TECHNET Archives

September 2002

TechNet@IPC.ORG

Options: Use Monospaced Font
Show Text Part by Default
Show All Mail Headers

Message: [<< First] [< Prev] [Next >] [Last >>]
Topic: [<< First] [< Prev] [Next >] [Last >>]
Author: [<< First] [< Prev] [Next >] [Last >>]

Print Reply
Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Sun, 8 Sep 2002 10:07:08 +0300
Content-Type:
text/plain
Parts/Attachments:
text/plain (88 lines)
Sorry to disagree with the others. If you have 15 µm of plating before
reflow, it will "shrink" during reflow as tin/lead intermetallics form
and you will probably get typically an average of 12 µm over the surface
and this will be 15 µm on the flat surfaces but may be less than 1 µm on
the corners. However, depending on your tin/lead ratio (which may also
vary somewhat on the corners as the electric field during plating may be
slightly more intense - do Hull cell tests), the reflow time and
temperature, the flux chemistry and any interaction with the reflow
fluid and the thermal mass of the board, your tin-copper
(bronze)intermetallic layer may also be of the same order of thickness.
If this happens, I would expect a distinct deterioration of
solderability (and bagging the circuits will do nothing to help or
hinder) over a 5 year storage, especially with an RMA flux which may not
have enough oomph to ensure soldering to aged and oxidised intermetallic
layers, where they are not protected by fused tin/lead. In the worst
case, you would get non-wetting but I would consider dewetting a very
distinct possibility, where copper atoms actually are on the surface.
IMHO, there can be no guarantee of solderability.

In a past life, when I directed a PC fab plant (over 30 years ago), we
had a customer for whom we made some 4-layer boards which were very
sophisticated for that epoch with 0.2 mm tracks and spacings. His
requirements were only 10 to 15/year but he wished to enjoy some economy
of scale by ordering 100. In order to overcome the solderability problem
over time, we mutually decided (Earl, we were practising concurrent
engineering even then !!!) to deliver the circuits, wrapped individually
in a special aluminium foil, with an unprotected bare copper finish,
along with a few litres of Lonco Copperbrite (remember it?) and
instructions. When he was going to do an annual assembly batch, he would
make up a bucket of Copperbrite solution, dip the circuits in it, rinse
them in DI water and dry them, soldering them within a day or two. He
had no problems. As far as I was aware, he was happily doing this a
number of years later because, by then, I had started my own company
(Protonique SA) and had become the distributor for Lonco and I remember
we supplied him with some Copperbrite (it sticks in my mind because this
was not something that assembly shops asked for!).

Brian

Ted Edwards wrote:
> I have received a question that I need to answer from a customer who is
> making a long term buy of solder reflowed tin lead finish boards. Plated
> tin lead seems to read in the .0006 inch range before hot oil reflow on
> the boards in question.
> One of the customers concerns is the possibility of growth of the tin
> lead intermetallics that might in their mind result in an unsolderable
> condition later.  Projected storage is at least 5 years at room
> temperature.  Final assembly when done will probably be RMA flux
> and wave solder in most cases. Any help would be appreciated.  Boards
> are individually bagged with a desiccant package and humidity indicator
> strips and heat sealed.
> I am looking for technical papers, other literature, etc.
>
> Werner:  I printed out and kept all your TechNet e mails, and in your
> September 3, 1997 Assembly: Dewetting; you mention a temperature/time
> dependency, are there charts or other reference materials?
>
> Thanks to all:
>
> Ted Edwards
> Process Engineering manager
> Dynaco Corp.
> Tempe, AZ.
> ---------------------------------------------------------------------------------
> Technet Mail List provided as a free service by IPC using LISTSERV 1.8e
> To unsubscribe, send a message to [log in to unmask] with following text
> in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt
> or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET
> Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the
> posts: send e-mail to [log in to unmask]: SET Technet Digest Search the
> archives of previous posts at: http://listserv.ipc.org/archives Please
> visit IPC web site http://www.ipc.org/html/forum.htm for additional
> information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700
> ext.5315
> ---------------------------------------------------------------------------------
>

---------------------------------------------------------------------------------
Technet Mail List provided as a free service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/html/forum.htm for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
---------------------------------------------------------------------------------

ATOM RSS1 RSS2