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Uncovering Assembly Problems of High-Speed PCBs

High-speed PCBs have become much more common during the past five years. However, special challenges arise due to unanticipated high-speed issues. Some relate to manufacturing or processes, while others are pure component- or fabrication-based. In particular, highly complex, highly integrated components like FPGAs need close scrutiny. Also, it’s important to note that PCBs may use mixed materials like Rogers and FR-4 cores in order to incorporate those high speeds.

The mixed-material board may need pre-preg special processing and temperature treatment compared to conventional FR-4 boards. In such cases, extra care must be applied to watch for certain manufacturing aspects. For example, high-speed boards might pass at certain lower spectrum speeds, but begin failing at certain other higher speeds. However, when it comes to regular investigation, those same boards might pass all the run-of-the-mill tests like flying probe, functional tests and ground-to-power short tests.

It becomes worrisome when these boards fail at certain high-speed level bands. Investigation as to the reason(s) why they fail include checking out the process and dissecting it step-by-step and scrubbing the bill of materials (BOM) all the way from A to Z to make sure there are no issues during verification testing.

The high-speed board may be perfect when it comes to BGA assembly. All the balls properly collapse; all the thermal profiles are accurately determined and performed. All soak temperatures, pre-heat, soak and cool-off periods fall within manufacturer limits and ranges. Yet, this high-speed board fails at high speed at the time of system functional level testing in the system.

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Editor's Note: This column originally appeared in the February 2014 issue of SMT Magazine.