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November 1998

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Subject:
From:
"Jindra, Larry" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 19 Nov 1998 10:11:28 -0800
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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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