Poh Kong Hui wrote:
> 
> I have a serious problem on some of the board that stucked in the store.
> All of them is at least 2 years.
> When I  wave-soldered the bare board, it has a lot of blow-holes, solder
> balls, pin-holes.
> 
> Anyway, I try to purge all of them, but there are 6000 pieces in the Store.
> 
> I try to bake at at 100 deg. C for 4 hours. Too bad, it doesn't work.
> 
> I try to bake for any 4 hours just before wave-soldering. The result is
> terrible. Not even a slight Help !!!
> 
> Can anybody give me some advises what should I do ???
> 
> Thanks in Advance
> 
> Poh ..

Dear Poh:

In the past, I have observed several steps in the manufacture of pcb's
which seemed to increase the potential for blow holes 9and other
soldering defects) at solder reflow.  While this is different process
than wave soldering, I would presume that the underlying causes may be
similar.

As I understand the problem, blow holes are the result foreign matter 
within the solder which is volatile at soldering temperatures.  This
contamination may occur:

1. on the surface of the copper prior to solder deposition due to
improper copper surface preparation;

2.  organic inclusion (presumably from peptone or other solder
electroplating additives) during solder plating;

3.  porous solder plating increasing the probability of chemical
entrapment within the solder plate;

4.  improper solder brightening and/or rinsing after final etch;

5.  inadequate baking to reduce moisture after solder brightening;

6.  contaminated flux and/or improper flux treatment of tin/lead plate
prior to reflow; and

7.  contaminated reflow oil, contaminated solder pot, or improper time/
temperature or uniformity of temperature during reflow.

Specific to your problem, I will assume the parts in question (because
they are two years old) have been previously soldered.  It is likely the
solder is heavily oxidized.  Additionally, the copper/tin intermetallic
layer will be thick due to the time in storage.  If the solder on the
surface of the board was thin to begin with, the copper/tin
intermetallic layer will be more problematic during wave solder.

I would suggest cleaning the boards in an agressive flux (non-alcohol
type) prior to wave soldering, replacing the oil (if employed) in the
solder pot, and assuring purity and solder composition of the molton
solder.  (It has been my experience that ethoxylated glycerine based
reflow "oils" provide the best performance in wave soldering
applications.)  Good Luck!

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