Julie,
Soldering processes, by nature, are considered complicated and involved due
to the various factors that must be maintained and monitored. Most of these
factors are indigenous to the process, such as pot temperature, dwell times,
and contamination levels. Items such as temperature and dwell times are
readily measured, adjusted, and understood, making them some of the more
comfortable aspects of the soldering process. Solder contamination, on the
other hand, tends to be a less understood phenomenon.
To accurately determine trace contamination levels in solder requires very
specific analytical equipment and precise analytical methods. I agree with
Franklin, You need to collect data and establidh the trends for your
production. In our solder service we send out a sample daily to ensure
compliance. In a board shop you can reduce that considerably after you
establish how many surface square feet you are processing and the the rate
at which the copper is rising. Most solder suppliers provide multielement
analysis beyond tin, lead, and copper. Once levels of contaminants have been
determined, interpretation of these results may require assistance based on
your particular problem. Your solder supplier should be able to assist you.
Typically, copper contamination and tin concentration are targeted as the
gauges of the relative health of a solder bath. The introduction of gold is
also a critical contaminant requires watching.
Copper, being the most typical contaminant, is probably the best understood
in soldering circles. Manifesting itself as an intermetallic along with tin,
it is necessary to deal with it. A normal by-product of the formation of the
solder bond on the board surface, it migrates outward from the board into
the solder pot, where it typically becomes soluble and homogeneous. There
are two forms of this material, Cu3Sn and CusSn6, of which the latter is
more prevalent. There will come a point in the lifetime of any solder bath
where the solubility of the copper/tin intermetallic will be at or near the
saturation point. Boards processed under these conditions [approximately
0.30 percent by weight at 470° F (243° C)] will show a pronounced gritty
appearance. The reason for this is that the intermetallic, being less dense
than molten tin/lead, will migrate to the top of the deposit where the
characteristic dendritic crystalline structure will create the gritty
surface.
This poses a problem, both in terms of esthetics and in solderability. The
presence of a concentrated copper in the relatively small volume of solder
located on a pad will raise the thermal demand requirement on a local level,
compromising the reflow characteristics of the solder deposit.
Gold is another typical contaminant of a solder bath if it is being used to
process boards that have been previously tab plated. AuSn4 is another
intermetallic that will form if solder is allowed to contact gold-plated
surfaces. Utmost caution must be taken to avoid this occurrence, because
gold is approximately six times more soluble than copper and can be even
more detrimental to the solder joint. Gold contamination will be visible as
a frostiness of the solder deposit with an accompanying embrittlement of the
solder joint. A joint produced under these conditions can be prone to
failure under thermal cycling or high vibration environments. Adequate
protection of the gold from solder contact or elimination of pre-gold
plating are the keys to controlling gold contamination. It can not be
removed by any user-available methods--only by sophisticated metal treatment
systems or solder bath replacement.
Variation of tin concentration can have a pronounced effect on solder
performance, but there is a reasonable window in which to operate that is
not difficult to maintain. Typically, most processes will run well between
61.5 and 63.5 percent tin concentrations. The danger in running above or
below this range is the change in melting points and subsequent variation in
surface tension characteristics. Surface mount processing poses particular
problems primarily in that surfaces devoid of a hole demonstrate low
surface-energy characteristics. As such, they resist the efforts of fluxes
and solder to achieve lower free-energy states and wet the copper surface.
Sufficient deviation from the eutectic alloy ratio can affect later reflow
performance in high-speed, low-temperature soldering applications.
An important factor to consider when examining the soldering of a board or a
given area is that, although a pot might exceed the melting point of the
solder deposit, there is a heat sink effect of the board and flux that will
lower the overall amount of energy transferred to the board, decreasing the
ability to melt the solder deposit. The physical scale we are dealing with
in terms of thermodynamics is small enough to deviate from what we would
consider correct and to force us to reevaluate our way of troubleshooting
and investigating problems. Contamination levels that are unknown can play a
huge role in the repeatability of the process.
Julie, if I can be any more help please contact me off line and I can get
you some additional information.
Good luck
Sherry
phone: 603-437-8653
Fax: 603-43404156
e-mail: [log in to unmask] or [log in to unmask]
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]]On Behalf Of Julie Dixon
> Sent: Thursday, October 07, 1999 2:59 PM
> To: [log in to unmask]
> Subject: [TN] HASL-contaminated solder pot
>
>
> TechNet,
>
> We have sporadic problems with the copper level rising in our
> solder pot. We
> reduce the temp and dross each morning. Could you recommend a specific PM
> program to keep our SnPb clean? We believe per the certicates of analyses
> that
> our supplier gives us good product. Any other reasons it might rise?
>
> Thanks,
> Green
>
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