Thursday, 14 March 2013

Design for life - part 2

Last month we looked at electrolytic capacitors, their limitations and why you need to pay careful attention to ripple current rating.

This week we will look briefly at film capacitors and see how selecting the wrong material for the application can have very serious consequences.

Part 2—Film capacitors

It is not often appreciated that the ac rms voltage rating of film capacitors must be greatly de-rated for frequencies above approximately 1kHz. A popular cap, rated at 400Vdc & 250Vac is specified at just 1Vac maximum at 100kHz, so it can be easy to exceed the high frequency ac voltage rating in a power circuit. The image below is the result of exceeding the HF ac voltage rating of a 470nF 250Vac cap which occurred suddenly after 38 months in the field.

Film caps are also vulnerable to failure as a result of exceeding the repetitive  rate of change of voltage (dV/dt).

Metallised polyester snubber caps across switching semiconductors have been found to fail due to excessive dV/ dt, where the use of polypropylene, ceramic or foil film would have been preferable.

Surface Mount Multlayer Ceramic Capacitors (SMD MLC caps)

The larger sizes (1812, 2220) of SMD multilayer caps are prone to failure when mounted on fibre glass or composite PCBs due to the different coefficients of thermal expansion of the cap and the substrate. These components can fail short circuit with devastating consequences if they are connected across a power rail.

All sizes, but more especially the larger ones, are prone to failure due to mechanical stress. An example encountered recently used several SMD MLC caps under a dc power output screw terminal block which was subject to flex whenever the terminal was pressed down by tightening or loosening the screws. The subsequent fracture of the cap burnt a hole right through the pcb, as the ceramic cap body remains mostly intact even when red hot.

Avoid large SMD MLC caps on circuit boards involved with intense thermal cycling unless substrates are matched, and never place in areas of mechanical flex or stress.

Next time we will look at MOSFETS and uncover some very surprising factors which have a big impact on long term reliability.

Advance Product Services Ltd
Paul Horner is Managing Director at 
Advance Product Services Ltd.