What is Cracking
Cracking, in the context of the PCB, refers to a significant failure mechanism that can occur in conformal coatings, solder joints, or laminates on a printed circuit board. It involves the development of fractures or cracks in the coating or laminate, which can have detrimental effects on the performance and reliability of the PCB.
In conformal coatings, cracking typically happens when the smooth surface of the coating fractures into sections, leaving behind visible cracks that expose the underlying area to potential contaminants. This can compromise the functionality of the components on the PCB. The causes of cracking in conformal coatings can be attributed to temperature inconsistencies during the curing or drying processes. Excessively high cure temperatures, especially if the coating cures too quickly, can lead to premature curing of the outer surface without sufficient time for room temperature drying, resulting in a cracked surface. Conversely, very low temperatures, especially following extreme heat, can also induce cracking.
Cracking can also occur in solder joints, which are responsible for connecting components to the PCB. The cracking of solder joints is relatively uncommon but can happen due to factors such as poor design, inadequate solderability, repeated movement during processing, or thermal expansion. These cracks can lead to electrical connectivity issues and potential failures.
Furthermore, cracking can be observed in the laminates of a PCB, particularly when lead-free materials are introduced. Some base materials used in PCBs can be brittle, leading to the cracking of the epoxy in the laminate. This can result in pad lifting on BGA packages, where the pads used for connecting the BGA package to the PCB become detached. Cracking can also occur during flexure of boards or drop testing, particularly with certain laminates. Flexure refers to the bending or flexing of the PCB, which can happen during normal use or when subjected to mechanical stress. Drop testing involves intentionally dropping the PCB to simulate the impact it may experience during transportation or handling. The presence of cracking during these tests suggests that the laminates used in the PCBs may be prone to cracking under mechanical stress.