teeth of 
the gear mounted on the shaft 22 mesh with the teeth 14. 
A pair of brackets 28 are mounted on the end of the 
mounting arm 16 at a point remote from the standard 
10 and serve to secure a motor 30 to the arm. This 
motor may be of any type which will produce a rotary 
output motion, but is preferably a pneumatic motor pro- 
viding a high starting torque. 
Depending from the motor 30 is an output shaft 32 
having a mounting plate 34 secured to its lower end. A 
plurality of circuit board holders 36 and 36' are secured 
along the periphery of the mounting plate 34. Slots 38 
in the plate 34 accommodate bolts 40 which are threaded 
into the circuit board holders 36' so as to provide an 
adjustment that will allow circuit boards 43 of different 
sizes to be held in position. These adjustable circuit 
board holders also are moved outwardly to allow in- 
sertion and removal of the boards 42. 
An open top container 44 constructed of a suitable 
insulating material, is positioned immediately beneath the 
niounting plate 34. A quantity of molten solder 56 is 
held in the container 44 and maintained in a liquid state 
by a heating element 48. The temperature of the heating 
element, and hence the molten solder 46, is controlled 
by a variable thermostat 50. 
Floating on the surface of the molten solder 46 is 
a layer of peanut oil 52 which is used to reduce the 
wrface tension of the molten solder and to provide a 
barrier to air to prevent oxidation of the copper during 
the coating process. A shield 54 extends above and en- 
closes tqe container 44 to prevent spreading of molten 
solder during the spin cqcle. An aperture 56 in the shield 
54 provides an entrance way for the heating element 48. 
While not shown it is obvious that motors 24 and 30 
can be controlled either manually or on an automatic 
cycle. If the latter system is u e d ,  the motors will be 
interlocked by a series of suitable valves and time con- 
trolled switches. 
The method 
In operation, after a circuit board 42 to which flux 
has been applied has been securefy fastened to the mount- 
ing plate 34 by means of circuit board holders 36 and 
36', the motor 24 is actuated to drive the gear mounted 
on shaft 22 in such a manner 'that the horizontal arm 
16 is lowered. The motor 24 is stopped when the circuit 
board 42 is disposed in close proximity to a layer of 
oil 52 which is resistant to high 'temperature such as, 
for example, peanut oil. The board is held in this po- 
sition and slowly spun for a short period of time pntil 
it is sufficiently heated. The slow spin of the board is 
halted and it is then submergcd,in the molten solder for 
a predetermined time before being withdrawn by re- 
versing motor 24. As the board emerges from the oil 
film 52, i t  is immediatcly spun to both remove excess 
solder and to provide for rapid cooling. 
Obviousiy many combin:itions o f  times and spinning 
speeds for the various cycles of this operation may be 
used. Satisfactory results, howeycr, have been obtained 
by holding the circuit board about one inch above the 
burface of the peanut oil for appro.~imately 5 seconds 
to accomplish preheating. A submersion time Of 1-3 sec- 
0 
10 
15 
20 
25 
30 
35 
40 
45 
50 
55 
60 
65 
70 
75 
onds and a spinning speed of 1200-1800 r.p.m. hcld for 
1 or 2 second? produccs a \mooth uniioim lini\h. OF 
coune, the spinning spccd is limited by the iihi1t.y o i  
the board to withL,tand centriful;al forces and tiic !Li;\Ii 
hickne\s of coating desild.  A boicier havinp a coni- 
on of 60% lin and 40% lead ni:iintaincd at i i  tcm- 
perature of 500" F. has been founu to produce excellent 
results. 
As best shown in FIGURE 2, an alloy 62 which is 
believed to consis! of CuOSn:,, is formed at the ititcrf.,ce 
between the coppcr circuit path 58 and the wlticr coat- 
ing 60. This formation IS highly desirable i n  t h d  i t  in- 
creases the adhesion between these two layers. Thi\ 
alloy layer is uniform and contains no corroded spa;\ 
due to the rapid cooling and uniform dibperbion produccd 
by the spin cycle. 
It will be apparent that by utilizing the teachings 
this invention, a coating which i.; uniform in thickn 
and in finish can be produced. The thichnes, of the CL 
ing is controlled by varying the spinning speeds. The 
rapidity of the spin produces a cooling effect which 
eliminates the possibility of corrosion spots on the inter- 
face between the cooper circuit path and the coating. 
