Lou,
Although Bev asked some good questions, the one question that isn't obvious
and can be missed very easily is were there any changes to your incoming
boards and I don't mean did you switch to OSP coated copper boards. It has
been our experience that the copper concentration in a wave soldering pot is
controlled by two factors: the number of boards you wave solder and the
copper concentration in your vendors HASL machine. When we converted many
boards from HASL surface finish to OSP coated copper we were concerned about
the increase in copper concentration in our wave soldering facilities. What
we found out is that the copper concentration in a wave soldering facility
does not increase when OSP boards are soldered. In fact there is a decrease
in copper concentration. Yes it is true that copper dissolves in solder and
a copper -tin intermetallic is formed when tin and copper come in content.
However, when a copper board is wave soldered the solder wets to the copper
forming an intermetallic. The intermetallic is necessary because it is the
"glue layer" holding the solder onto the copper. Some copper is dissolved
into solder as the flowing solder contacts the board but it is a very small
amount because the time is small (i.e., a few.
Regards,
George
George M. Wenger DMTS
Bell Laboratories Princeton, Supply Network Solutions
Engineering Research Center FMA / AQA / RCA Lab
(609) 639-2769 (Office); 3210 (Lab); 2343 (Fax)
[log in to unmask]
-----Original Message-----
From: Bev Christian [mailto:[log in to unmask]]
Sent: Wednesday, December 06, 2000 3:33 PM
To: [log in to unmask]
Subject: Re: [TN] Copper contamination is wave solder
Lou,
You need to ask yourself what changed seven months ago - either a one time
event or a recurring event.
Did you change to an OSP that burns off prewave (if there is such a beast)
or super quickly leaving a few factions of a second for copper pads to
dissolve?
Are you using OSP boards where the pads are not covered very well?
Any change in solder mask?
Did you change your solder supplier?
Did you change the purity level of the solder you buy from your present
supplier?
Have you checked the certificates of analysis from your solder supplier?
Did a massive SMT component with a copper slug somehow get knocked off the
top of the board into the pot?
Did someone drop in some wire, a pin, bolt, penny?
Have you checked the analyses? Control? Standards? Standards lifetime?
Standard dev?
Are you running a different set of boards with massive numbers of cut pins
with uncovered copper?
Did you change your solder pot liner? Your skimmer? Pump? The type of
gloves used by the operator?
Any copper industries close by? Any chance of downdraft in your fume
extractor over the pot?
I know some of these are pretty far-fetched. I was brainstorming.
I will be very interested to hear the resolution to your investigation.
Bev Christian
-----Original Message-----
From: Lou Hart [mailto:[log in to unmask]]
Sent: Wednesday, December 06, 2000 3:03 PM
To: [log in to unmask]
Subject: [TN] Copper contamination is wave solder
TechNetters,
I've made SPC charts to track tin, copper, gold, and antimony in our wave
solder machine. The operator skims dross from the solder pot every morning.
We send a solder sample the first of each month to Kester for analysis.
Over the past 7 months, there has been a monotonic increase in copper
concentration and the past two months have been above the upper control
limit. The copper concentration is still well within spec (0.3%, if I am
correct, according to the relevant IPC standard) at 0.14%, but I want to
make sure we know what is happening.
Any comments on these observations? Some years back I had charts for a HASL
machine and over perhaps 12 months the semimonthly copper concentration
wandered between 0.21 and 0.24 %.
Lou Hart
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