Jack Crawford posted a concern about the effect of unintentional 
     migration of silicone from a conformal coating operation to where 
     soldering is being done.  He asked whether the concern is valid and 
     whether anyone had a history of a similar situation.
     
     I hadn't been in the soldering business very long before I heard that 
     silicone can be death to solderability.  Since I had a wetting bal- 
     ance, I decided to investigate.  I was amazed to discover that I could 
     smear silicone grease on a component lead and not have any measurable 
     effect on the wetting time or final wetting force.  After the fact, I 
     came to see that if the component lead's basis metal is covered with a 
     layer of solder, it doesn't matter what is on the solder, because the 
     solder melts and breaks up the contamination layer (which just floats 
     away).  The only exception is if the contaminating layer is solid and 
     so thick that it persists as a sleeve even after the underlying solder 
     melts.  (A thin solid layer breaks up because the solder expands as it 
     melts.)
     
     As for a contaminant in a plated-through hole, unless it is massive 
     enough to stay stuck, it will simply be pushed along to the component 
     (destination) side as the hole fills with solder, where it may be 
     visible but will not reduce the reliability of the connection.
     
     I have not investigated the effect of silicone contamination on 
     surfaces whose solderability is protected by means other than solder, 
     such as gold or an OSP. My guess is that here, trace amounts do get in 
     the way, and that removing them would be very difficult (plasma 
     cleaning?).
     
     As for solder-coated surfaces, my take on the matter is that if 
     silicone is able to interfere with soldering, then the solderability 
     was marginal to begin with.  In any case, it is a very simple matter 
     to investigate - even without a wetting balance.  Just spray conformal 
     coating in varying amounts on boards (or dip in solutions of varying 
     concentrations) and see what happens when you try to solder. I predict 
     that anything short of deliberate coverage will not interfere, except 
     for possible changes in appearance (but not fillet shape).  (If I'm 
     wrong, then we have an opportunity to learn what is going on, and put 
     the matter beyond where I see it today, which is at the anecdotal 
     level.)
     
     Gordon Davy

***************************************************************************
* TechNet mail list is provided as a service by IPC using SmartList v3.05 *
***************************************************************************
* To subscribe/unsubscribe send a message <to: [log in to unmask]>   *
* with <subject: subscribe/unsubscribe> and no text in the body.          *
***************************************************************************
* If you are having a problem with the IPC TechNet forum please contact   *
* Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask]      *
***************************************************************************