Para 5.4.1 of J-std-001 revs B and C specifically mandates gold removal from
surface mount components regardless of thickness. I would also be cautious
of taking manufacturers stated plated thickness as evidence that the leads
have less than 2.5 microns of Au. That figure is a reasonable thickness for
solderability maintenance. Increased current density on the lead compared to
the body or tie bar mean that the actual plating thickness on the lead is
greater than that measured on a flat surface, where it usually determined.

With regard to the extent of Au removal, if you can determine that the
tinning operation has not compromised the package integrity, and provided
the body hasn't been immersed into the solder bath, then it shouldn't, then
solder coating up to the body should not be an issue. Frit sealed QFP
packages are often used by semiconductor manufacturers with bare alloy 42
leads. The leads are cleaned of oxide and tinned prior test and despatch.
These have a solder coating over bare alloy 42 right up to the body.

Regards
Ian Fox
Goodrich Engine Controls

-----Original Message-----
From: Charles Caswell [mailto:[log in to unmask]]
Sent: 01 December 2003 22:16
To: [log in to unmask]
Subject: Re: [TN] Pretinning height on fine-pitch gull wing leads


I sympathize. I do not believe the spec applies to tinning although I do
have a customer with a req. of visible clearance from the tin line to the
body. However J-STD 5.4.1 says gold shall be removed from 95% of the surface
to be soldered on THROUGH HOLE components with more than 2.5 micro meters of
gold. It does not mention SMT parts. We have eliminated tinning on most
through hole parts by checking the Manuf. data sheet for gold thickness.
Most are less than 2.5 uM.

-----Original Message-----
From: d. terstegge [mailto:[log in to unmask]]
Sent: Wednesday, November 26, 2003 9:29 AM
To: [log in to unmask]
Subject: Re: [TN] Pretinning height on fine-pitch gull wing leads


Hi Steve,

I forgot to tell that my problem is that our QA rejects parts because the
solder has wicked to far onto the leads during pretinning. It's treated like
a solder fillet that touches the body, which to my opinion makes no sense.
It's not a problem with gold embrittlement.

Daan Terstegge
Thales Communications
Unclassified mail
Personal Website: http://www.smtinfo.net

>>> "Sauer, Steven T." <[log in to unmask]> 11/26/03 03:44pm >>>
Hi Daan,
I'll go back to the old testament (MIL-STD-2000 para 5.4.17.2 on p.
114)
that states "...Only the portion of the lead subsequently to be soldered
need be..." Similarly, J-STD-001 para 5.4.1 states that gold shall be
removed from at least 95% of the to-be-soldered surface.

The elimination of gold removal prior to soldering can be  done with
objective evidence that solder embrittlement is not compromised. Depending
on your customer and quantifying how much gold is in the final solder
connection are the bumps on that road.  Good luck.

Steve Sauer
NG Xetron
Cincinnati, OH

-----Original Message-----
From: d. terstegge
Subject: [TN] Pretinning height on fine-pitch gull wing leads


Hi Technet,

We're using some ceramic QFP's here with gold-plated leads that need to be
pretinned prior to assembly. Does anyone know a specification that
prescribes how far the pretinning solder coverage for these parts may be for
boards that have to meet ANSI-J-STD-001C class 3 requirements ?

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