Steve Gregory asked:

>>     I've got a question, how many of you out there check whether or not
>>a fine
>>pitch solder joint is good by dragging the tip of an exacto-knife blade
>>or a
>>steel pick across the foot of the leads?
>>
>>     I have seen it done back in the past by a few inspectors maybe 4 or
>>5 years
>>ago, but not anytime recently...until I heard about it today. Is this
>>still a
>>common practice?

Yes, so is nose picking. This means nothing.

>>     I'm pretty sure it's not the smart thing to do, but when you have
>>someone
>>call and tell you that the fine pitch solder joints on the board you
>>built for
>>them are insufficient because the leads come loose when they check them
>>with an
>>exacto-knife...they sure don't want to hear you tell them that they
>>shouldn't be
>>dragging anything across the leads.

I'd sure as hell tell them that, and with minimal amounts of polite
sentence construction and a keen edge on my voice. This may be dangerous,
and an invitation to unpleasant repercussions, but it is a valid attack on
ignorance. Of course take my advice at your own risk.

>>     I'm almost sure it's not one of the cases where sometimes a solder
>>joint
>>will become plastic during wave soldering and separate from the pad,
>>some of you
>>may have seen that before. You know, where the fillet stays attached to
>>the foot
>>of the lead with a nice wetting angle and all that stuff, but it's not
>>attached
>>to the pad...but I haven't actually seen the actual boards that
>>supposedly has
>>the insufficient solder yet, the customer has to bring them to me...

Suggest that the cause of this is predominantly bad coplanarity of leads,
or failure of the paste to release from the stencil aperture. I have seen
many joints which had suffcient solder, with the lead resting on top of the
relfowed meniscus of solder. You can have sufficently low resistance from
the contact of the lead to the solder to pass test, and fail after the
oxidation grows.

>>     Oh, another thing, these boards have all been through and passed
>>ICT, a
>>24-hour burn-in, and a functional test after burn-in. So it would seem
>>to me
>>that if I have a problem with my solder joints, it would have showed
>>itself for
>>sure in burn-in...

I think thermal burn-in is a waste of time for stressing solder joints. You
need random vibration to find this type of defect.

>>     But before they get here, I'd thought I'd ask and see if it's a
>>common
>>practice to rake fine pitch leads with a steel object...by the way,
>>we're
>>talking about .020" pitch QFP devices here...

I had an inspector do this on some QFP's and show me how his dental pick
could break my 0.5 mm pitch QFP joints. I then demonstrated my annoyance by
ripping hell out of virtually every joint on that board with the pick and
telling him any board he picked up would fail that test. He was later
instructed by his management never to do this.

IMO, this means nothing. The shearing force being exerted is not
controlled. The force which the joint "should" withstand is much less than
that which can be easily exceeded with a dental pick or exacto blade.

I have instructed inspectors to use a wooden toothpick held lightly, and
moved over the tops of the gullwing leads while observing the leads under a
10X scope (observing motion as the toothpick is "raked". Even this is a
crude and dangerous method, to be reserved for situations where a known
problem exists.

Sorry, I have no data to provide, only a firm opinion that this is an
invalid measurement of joint strength on fine-pitch SMT joints, based on
trial and observation.


regards,


Jerry Cupples
Interphase Corporation
Dallas, TX USA
http://www.iphase.com


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