Passives

HolyStone – New multilayer ceramic capacitors solve SMD cracking problem

Sep 24 2007 - Passives [More Passives Articles]

A new range of multilayer ceramic capacitors has been introduced by HolyStone (Europe). The devices provide a cost-effective and reliable solution to the common problem of cracking that occurs in applications where surface-mount components are subjected to excessive mechanical or thermal stress, says the company.

The SuperTerm Series capacitors incorporate a ‘flexible’ polymer layer in the termination structure, which effectively absorbs mechanical stress. This greatly reduces the incidence of cracking and improves long-term product reliability.

SuperTerm capacitors also benefit from an ability to withstand repeated thermal shocks. The company says most standard termination systems start to fail after approximately 500 thermal cycles, whereas SuperTerm units can typically withstand 3,000 cycles without failure.

Multilayer ceramic capacitors are more prone to the excesses of mechanical stress than most other types of surface-mount components due to the brittle nature of the dielectric material. Dielectric materials are strong in compression, but relatively weak in tension. This weakness causes the industry’s most common type of capacitor failure – mechanical cracking from excess bending of the PCB after soldering. Where this crack crosses two internal electrodes of opposing polarity, a potential short-circuit failure can occur. Mechanically-cracked capacitors are extremely difficult to detect with end-of-line testing. Failures can happen within hours, but more commonly it is weeks or even months before loss of Insulation Resistance results in a short-circuit failure, influenced mainly by the speed at which moisture seeps into the crack, says the company.

“SuperTerm capacitors offer a broader range of products with significantly improved performance, and at a lower cost, compared to similar products from most western manufacturers,” said Clive Youngs, technical and applications manager, HolyStone (Europe) Limited. “We have conducted extensive destructive bend testing, where the PCB is subjected to increased bending until capacitor failure. Standard termination systems will begin to fail at approximately 2mm of bend, but SuperTerm can normally withstand up to 7mm, with no cracking of the ceramic body. “SuperTerm failures resulting from destructive bending tend to occur in ‘open’ circuit mode when the termination fails, not in ‘short’ circuit mode typical of standard termination failures.”

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