PCB reliability problems due to the growth of CAF (Conductive Anodic Filaments)

03 Apr 2018

CAF is metal filaments that can grow from copper via-hole plating along the glass fibres embedded in PCB materials such as FR4. They can eventually short via holes out, which is bad news for the reliability of products in the field, and also for warranty costs and customer satisfaction.

This only happens for vias with a DC bias voltage between them (especially 0V(GND) and PWR) and which are also close together (say < 3mm).

Can get CAF-proof FR4, or use homogenous dielectrics such a LCP, but more common is to ensure ordinary FR4 and ensure that any via holes that are close together (with different DC voltages, especially decoupling capacitor vias) are not in line with the warp or weft of the fibreglass in the PCB substrate.

Could achieve this by printing a board at 45 degrees to the FR4 panel's length/width, but in volume manufacture this wastes some material and costs a little more.

What we did for a project where we wanted to put pairs of decoupling capacitors very close together in anti-parallel (so that their mutual inductance cancelled out as much as possible of their series inductance, see [1]) – was to put the decaps (and their plane vias) at 45 degrees to the board’s X and Y (horizontal and vertical) directions, i.e. at 45 degrees to the directions of the boards fibreglass’s warp and weft.

Here are some references…

[1] See slide 6A.5.42 in Module 6A: “Essential PCB design/layout techniques for cost-effective SI, PI and EMC in 2018”, available from www.emcstandards.co.uk/essential-pcb-designlayout-techniques-for-cost

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