Edward,

I agree with your assessment of the problems associated with IR
Reflow; it does require more control of the tin-lead plating process
and good plating distribution. That is a matter for us fabricators and
maybe some have lost the art of achieving a good result over the
years.

If you do have a cosmetic failure in reflow you can always use HASL as
a repair operation. The most difficult panels are those with large
solid copper ground planes which really should be HASL anyway and we
do advise customers when a requested finish is inappropriate. I have
converted many designs from reflow to HASL because of this.

Many companies have moved to tin plating as the etch resist and there
is also the photo-resist approach using panel plate. These
manufacturers cannot offer a reflowed tin-lead finish. (I don't know
why we call it reflowed when it wasn't flowed in the first place??)

There are going to be many different opinions on this subject but all
I would say is that IR reflow is a valid finish although some
manufacturers no longer choose to offer it.

Paul Gould
Teknacron Circuits Ltd
UK
www.teknacron.com


-----Original Message-----
From: Eltek Ltd. - Process Engineering <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: 02 December 1998 08:29
Subject: Re: [TN] HASL vs. Fused Solder For CCA Assembly


>Paul ,
>This time I do not agree with with you :
>
>>1.Plated tin-lead post-etch fused is still the cheapest finish.
Reasons are
>>no need to strip the etch resist, no need to pre-clean and pumice
the copper
>>before solder mask application, no HASL process. All that is needed
is to
>>solder condition, flux and IR reflow which believe me is much
cheaper.
>Most PCB manufacturers forgot how to keep the plated Tin-Lead
solderable.
>There is a lot of mess concerning distribution of composition of the
plated
>solder , thickness distribution . Than photoresist stripper and
alkaline
>etcher affect the tin-lead . For multilayers thermal treatment is
required
>and then all impurities are being fixed .
>The most crucial point : for IR reflow is only one pass : in my life
I never
>was able to fix the boards , than once failed in IR reflow .
>What about polyimide boards ?
>
>
>>
>>2.It is possible to fuse thinner deposits if you are not concerned
with
>>cosmetics. Very good results are achieved with thinner deposits on
fine
>>line.
>>circuits because of the etch undercut which increases the amount of
tin-lead
>>over the copper trace.
>>
>>3.It should be possible to get close to 63/37 otherwise, as you say,
the
>>deposit may get into the pasty range and spoil the cosmetic
appearance.
>>
>>The only drawback to fused tin-lead is that solder mask has to be
applied
>>over it and this can cause breakdown through wave soldering. More
and more
>>assembly is now IR fused solder paste so this finish could well be
worth
>>looking
>>at again instead of going for even more expensive alternative
>>finishes.
>>
>>Paul Gould
>>Teknacron Circuits Ltd
>>UK
>Edward Szpruch
>Eltek Ltd - Israel
>Tel  972 3 9395050
>Fax 972 3 9309581
>E-mail :  [log in to unmask]
>
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