Hi Steve,
In general I agree with what you say, and would like to add two small
remarks.
First of all - the thermal shock. Although the HASL is a nasty process, the
reflow oven is not picnic either. The board still heats up in there and
stays there more than those 2-3 seconds of HASL. It's more or less like the
assembly reflow oven but a with a little higher temperatures and less
control of profile.
I also find it difficult to understand why they called out both. Years ago,
when soldermask was not that popular they could be viable alternatives and
at that time it didn't matter too much that fused tin-lead had a higher
miniscus than the HASL.
Since you first electroplate the Tin-Lead (as Werner explained) and then go
through the fusing oven - all the surface is solder coated. If you apply
soldermask - it will go over the solder and that can cause peeling when you
go into the real soldering process (since the solder will remelt under the
soldermask).
For a fine pitch BGA it doesn't sound to me like a good option. I think they
copied this requirement from another spec of a simpler board.
Best regards,
Yehuda Weisz
Tel: (972)-52-3556897
----- Original Message -----
From: "Stephen R Gregory" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, September 20, 2005 3:24 PM
Subject: Re: [TN] Fused tin lead plating versus HASL
> Hi Richard!
>
> We have a few boards that call out that
too. From my point of
> view, as far as appearance, it looks the
same as HASL.
>
> I've tried to think why Fused tin/lead
would be called out instead
> of HASL, and my only guess is that someone
didn't want the boards
> to be themally shocked by the HASL
process. I might also add these
> were on 61-20 layer VME cards where
everything was SMT, and the
> only PTH was press-fit connectors...so the
boards didn't even see wave
> solder.
>
> Kind regards,
>
> -Steve Gregory-
> Senior Process Engineer
> LaBarge Incorporated
> Tulsa, Oklahoma
> (918) 459-2285
> (918) 459-2350 FAX
>
>
>
>
>
> "Stadem, Richard" <[log in to unmask]>
> Sent by: TechNet <[log in to unmask]>
> 09/20/2005 07:51 AM
> Please respond to TechNet E-Mail Forum
<[log in to unmask]>; Please respond to "Stadem, Richard"
<[log in to unmask]>
>
> To: [log in to unmask]@SMTP@Exchange
> cc: (bcc: Stephen R Gregory/LABARGE)
> Subject: [TN] Fused tin lead plating versus HASL
>
>
> Hi, folks
> I have a pwb fab drawing that calls out
fused tin-lead plating or solder
> coating. I know that the solder coating is
HASL, but what is fused
> tin/lead? My understanding is that it is a
tin/lead coating that is
> applied to the bare pwb using a reflow
oven process. Does this provide a
> flat surface of tin/lead? If my only
choice of finish is tin/lead, which
> process is better in terms of flatness and
solderability? I have a
> fine-pitch BGA that is going to be
assembled on this pwb.
>
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