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The printed circuit board is the fundamental substrate of virtually every electronic assembly. The physical limitations of printed circuit materials and the complexity of the bare board manufacturing process often go unappreciated by the assembly engineer, who generally expects PCBs to conform to their purchase specification, to be totally compatible with his assembly process and to be totally reliable in service.
But PCBs can fail for a multitude of reasons. And the ability to detect and identify failures during assembly and final test is preferable to having to accept the consequences of field returns.
For over 30 years, a leading authority on electronics assembly technology, Bob Willis shared his encyclopaedic knowledge and practical experience of printed circuit defects with a responsive and interactive audience in a SMART Group webinar titled "Printed Circuit Board Failures – Causes and Cures", clarifying much of the mystery and mythology, describing failure mechanisms, demonstrating the use of standard test methods and tricks of the trade and explaining how to eliminate many of the common causes.
Willis began by recommending works of reference, particularly Clyde Coombs’ Printed Circuit Handbook, Preben Lund’s Quality Assurance of Printed Boards, IPC A-600H Acceptability of Printed Boards and IPC-TM-650 Test Methods Manual; and some relevant specifications: IPC-2221 Generic Standard on Printed Board Design, IPC-6011 Generic Performance Specification for Printed Boards, IPC-4552 Specification for Electroless Nickel/Immersion Gold (ENIG)Plating for Printed Circuit Boards, with the wise precautionary advice: "Never quote a standard unless you know what it says!"
He emphasised the importance of choosing a suitable PCB supplier, and gave clear instructions on how to assess the supplier’s documentation and technical capability, how to conduct a proper supplier audit, procure and evaluate samples and prepare a Supplier Audit report.
Delivery packaging was a significant factor in protecting PCBs from mechanical and environmental damage in transit and storage. Vacuum sealing or moisture-barrier bagging are currently popular.
Once PCBs had been received at goods inwards, Willis described the successive stages of mechanical inspection and dimensional inspection, and demonstrated how simple optical aids could be used to view the insides of plated holes. Solder mask coverage, thickness and adhesion needed to be to an agreed specification, as was freedom from undercutting which could lead to flux or paste entrapment. Willis preferred to see batch and date code information etched into the board surface, either in the solder mask or in the copper.
With reference to IPC's "Nightmare Microsection" multi-issue wall-poster, and to many of his own microsections and x-ray photographs, Willis illustrated and described a whole series of interconnection defects in plated-through holes and blind and buried vias, resulting from problems in the PCB manufacturing process – drilling, de-smearing, metallisation and electroplating – which may have passed electrical test at the bare-board stage, but which could go open-circuit – either permanent or worst-case intermittent – as a result of the thermal shock of soldering. He had developed his own innovative techniques for examining the integrity of laser vias and isolating sources of outgassing. He went on to describe methods for thermal shock testing and subsequently examining microsections for evidence of copper cracking, inner-layer separation from through-hole plating, and de-lamination.
Blind via capture pad with no sign of brittle fracture during mechanical testing. The plating of the via to the inner pad was never satisfactory.
Surface of blind capture pad correctly prepared for plating.
