Case Study (Page 1)
A recent example of a new project we were involved with.
A new customer approached us with a prototype of a PCB that had just been delivered to them.
The Problem:
The board was an 18 layer backplane utilising 1 oz copper planes and 0.5oz copper signal layers, all the signal layers had controlled impedance requirements and the board was approx. 400mm square. The boards had been assembled and there were a number of issues. These were not specified and the customer asked us to evaluate as if it were a new design. The original manufacturer had been contacted but insisted the product was as per the specification and refused any further comments.
Our Initial Opinion:
We analysed the data and accompanying specification. We verified the impedance calculations and gave our opinion on the design:
- The board had an unsymmetrical copper balance and was likely to exhibit either bow or twist either as delivered or post assembly.
- The signal layers had large areas without any copper features and to aid flatness and allow proper control of the impedance requirements and registration of the holes to the copper pattern we advised a non connected copper pour be applied to the isolated areas.
- Some minor changes to the solder resist geometries and the component notation were also advised.
- The purchasing specification had no formal quality standard requirement and we gave advice as to the standards that should be referenced in line with the nature of the project.
The customer responded that they had indeed experienced flatness issues with the assembled boards, and also that the board had a registration problem. The outer layer pattern did not align with the solder paste stencil and that the impedances were not as they expected. One of the assembled boards had exhibited intermittent opens when tested.
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