To:  Karl Sauter, Pat Kane, Dave Hillman and Jack Crawford.

The subjects included Heat Seal Packaging and Board Baking.  We can 
discuss these all together.  Though much has been discussed on board 
pre-bake on this forum, let's look at it again.

Heat Seal Packaging:  A study was done by the military in the early 
1970's on packaging.  I really have trouble remembering back that far 
but I think Motorola also did a study on this subject.  I remember 
that when heat sealing the bag, a gas is created from the heat source 
when melting the bag.  The gas attacks the tin, resulting in 
soldering problems after a short storage.  This study also showed 
that the sulphur in paper attacked the lead.  

Question:  name of specific gas?  Can't remember the specifc gas, but 
any chemist should be able to answer this for you.

Question:  Can drawing a vacuum adequately remove the gas from the 
bag during heat sealing?  Sorry, I don't know.  I would suspect that 
the gas would be eliminated from the surfaces but not the holes.

Question:  Can the sealing be done in such a way that this gas is not 
present or cause a significant problem.  Sorry, I again don't know.  


Question:  Do I have any published data on heat seal effects 
mentioned?  I did have a copy of this data many years ago but am 
unable to find it now.  You may try checking with the Military and/or 
Motorola to see if it can be located.  It was mentioned by Dave 
Hillman that the type of bag, sealing environment and processing 
factors may play a roll in this.  That may be true as the bag 
materials may have changed over the years.  However, I don't think so.


Board Baking:  Board baking is probably part of your process for the 
same reason you want to heat seal the bags.  To eliminate or reduce 
moisture in the board material prior to assembly or soldering????  
This subject began with someone asking about eliminating the pre-bake 
cycle on wave soldering because they had no problem in IR reflow.  

The return comments made many suggestions that the vendors process 
should be monitored and controlled to control moisture.  Let's look 
at some facts:

1-  Baking boards increases the intermetallic and reduces 
solderability.  The higher the temperature               and the 
longer the bake cycle, the faster solderability is diminished. - 
FACT
2.  Board material has very high moisture absorbtion properties.  In 
much less than 24 hours, the board totally absorbs any moisture baked 
out back into the board.  Therefore, any baking cycle will require 
you to complete all of your soldering processes within 3 to 4 hours 
just prior to soldering.  Otherwise, the only thing accomplished is 
diminished solderability - FACT

What is your reason to bake Boards?  Eliminate or reduce solder joint 
pin holes/blow holes?  Reduce the possibility of measles or 
delamination?  Solder joint pin holes and blow holes are not caused 
from entrapped moisture.  Pin holes are caused by plating voids or 
flux entrapment in the holes.  Blow holes are caused from a wetting 
problem.  Baking may help pin holes but increase blow holes.  The 
answer is not whether or not to bake.  The answer is to eliminate the 
source of the problem.  

As for measles or delamination, when was the last time you have seen 
either of these problems?  Not for many years, I'm sure, except for 
very isolated cases.

Baking may or may not be  a solution for popcorning.  Can it be 
assumed that there was an intensive study in this area?

My Question to you:

1.  Is heat sealing a vendor convenience or your choice?
2.  Is the reason for bagging solely to keep the boards clean?     
3.  Does heat sealing keep the boards moisture free during storage?  

4.  Is the heat seal bag material moisture proof or does it also have 
moisture absorbtion properties?
5.  After you remove the boards from the bags, do you keep them in a 
desicator during assembly and prior to soldering?  If not, the boards 
have your assembly environment moisture throughout the board material 
prior to soldering.  Then, has heat sealing accomplished anything 
what-so-ever?

As for monitoring the vendors process to keep moisture at a minimum.  
The board is submerged in moisture through most of its process 
(srubbing, chemicals, water rinses, more scrubbing, etc.).  No amount 
of vendor monitoring will help moisture content.  The only thing a 
vendor can do for you is bake the boards prior to packaging.  Based 
on everything discussed here, that would also be a waste and further 
diminish solderability.

If you feel that heat sealing is the way to go, then a new study is 
in order.  Lot's of money and time required.  In my opinion, there is 
no need to heat seal boards in a bag and no reason for board baking, 
unless you have set up an assembly and pre-soldering environment 
conducive to very low humidity conditions.  Then watch your employees 
complain about dry skin and rashes.

Sorry for the long reply.

Regards,

Norm Einarson
Printed Circuit Technology