On a PCB pad cratering sounds like it should be caused by tiny flaming meteorites, or something comparably dramatic. In reality, the “craters” result from a pad separating from the surface of the PCB, and leaving behind a divot in the laminate.
This article was originally published in the Altium Blog
WHAT IS PAD CRATERING?
Pad cratering occurs when a copper termination pad separates from the PCB. The separation can be a partial or entire disconnect from the board, which often makes it harder to identify the fault. In a total separation, the solder usually remains attached to the component and looks like the pad is intact.
Cratering most commonly happens on area array designs, like ball grids (BGAs), and less commonly to other components with many connections. A variety of processing parameters can increase your risk of cratering, but the ultimate cause is almost always mechanical strain. Cracking starts in the laminate when the board is bent during testing, manufacturing, vibration during transport, or even having connectors attached. When the PCB under an array of solder beads flexes, the board and component don’t bend at the same rate, and the copper pads are ripped away from the PCB as the board is strained.
Ball grid arrays are the most prone to pad cratering
WHAT DOES IT DO TO MY PCB?
Because cratering doesn’t always result in a complete separation between the pad and the rest of the PCB, faults are difficult to identify. Electrical failures from open circuits may be inconsistent, especially between different boards from the same batch. You may find odd performance issues due to increased resistance at the joint, or intermittent contact, as well as completely open circuits.
Sometimes, you can see the damage during optical inspection (some good examples are on page 31 of this presentation). Often, extensive and destructive testing is required to positively identify cratering as a fundamental issue with PCB performance.
WHAT CAUSES PAD CRATERING?
Many factors in your board design and manufacturing process affect the risk of cratering on your PCB. Obviously, array area components are at the greatest risk, but components with smaller trace pitch, larger size, or rigid packaging are also more prone to cratering issues.
The PCB materials can also affect outcomes. The thickness of the board, and hardness of epoxy change how the board flexes, and increase the likelihood of cratering. Brittle laminates are also more likely to crack and introduce cratering.
HOW DO I PREVENT IT?
Since cratering can be so difficult to detect, prevention is definitely the best path. If you have a RoHS restriction and are limited in your solder and packaging choices, you still have plenty of ways to improve outcomes for your PCBs.
Plan your layout carefully. Don’t place large components or BGAs near edges, or areas with connectors that might have to flex under pressure. Also, keep large heat sinks away, so the heat won’t cause repeated shrinking and expanding to strain the laminate or components.
Handle your boards carefully. Repeated vibration can be just as damaging as forcing a connector. And, it should go without saying, don’t drop the boards! The physical shock can damage more than just your copper pads, and make it difficult to identify original failure causes.
Heat sinks can introduce thermal mismatches that strain materials and lead to cracking and cratering.
Have other questions about cratering? Contact an expert at Coindustrio.
The production of electronic devices involves a series of steps and different manufacturers. Coindustrio.com is a manufacturing as a service company that improves and implements innovative processes to optimize PCB production and offer a best time-to-market.