X400-Content-Type: |
P2-1984 (2) |
Old-Return-Path: |
|
Date: |
Fri, 13 Jun 1997 15:18:25 +0100 |
MIME-version: |
1.0 |
Precedence: |
list |
Resent-From: |
|
X-Loop: |
|
Status: |
O |
Resent-Sender: |
|
X-Status: |
|
From [log in to unmask] Tue Jun 17 11: |
10:58 1997 |
X-Mailing-List: |
|
Resent-Message-ID: |
<"RXkVm3.0.5tE.TKLep"@ipc> |
Alternate-Recipient: |
Allowed |
>From willli Fri Jun 13 09: |
11:58 1997 |
X400-MTS-Identifier: |
[/PRMD=icl/ADMD=gold 400/C=GB/;kid0139 0000033200000966] |
TO: |
|
References: |
<H000022200db0275@MHS> |
X400-Originator: |
|
In-Reply-To: |
<H000022200db0275@MHS> |
X400-Recipients: |
non-disclosure:; |
Message-ID: |
<"966*/I=J/S=Drake/OU=kid0139/O=icl/PRMD=icl/ADMD=gold 400/C=GB/"@MHS> |
Received: |
by ipc.org (Smail3.1.28.1 #2)
id m0wcX4o-000BjQC; Fri, 13 Jun 97 09:11 CDT |
Content-type: |
multipart/mixed; boundary="4ianqyhk0k4VhYsYD5GzE3vV4RiYZhfc" |
Subject: |
|
From: |
|
Return-Path: |
<TechNet-request> |
X400-Received: |
by mta bath.mail.pipex.net in /PRMD=pipex/ADMD=pipex/C=gb/;
Relayed; Fri, 13 Jun 1997 15:19:24 +0100
by mta fel01m2 in /PRMD=pipex/ADMD=pipex/C=gb/; Relayed;
Fri, 13 Jun 1997 15:19:35 +0100
by mta kid01c1 in /PRMD=pipex/ADMD=pipex/C=gb/; Relayed;
Fri, 13 Jun 1997 15:20:55 +0100
by /PRMD=icl/ADMD=gold 400/C=GB/; converted (ia5-text); Relayed;
Fri, 13 Jun 1997 15:18:25 +0100 |
Original-Encoded-Information-Types: |
ia5-text |
Content-Identifier: |
966 |
Parts/Attachments: |
|
|
On the subject of 80/20 Au/Sn. In a previous existance I used to
do a lot of this on PGA and other semiconductor package
assemblies. Our assemblies where gold over nickel over tungsten
on Alumina packages, gold over nickel over Kovar lids with a
gold/tin solder preform tacked to the lid. Gold plating on the
surfaces was specified at 1.5 to 4.5 microns and was typically
kept at the low end of the specification by the package and lid
suppliers. Reflow was done without flux in an inert nitrogen
atmosphere in multi zone belt furnaces. Typical peak sealing
temperatures where 340 to 350 deg C. I forget reflow times.
This system will reflow without flux provided that certain
conditions are met. The surfaces must not be contaminated to badly
with organic or other materials. We stored packages lids and
preforms in Nitrogen cupboards before use. The temperature/time
exposure of the packages prior to sealing must be controlled to
prevent Nickel diffusing through the gold and oxidising. Nickel
oxides are not solderable. If nickel oxides do develop on the
surface the situation can be recovered by doing a forming gas
scrub with N2 12%H2 at 350 deg C. This reduces the Nickel oxides
on the gold surface to nickel and makes the surface solderable for
a reasonable length of time. We found that if processing
temperatures prior to seal were kept below about 350C during die
attach then forming gas scrub was not needed. You want to avoid a
forming gas scrub due to the extra process step and the potential
dangers of hot Hydrogen atmospheres (Kaboom). Don't try sealing
parts in an atmosphere containing Hydrogen. Oxides will be
converted to water, which will be trapped inside your package,
perhaps leading to corrosion problems.
The process described produced excellent wetting with minimal
voiding. The times when we found voiding and poor wetting where as
follows:
Oxidised nickel on the surface and/or poor gold plating.
Poor design of seal ring and preform size, leading to there not
being enough solder to fully wet the surfaces.
Contamination, dust, fibres, etc on the sealing surfaces. We
didn't find a need for plasma or other cleaning. Typically we
found that if we could aluminium wire bond succesfully with high
yields we could seal the packages.
I would worry about using a paste to seal an MCM. I assume
that there is some sort of flux used as part of the paste vehicle.
Residues from this could be sealed within the package, to
potentially cause problems latter. A liquid paste is more likely
to give voiding as the gas from evaporated liquid has to travel
out across a seal width of a mm or two with a thickness of only
perhaps 30 to 40 micron. Gas can easily be trapped leading to
voids. Another source of gas for voids could be outgassing from
materials within the MCM cavity.
Batch ovens may be got to work, but in my experience would give
very long times above liquidus for the solder due to the
relatively slow temperature response compared with a belt furnace.
Vacuum soldering can also be made to work, but I don't have any
experience with it. Choice of process depends partly on what is
contained within the MCM. If the contents are temperature
sensitive then vacuum seam sealing might be used which
concentrates the heat in the lid joint, without heating the whole
package.
You do need to minimise voids in the seal. These can cause
leaks over time due to thermal cycling stresses.
Talk to your lid vendor and package vendor. They should have
a lot of knowledge on this subject.
best regards, Jeremy Drake
|
|
|