If there is a p:eviously existing corroded spot on a cir- 
cuit path, the solder will be cast from that point by the 
centrifugal force and, therefore, the circuit boards will be 
of a uniformly high quality. Such has not been hercto- 
fore possible in existing methods of coating which pro- 
duced bare spots subject to corrosion, and very rough 
finishes, Circuit boards coated by utilizing the method 
which is the subject of this invention, may be stored 
many months and will provide for strong and reliable 
connections with components mounted on the board. 
The invention may be embodied in other specific forms 
without departing from the spirit or essential charac- 
teristics thereof. The present embodiment is thercfore 
to be considered in all respect as iilustrative and not 
restrictive, the scope of the invention being diciated by 
the appended claims rather than by the foregoing descrip- 
tion, and all changes which come within the meaning 
and range of equivalency of the claims are therefore 
intended to be embraced therein. 
What is claimed and desired to be secured by Unitcd 
States Letters Patent is: 
1. The method of coating the circuit paths of a printed 
circuit board with solder comprising: 
(a )  positioning said circuit board in a holder; 
(b)  lowering said holder and said circuit board into 
a bath of molten solder; 
(c) allowing said holder and said circuit board to dwell 
in said bath for a short period of time whereby aq 
intermetallic bond between said circuit paths and said 
coating is produced; and 
(d)  spinning said holder and circuit board immediately 
upon withdrawal to remove excess solder from said 
circuit board to thereby produce a coating on said 
circuit board having a uniformly smooth finish and 
the desired thickness. 
2. The method of claim 1 wherein flux is npplicd to 
said circuit board before lowering said circuit board into 
said bath of molten solder. 
3, A method according to claim 1 whcrein said holtlcr 
and said circuit board are held above said molten Saih 
before lowering whereby said circuit board is prehca!cd. 
4. A method according to claim 3 whcrein said holicr 
and said circuit board are slowly rotated during the prc- 
heating cycle. 
5. The method of coating the circuit paths of a printed 
circuit board with solder comprising: 
(a) positioning said circuit board in a holder; 
(b)  lowering said holder and said circuit board to a 
point just above a bath of molten solder and holrling 
said holder and said circuit board above said molten 
bath for approximately 5 seconds; 
(c) lowering said holder and said circuit boaid into 
said molten bath; 
3,359,132 
5 
(d)  allowing said holder and said circuit board to 
dwell in  said bath for a short period of time whereby 
a intermetallic bond between said circuit paths and 
said coating i s  produced; 
(e) withdrawing said holder and circuit board from said 
bath; and 
( f )  spinning said holder and circuit board immediate- 
ly upon withdrawal to remove excess solder from said 
circuit board to thereby produce a coating on said 
circuit board having a uniformly smooth finish and 
the desired thickness. 
6, The method of claim 5 wherein flux is applied to 
said circuit board before lowering said circuit board into 
said bath of molten solder. 
7. A method according to claim 5 wherein said holder 
and said circuit board are slowly rotated during the pre- 
heating cycle. 
8. A method according to claim 5 wherein said holder 
and said circuit board are allowed to dwell in said bath 
for from 1 to 3 seconds. 
9. A method according to claim 5 wherein said holder 
and circuit board are spun at a speed of from 1200 to  
1800 r.p.m. immediately upon withdrawal from said mol- 
ten bath. 
10. The method of coating the circuit paths of a printed 
circuit board with solder comprising the steps of: 
(a) positioning said circuit board in a holder; 
(b) lowering said holder and said circuit board to a 
poini approximately 1 inch above the surfacc of a 
bath of molten solder; 
( c )  slowly rotating said holder and said circuit board 
for approxiinatcly five seconds as they are held above 
said molten bath; 
(d)  lowering said holder and said circuit board into 
said bath of molten solder; 
(e)  allowing said holder and said circuit board to dwell 
in said bath for from 1 to 3 seconds whereby an inter- 
metallic bond between said circuit parhs and said coat- 
ing is produced; 
( f )  withdrawing said holder and circuit board from 
said bath; and 
(9)  spinning said holder and circuit board immediately 
upon withdrawal at a speed of from 1200 to  1800 
r.p.m. for approximately 1 to 2 seconds to remove 
excess solder from said circuit board to thereby pro- 
duce a coating on said circuit board having a uni- 
formly smooth finish and the desired thickness. 
References Cited 
UNITED STATES PATENTS 
2,273,250 211942 Charlton _ _ _ _ _ _ _ _ _ _ _  117-101 
3,200,788 811965 Tardoskegyu _ _ _ _ _ _ _ _ _  228-36 
WILLIAM L. JARVIS, Primary Examiner. 


Method of coating circuit paths on printed circuit boards with solder Patent

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