Good Morning TechNet (depending on your timezone)!
Specification confusion is such a wonderful thing - it shows that the
specification committees need to write as clearly as possible. As for my
comments:
"Assuming that the boards are ordered and manufactured according to
IPC-J-STD-003,but stored in a noncontrolled environment for more
than 1 year"
*Gaby - the ANSIJ-STD-003 is not a printed wiring board manufacturing
specification! The 003 specification is a solderability test document which
boards can be tested to and meet different criteria. You would be ordering
boards per an IPC board fabrication specification (e.g. IPC-6012) and one
requirement would be that the boards meet the solderability criteria of
003. I know this might be explanation overkill but many people have been
lead to believe that the 003 is a fabrication document when it is not.
"1.What is the known growth rate of the intermetallic,and what is the
minimum tin/lead thickness left?"
*Dr. Chis Hunt of NPL published a good paper on oxide and intermetallic
growth at in the 1996 SMI Proceedings titled "A Model For Solderability
Degradation" pages 650-655. Intermetallic growth is a diffusion phenomena
which means that time and temperature are controlling factors. Most of the
intermetallic you see on a printed wiring board is formed at molten solder
temperatures (e.g. HASL, assembly reflow, etc.). The amount of
intermetallic growth you get at room temperature storage is very small.
"2.If there is moisture absorbed,but the plating in holes is without
defects concerning thickness,voids,drilling,a.s.o.,should I expect
blowholes?"
*If the plated thru hole has no plating wall defects then moisture due to
storage humidity will not blow holes (we haven't figure out a way to get
moisture to penetrate a metallurgical joint yet). However the storage
humidity will contribute to a degradation/conversion of tin oxide from a
SnO to a SnO2 species. Most fluxes are not very good at breaking up the
SnO2 species. Look back in the TechNet achieves for an explanation of the
sources of blowholes - several folks have commented on their formation in
detail.
"3.What is the maximum intermetallic growth after drying for 3 hrs at
120C?"
*See Dr. Hunt's paper - you can make a calculation based on the data
listed. Again, 3 hours at 120C isn't going to grow very much intermetallic.
"4.Are delamination problems encountered in almost all or few boards
which absorbed moisture ,and is this related more to the manufacturer's
workmanship?"
*I'll leave that question alone! Other TechNetee's will give you better
info than I. We don't bake boards prior to soldering operations but have
worked with our board suppliers to get to where we are today. The board
fabricator's process has a direct impact on the amount of moisture a board
can absorb.
"5.Finally would drying decrease solderability so much as to render
boards unsolderable with no-clean fluxes?"
*The definitive answer is maybe! Dry baking will degrade the solderability
of a board by increasing the surface oxidation of the tin or of exposure
copper/tin intermetallic. The impact of baking is dependent on the amount
of solder coverage/thickness on the board. Also, many low residue (no clean
is a process choice, not a material characteristic) fluxes formulated to
have lower or limited flux activation potentials so they could have
problems with some "baking" recipes if sufficient oxidation occurs.
Good Luck
Dave Hillman
JSTD 003 CoChairman
[log in to unmask]
[log in to unmask] on 02/02/98 03:05:54 AM
Please respond to [log in to unmask]; Please respond to
[log in to unmask]
To: [log in to unmask]
cc: (bcc: David D Hillman/CedarRapids/Collins/Rockwell)
Subject: [TN] Help on drying boards
Dear TechNetters !
Assuming that the boards are ordered and manufactured according to
IPC-J-STD-003,but stored in a noncontrolled environment for more
than 1 year, I would like help on the following-(for eutectic
tin/lead):
1.What is the known growth rate of the intermetallic,and what is the
minimum tin/lead thickness left?
2.If there is moisture absorbed,but the plating in holes is without
defects
concerning thickness,voids,drilling,a.s.o.,should I expect blowholes?
3.What is the maximum intermetallic growth after drying for 3 hrs at
120C?
4.Are delamination problems encountered in almost all or few boards
which absorbed moisture ,and is this related more to the manufacturer's
workmanship?
5.Finally would drying decrease solderability so much as to render
boards
unsolderable with no-clean fluxes?
Could somebody give me some information very QUICKLY?
Thank you all,
GABY
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