Most PCB materials like FR2 and FR4 are very resilient to the application of heat during the hot bar reflow soldering (or hot bar bonding) process. But some materials - ceramic substrates in particular - need 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 being joined 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 may not have much effect on quality, but any large thermal mass change along the joint area will cause inconsistency of temperature and result in a poor solder joint.
Following are some of the common part design problems we run into and possible solutions to ensure successful hot bar bonding.
Referring to the image above:
A. Heat is transferred away from the joint area to the large pad area, which is positioned too close to the joint area.
B. Increased trace width and plated through-hole draw heat from the joint area.
C. Reduced-width trace acts as a thermal dam and prevents any heat sinking of the pad.
D. 0.08” is the effective minimum area in which there must be no heat sinks if small trace heat dams are used.
E. Equally sized small traces act as a thermal dam and ensure equal heating across joint area.
Note that the traces leading from the pads should be of equal width and be as narrow as possible. This design will act as a thermal dam, preventing excessive heat drain from the pad area during the hot bar reflow soldering process.
For multi-layer boards, restrict the traces under the bonding area to the smallest width possible and spread equally under the pads on the PCB. Any shielding on the PCB should have an equal effect along the joint area.
For more information, read our whitepaper Part Design Guidelines for Reflow Soldering Flex to PCB's, Reflow Soldering Process Overview, and Pulse Heated, Hot Bar Reflow Soldering & Bonding Fundamentals