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Tin whisker failure. M Dranfield 2011 Television-Magazine-Forum.co.uk

The consumer electronics industry is heading for melt down , its already started and

there is seemingly nothing we can do about it , I am of course referring to Tin

Whiskers Never herd of them ? you soon will do as they are causing havoc inelectronic equipment , tin whiskers are not new , they have been around since the

1940s and were known as dendritic growth,microscopic tiny hairs that erupt in all

directions from tin based materials such as solder, back in the 1950 s a physicist

discovered that mixing small amounts of lead with solder cured the dendritic growth

and it no longer posed a problem.Tin whiskers typically measure 5 microns in

diameter and have a sharp pointed tip that given time can puncture any surface ,a

single whisker could carry up to 30 milliamps of current and heat seems to accelerate

there growth ,so why after so many years has the problem suddenly cropped up again.

The answer to that is simple, since 2006 under the (RoHS) directive ,Reduction Of

Hazardous Substance the use of lead in solder has been banned in consumer

electronics, interestingly enough though, military and medical applications are exemptfrom this legalisation, so while they have a zero failure rate the poor old consumer has

to put up with substandard equipment which will invariably have a very short life.

Anyone who services sky digital boxes will know the reputation the ZIF tuner has

acquired for intermittent signal faults in pace manufactured boxes , the cause is the 48

pin QPSK chip MAX2104CCM on the tuner and the symptoms are typical of tin

whisker growth inside the chip itself, however the growth inside the chip seems to

have been caused by a chemical that was added to the chips plastic moulding as a

flame retardant, the problem is every single chip made will eventually fail, in fact I

never sell a pace digibox without first replacing this chip regardless of whether its

faulty or not , in later production chips the chemical was removed to cure the

problem, however with the removal of lead in solder from 2006 we could well be

heading for a major disaster , now while I dont profess to be a chemist and the failure

mechanism is complex and far beyond the realms of this magazine I have observed

the following simple points which you might like to bare in mind.

.

Tin whisker growth seems to be accelerated by heat, the hotter a chip runs the quicker

it will fail.

Tin whisker growth causes virtually untraceable intermittent faults, equipment may

run for a week without problem then suddenly drop into ST/BY, it can then comeback on its own and perhaps run for hours on end before the fault shows up again, in

some cases the sheer intermittency can make the equipment un economical to repair,

it may even give the impression you are looking for a dry joint.

Tin whiskers can be destroyed by heating the suspect chip up to solder melting

point , in the case of BGA packaged chips reflowing the chip itself can cure a fault

leading you to believe it had a dry joint ,however this is only a short term cure , the

whiskers will grow again and the chip will fail again, the more functions a chip carries

out the more different fault symptoms will be displayed.

Lastly, heres food for thought, the painter chip in Phillips TVs can cause no end ofdifferent fault symptoms , and someone in a past issue of Television magazine

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reported reflowing the pins with solder cured the problem and he thought it was a dry

joint, I havent had one of these sets in for ages but I wouldnt mind betting heating

up the entire chip will cure the fault for a while.

Michael Dranfield.

DigifixLtd.