March 28, 2011
by Terry Costlow, IPC online editor
Over the past few years, pad cratering has become a vexing problem in lead-free assemblies. It’s difficult to spot, and there haven’t been any approved techniques for finding these problems.
Pad cratering occurs when a solder pad lifts up, pulling away part of the board material so a small crater appears. The defect is largely attributed to problems with the resin used to make a board.
However, it’s been difficult to determine what materials will work best in varied applications. “In the beginning, there wasn’t much attention to these types of failure because they were almost unheard of. But more companies started getting products ready to ship and realizing they had a problem that was not with one or two parts, but with many. These are catastrophic failures that are sometime latent. The defect doesn’t always show itself in testing,” said Reza Ghaffarian, a NASA Jet Propulsion Lab engineer.
Ghaffarian chairs the IPC 6-10d SMT Attachment Reliability Test Methods Task Group that created a standard that provides test methodologies that let product developers examine a few test coupons to determine the best material for their application. IPC-9708, Test Methods for Characterization of PCB Pad Cratering, provides three standardized test methodologies. They will ensure that tests of different materials are uniform.
That will help alleviate an issue that has become a major challenge for many manufacturers. “With the transition to lead-free solder & halogen-free PCB materials, one of the primary failure modes for lead-free BGAs is when copper pads on the board are pulled up, which causes pad cratering,” said Satish Parupalli, an Intel engineer who helped coordinate the development of the IPC-9708 within the IPC 6-10d task group. “Assuming no manufacturing defects and test issues, people agree that the resin material is a driving factor for this failure mode. But with the transition to lead-free, there’s been no one technique that helped us identify what changed in resin materials that caused this problem.”
The standard describes three techniques: cold ball pull, ball shear and hard pin pull tests. When one or more are used, they can help eliminate the problem while cutting costs.
“Substantial cost reductions can be gained when you reduce the burden of verification and qualification. Testing is costly and time consuming. If you don’t have standardized test methodologies, you can’t quantify which material performs better,” said Mudasir Ahmad, a Cisco engineer who led the development of IPC-9708 along with Parupalli.
In application, manufacturers can begin by picking a few board materials that seem to meet their reliability and cost requirements. They can then build standardized sample boards that can be run through the test they choose.
“The standard will help people select, say, four materials that seem to meet their needs. Then they can build test coupons defined by IPC-9708 and run tests on these coupons,” Parupalli said. “That lets them make a decision before they build any real products.”
Some companies are making IPC-9708 a mainstay in their material selection process. Cisco, for example, is setting up its infrastructure.
“We have decided to be proactive on this. We’re working with multiple suppliers and testing multiple materials. Investing our time and money highlights to suppliers that we believe in this standard and its benefits,” Ahmad said.
As with many standards, those who helped create the documents feel there will be significant benefits if adoption becomes widespread. Proponents hope that resin suppliers and board fabricators will begin reporting data based on standardized testing.
“Now there is a common base line for measurements. We hope material providers will supply this information and make it simpler for their customers to select the right material for their applications,” Parupalli said.
Companies that don’t want to establish their own testing infrastructures may be able to leverage the standard if it gets acceptance from specialized companies.
“We hope third party labs will see the benefits and utilize this. The infrastructure for each test is not that difficult or expensive to set up,” Ahmad said.
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