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APPENDIX E: GUIDELINES FOR REFLOW SOLDERING
UNIFLOW
®
4 PULSED HEAT CONTROL
990-908 E-3
3
Flex and PCB Trace Sizes
Ideally, the flexible circuit pads should be narrower in width than that of the pads on the printed circuit
board. As the solder melts and the parts compress, solder is forced to the side. This design will allow
space for the solder to flow on either side of the flex pad and will be more tolerant of solder quantity on
the PCB, avoiding solder bridging problems.
A smaller pad width on the flex will help with registration and alignment of the two parts. For fine pitch
applications, the width of the PCB trace is designed to be 50% of the pitch. This reduces the risk of
short circuits due to misalignment.
Printed Circuit Boards (Pcb) And Other Substrates:
General Part Design Guidelines
Most PCB materials such as FR2 and FR4 are very resilient to the local application of heat during the
process. Materials such as ceramic substrates have to be heated in a more controlled fashion to
minimize the chance of cracking. Excessive differences in the heat sinking capability of the two parts
can also cause solder cracking during cooling.
Heat sinking differentials along the solder joint length are the most common design problem to
overcome. Small differences can have minimal effect, but any large thermal mass change along the joint
area will cause inconsistency of temperature and solder joint quality.
Heat Sinks And Land Areas
Common problems and their possible solutions are detailed below:
Design Problems And Solutions Notes