Printed circuit board design allows for thermal stressing of the soldered joints during many thermal cycles. Components' leads should be cleaned, tinned, and freed of particles that prevent solder adherence, have a thin conformal coating, and have an air space between the coating and the body of the component

Beasley, B.

English

NASA Technical Reports Server

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August 1970
	
Brief 70-10241 
NASA TECH. BRIEF 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage commercial 
application. Tech Briefs are available on a subscription basis from the Clearinghouse for Federal Scientific and Technical 
Information, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Division, NASA, Code UT, Washington, D.C. 20546. 
Prevention of Cracking of Soldered Joints in Electronic Assemblies 
.
Transistor Configurations with Clinched Solder Joints: Standard (left) and with Resilient Pad (right) 
Cracks have developed in soldered joints on printed-
circuit boards in electronic assemblies at high alti- 
tudes. When in service such assemblies are subjected 
to much thermal cycling; the difference in coefficient 
of expansion between the boards and the components 
in the assemblies is reflected as force in the com-
ponents' leads that causes stress and cracking. 
An investigation of the cracking followed two ap-
proaches: analytical and empirical. In the analytic 
investigation, a mathematical analysis was made of 
the joints in question, especially the transistor joints 
which seemed most subject to failure. A mathematical 
model of existing techniques for mounting components 
on printed-circuit boards was analyzed, and the force 
on critical components was calculated. This approach 
permitted insight into the basic problem and thus led 
to ideas that could provide solutions. Mathematical 
models based on the proposed solutions were then
analyzed for determination of the theoretical validity 
of the solutions. 
The empirical investigation entailed determination 
of the extent of damage caused by temperature cycling 
of the assembled boards, and of the practical validity 
of the proposed solutions to the problem. Boards were 
tested with components mounted either in the normal 
manner or in accordance with the proposed solutions. 
The results of the analytical and empirical investiga-
tions were compared. 
The investigations found no single cure for the 
cracking but have led to several recommendations: 
Future design of such board assemblies should in-
corporate techniques allowing for thermal stressing 
of the soldered joints during many thermal cycles. 
Fabricators should be warned against installation of 
components in a manner that builds strain into the 
joints.
(continued overleaf) 
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Components' leads (see fig.) should be thoroughly 
cleaned, tinned, and freed of particles that may pre-
vent adherence of solder—especially leads of encapsu-
lated pulse transformers and modules that have seal-
ant applied to the leads during the encapsulation. The 
conformal coating should be as thin as possible. There 
should be an air space between the conformal coating 
and the body of the component; in the absence of an 
air space there should be a resilient pad under the 
component. The pad should be such that there is little 
likelihood of the coating bridging under the compo-
nent. Components should be "ruggedized" with 
fillets of the resilient material to reduce the likelihood 
of the conformal coating bridging under them. 
Because of the primary cause of the cracking, the 
material of the conformal coating should be replaced 
by one having a lower coefficient of thermal expan-
sion and a lower modulus of elasticity; such a material 
should be sought by research. A better material for 
resilient pads should be found.
Notes: 
The following documentation may be obtained 
from:
Clearinghouse for Federal Scientific 
and Technical Information 
Springfield, Virginia 22151 
Single document price $3.00 
(or microfiche $0.65) 
Reference: 
NASA-TM-53824 (N64-81884). Investi-
gation of the Development of Cracks in 
Solder Joints 
Patent status: 
No patent action is contemplated by NASA. 
Source: Barnes Beasley 
Marshall Space Flight Center 
(M FS-20544)
S 
Brief 70-10241	 Category 08


Prevention of cracking of soldered joints in electronic assemblies

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