Larry,
Firstly I'm talking about HASL only. If a cross section were taken of a pad
along it's length, then I would like the minimum to be 2.5 microns. You
can't really persuade suppliers to live to this, (if I was in a more
influential company I would though), so 1.5 microns as the lowest is
probably fair.
To survive 4 years of storage, I would simply say, what is the depth of 4
years of intermetalic growth? Someone else will have to give us that figure,
who knows the rate of growth at 23 degrees C.
Regards
At 10:11 19/11/98 -0800, you wrote:
>Eddie-
>We typically spec solder plate and fuse to ensure solderability, or HASL to
>save on procurement costs. How do you spec your minimum thickness on
>surface mount pads? What thicknesses support solderability after 4 years of
>storage?
>
> Larry Jindra
> TRW/Avionics Systems Division
> [log in to unmask]
> (619) 592-3424
>
>
>> -----Original Message-----
>> From: Eddie Brunker [mailto:[log in to unmask]]
>> Sent: Thursday, November 19, 1998 2:33 AM
>> To: [log in to unmask]
>> Subject: Re: [TN] Tin/Lead Solder Coating Minimum Thickness
>> Requirements
>>
>>
>> Earl,
>>
>> >There can be no true thickness spec for HAL by process definition.
>>
>> What about controlling the Air knife pressure? If the minimum
>> thickness of
>> solder on any part of pads regularly comes in less than
>> specified, lets say
>> 1.5 microns, then if the pressure is reduced will it not increase the
>> quantity of solder left on the pad? We have used this in the
>> past ourselves,
>> the supplier put a document on the Vertical Hasl bath,
>> stating that our
>> boards should have a lower pressure reading on the gauge.
>>
>> >Simply, it must be capable
>> >of supporting soldering operations, solder wetting, acceptable solder
>> joints, and long term
>> >reliability. It often is not capable of doing so.
>>
>> We find no problems soldering to HASL boards after 4 years
>> of storage.
>>
>> If a fab shop tries to give a flat finish using HASL, then
>> there can be
>> problems, ie. it can be too thin. For reliable storage a
>> minimum thickness
>> should be specified. If you use HASL just accept that there's
>> a solder dome
>> and give it a decent thickness. Too many shops are trying to
>> give flat HASL
>> finish and compromising the reliability with regard to long
>> term storage or
>> regarding multiple thermal cycles.
>> (Any data I've seen presented, shows solderabiltiy of HASL as
>> better than
>> ENiIAu and OSP, after multiple thermal cycles.) I don't
>> recall data for long
>> term storage, although this should pressumably be qualified with HASL
>> thickness. Come to think of it so should the Multiple Thermal
>> Cycle data.
>>
>>
>> >This is why so many seek alternative solder
>> >termination area coatings.
>>
>> Which alternative coatings retain superior solderability
>> after long term
>> storage?
>>
>> The only reasons I can see for seeking alternatives are lower
>> SIR values,
>> (why can't the PCB industry use less aggressive/bromide free
>> fluxes?) or the
>> desire to remove lead from the process. Direct contact to pads such as
>> connectors in mobile phones are another reason.
>>
>>
>> >It's the big hump that's unacceptable to SMT.
>>
>> Assemblies with 5000 joints, with alot of devices at 20 mil
>> (thou) pitch,
>> and significant solder dome height,( we like lots of solder
>> ), present no
>> problems to us.
>>
>> Regards
>>
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