Hi Norm: Here are some answers to the questions you posed:
1. Is heat sealing a vendor convenience or your choice?
* The use of heat sealing is the choice of our board vendors. We do not
require heat sealing per some specification requirement.
2. Is the reason for bagging solely to keep the boards clean?
* The bagging is required to keep the boards clean and allow more robust
handling during the logistics of shipping.
3. Does heat sealing keep the boards moisture free during storage?
* Yes, I have done some weight measurements (which is no easy task in
itself due to the very low signal to noise ratio of the measurement) and have
shown that sealing the boards reduces the amount of moisture the boards pick up.
The boards are not moisture free due to the bagging material.
4. Is the heat seal bag material moisture proof or does it also have
moisture absorption properties?
* The bags are not moisture proof. They do have an absorption rate of
their own.
5. After you remove the boards from the bags, do you keep them in a =
desicator during assembly and prior to soldering?
* No, the boards are removed from the bags and assembled. And yes, they
don't always get assembled in a JIT manner.
I suspect that the Military study is now out of date with the materials that
exist in the electronics industry along with improved fabrication methods being
employed by the board fabricators. I have data and we both agree - if you don't
solder the boards within a couple of hours after baking or protected storage
then you just wasted alot of time - the moisture absorption of the boards from
the environment is very quick. Great discussion - just what TechNet is designed
to do!
Dave Hillman
Rockwell Collins
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______________________________ Reply Separator _________________________________
Subject: Re: Heat Seal/Bake
Author: [log in to unmask] at ccmgw1
Date: 5/14/96 5:38 PM
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=92s look at it again.
Heat Seal Packaging: A study was done by the military in the early =
1970=92s 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=92t 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=92t 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=92t 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=92t 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=92s 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=92m 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=92s 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 =
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