Tuesday, March 4, 2014

Reconsidering the PCB

As much as I love OSH Park's prices, I'm starting to reconsider using them for more complicated designs.  If it turns out that the bityExpress-C error is related to PCB manufacture (rather than design), then I will have gone thru two PCB spins without determining if the PERSTn misconnection on bityExpress-A was the problem.  This hasn't been too bad since I've learned a lot about home-brew PCB assembly during these spins, but looking forward OSH Park's cost advantage starts to look less appealing.

I've been casting about since I discovered the shorts on bityExpress-C.  Most prototype PCB houses don't support 5 mil traces/clearances and 8 mil drills, but I found one that does: PCB-Pool.  Their pricing model is a bit more complicated than OSH Park's simple $10 per square inch for a four layer PCB.  They charge a flat fee based on the design's complexity up to 16 square inches.  If the design is small you can get as many boards as it takes to fill up 16 square inches.  For comparison I used PCB-Pool's online price calculator to generate a quantity one price for various PCB sizes from 0.5 square inches to 25 square inches:

VendorClearanceDrillLayerFinishSilkE Testing
OSH Park5104ENIGTop/bottomNo
PCB Specs used in the comparison

For bityExpress-C, OSH Park costs $55.50 for 3 PCBs.  At PCB-Pool it would cost $155.37 for 2 PCBs (or $159.96 for 3).  At first glance $99.87 extra looks excessive, but if I drop the top silkscreen the price drops to $140.51.  PCB-Pool also provides a free metal stencil.  Since I paid $11.05 from OSH Stencil that brings the price difference down to $73.96.

So the question becomes is the free electrical testing at PCB-Pool worth $73.96.  For me the answer is yes.  I spent twice that amount on the FPGAs that I installed on bityExpress-B and bityExpress-C.

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