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.