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 dissolves 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 seconds). In contrast we see
a greater concentration build up in a wave soldering facility when we run
HASL boards especically if our board vendor doesn't controll the copper
concentration in his HASL solder pot. During HASL the same thing happens as
it does in wave soldering machines, the copper surface wets with solder,
forms an intermetallic and dissolves some copper. The HASL time is also
short but the HASL machine blows off the excess solder back into the solder
pot. The HASL solder pots get loaded with copper very rapidly. Most board
vendors keep their HASL solder pots below 0.3% copper concentration but not
well below. When a HASL pot approaches the 0.3% value they ussually don't
pump out the large amount of solder (usually 500-900 pounds)and exchange it
with fresh solder. It takes too long to pump it all out and remelt 900
pounds of bar stock. When the Cu concentration approaces 0.3% some vendors
cool their solder pot to just above the liquidus temperature of SnPb
eutectic solder. The solubility of Cu in SnPb solder decreases and the
copper perciptates at which point a portion of the solder is skimmed off and
some fresh solder added. This lowers the copper concentration quickly and
allows the board vendor to go back to process more boards. Now, as sfor why
HASL boards cause more of a Cu increase in wave soldering machines than OSP
coated copepr boards. When you process boards that were run through a HASL
machine that was operating a a high Cu concentration (lets say 0.2 to 0.28%,
there is a high concentration of Cu in the solder on the board. When you
wave solder the HASL board the solder that was put on the board during HASL
melts and co-mingles with the solder in your wave soldering machine. In
other words, every HASL board you process has 25 to 2500 microinches of
solder on every feature and that solder contains whatever concertraion of Cu
that was in the HASL machine when the board was process. When you wave
solder the board a large portion of the copper from the HASL operation winds
up in your wave soldering machine. When you wave solder a copper board,
there is no solder with copper in it to wind up in your wave soldering
machine.
If I were you I'd check the process control charts of your board vendor to
see if the increase in copper in your wave soldering machine was due to his
process.
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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