Richard,
US cleaning with appropriate solvent/modifiers[for your flux system] would be good/okay so long as there are no devices already mounted to the board that have cavities inside the packages [such as hybrid modules, crystals, etc].
An oxygen plasma cleaning will remove residual organics by oxidation [I presume this is what the hydrogen peroxide approach was aimed at]. If the gold is worth its keep [NOT porous], this will not oxidize the nickel beneath. The goal is that the gold should not be porous. Neither should the nickel barrier plate. Oxidized nickel is very difficult to gold wire bond to. An argon plasma will ablade the first few atomic layers to freshen up the surface. Alas, you have no plasma cleaner. How about an ozone generating PROM eraser. A really poor-mans oxygen plasma ...
In either case, if you are looking to remove visible flux residues, I don't feel that plasma will ever do it. We are not attempting to remove much more than just a wee smidgen of the surface with the plasmas we use.
For "onesie twosie, hafta have a part to test" type of thing you could try all manner of chemicals, etc. However, for a real live product, I would steer clear of such other approaches. Best to implement the training and handling procedures to prevent the situation.
Sometimes cleaning the pads manually with a foam tip swab and IPA helps, but certainly is not a production approach.
If the flux is aqueous based, try the DI and saphonifier approach. If rosin, try some IPA and the swabbies. The IPA should also work for finger oils, etc so long as they have not been baked on too much.
Regarding the 'drying' of the plating. That is not really something that I have worried about too much. In most/many cases, hybrid/MCM types will store WIP in nitrogen cabinets, or dried-air cabinets. When the device attach adhesive is cured things will get dried out during the cure. I expect that will be the same for your situation.
Whatever you do, be faithful on doing your sample wire pulls during set-up. Also consider performing an increased sample during the production run and make sure your ending wire pulls are still good. Do you have the ability to perform non-destruct wire pull? You may need to ...
40uinches of pure soft gold ought to be a dream to bond to!!!! How do the solder joints look with that much gold laying around? Am wondering what impact ultrasonics may have on a potentially gold embrittled solder joint..
At the very least, be faithful on monitoring the wire bond process with wire pulls and maybe start out with small batch sizes to limit your jeopardy, etc. Hopefully, the situation may not be as bad as it initially seems.
Just a few opinions. Not really strong on answers. Sorry!
Steve Creswick - Gentex Corporation
-----Original Message-----
From: Stadem, Richard [mailto:[log in to unmask]]
Sent: Tuesday, July 26, 2005 2:39 PM
To: [log in to unmask]
Subject: [TN] Cleaning of gold wirebond pads
Hi Technetters,
I have a wirebond application for a client where we need to perform
standard SMT, followed up with a wirebonding application. The process is
fairly straightforward, but I do not have a lot of experience doing the
wirebonding after the SMT. I am used to doing the wirebonding of the
chip and the encapsulation before the SMT, on the opposite side.
The question I have is, assuming the wirebond pads may be contaminated
with flux or fingerprints in spite of our best efforts to prevent it, is
there a good way of cleaning the gold pads after the SMT process is
finished, to assure there is no flux or other contamination, and that
the porous 40 uinches of soft gold over the nickel base is completely
dry in order to achieve a good wirebond? We do not have the capabilities
for plasma cleaning, we were wondering if there was another method such
as cleaning with the ultrasonic cleaner we have. I have read reports of
using a mixture of hydrogen peroxide mixed with DI water or weak
sulphuric acid in an ultrasonic bath, but cannot recall where I read
this. Any help in achieving clean and very dry wirebond pads would be
greatly appreciated.
---------------------------------------------------
Technet Mail List provided as a 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/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------
---------------------------------------------------
Technet Mail List provided as a 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/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------
|
