Important Considerations for Pads in PCB Design: Pad Cratering Prevention

November 8, 2017 CircuitStudio

Catapult drawing.

 

 

Every year after Halloween, my school used to have a pumpkin launch, probably so they could get the pumpkins on sale. Mostly, mechanical engineering students participated, but anyone was allowed to enter and see if they could launch their pumpkin the farthest. Sections of campus were blocked off and your path to class was rerouted if you went at all. Most of the engineers got (or took) the morning off for cheering and jeering.

 

Results were haphazard. We had occasional glorious pumpkin flights, but most pumpkins really didn’t make it very far. However, one year, a pumpkin somehow got launched straight up, and we all raced around shrieking in excitement, unsure of where to seek cover. The total lateral distance covered was about a meter, so the spectators were in no danger, but the perfectly coiffed lawn took some damage. New launch regulations were hotly debated with the administration for the rest of the year since apparently late fall is a difficult season for repairing pumpkin craters in the turf.

 

I think craters in all forms should be accompanied by that level of frantic energy. 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. You can be sure the aftermath is just a serious as a pumpkin crater, though.

What is Pad Cratering?

Pad cratering, as I mentioned, 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.

 

A microprocessor being lifted off a ball grid array of solder beads.

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.

 

The type of solder you use changes the risk for your board, too. While issues occur with both leaded and lead-free solders, Sn-Pb solder can handle almost twice the bending load as SAC solder before failure. Stiffer, less ductile alloys, which are usually lead-free, have a much higher cratering risk.

 

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.

 

First, 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.

 

Heatsink on a large PCB.

Heat sinks can introduce thermal mismatches that strain materials and lead to cracking and cratering.

 

 

Thermal mismatch during manufacturing can also cause issues. Talk with your manufacturer about the temperatures, ramp rates, and cooling times for all the materials in your PCB to minimize additional strain due to CTE mismatch. You can also ask about using copper needles to give better adhesion to the surface of the laminate, but you’ll sacrifice some electrical performance.

 

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. This isn’t a pumpkin launch, where destruction is part of the fun!

 

When you are ready to get started, use good tools. PCB design Software like Altium’s CircuitStudio allows you to customize your design rules, and plan spacing and material requirements from the beginning. The extra effort up front is much easier than the testing and rework you’d need otherwise!


Have a question about cratering? Contact an expert at Altium.

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