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| Date: | Fri, 30 Jan 1998 16:52:51 -0800 |
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See answers after your questions below.
-----Original Message-----
From: Thad McMillan [SMTP:[log in to unmask]]
Sent: Friday, January 30, 1998 3:39 PM
To: [log in to unmask]
Subject: [TN] Small holes, Thick Boards & Hole Reliability
Importance: High
Currently we are designing a board thicker than normal with
small
holes.
The board is a 10 layer board, 082" thick, and uses 13.5 mil
drilled/plated holes.
The board is large ~ 13 x 16" in size. It is HASL. During
component
assembly it will see 2 IR processes and a wave. We are
specifying
the board as IPC Class 2. It will be used in high end computer
product. Reliability is very important.
In light of the above I have several questions for this
distinguished
group:
We have had in production a 18 layer .105" thick
board for a year and a half with .013" drilled vias with no problems.
Not one via has opened! The assembly process is 2
reflow and partial wave. Finish is electroless nickel with immersion gold.
The nickel/gold also is in the holes. This also avoids the thermal shock of
HASL. (One less thermal cycle.)
Other boards in production are .155" thick with no
problems..with the 13 mil via. (12:1 ratio)
1. Considering the 6 to 1 aspect ratio, using conventional FR-4
will
the design above have reliable plated through holes?
You will have reliable plated holes at this ratio if
you have a supplier who is capable, not everyone is..
Ask him for data on % elongation of his copper. It
should be at least 10 or above. (our supplier is at
16%.)
2. We are considering specifying that the FR-4 be of Tg greater
than
170 Deg C. The hope is that this will improve hole relaibility.
The
problem is that this will add significant cost to the board. Is
it
necessary to do this?
We use 170 Tg material and at the moment, do not see a need
to change..
3. The design is dense so that we cannot grow the hole
significantly.
We are considering permitting hole breakout of 1 mil, so that
the
holes can be drilled larger. Is this a reliability risk?
No risk here, except with spacing violations possible if you
do not design for the break-out. Reliability of the hole is not effected
with break-out. With some suppliers, they charge extra for 13 mil drill.
Better check this out.
We currently use a .013 via +.000, -..013" in a .026" pad
without allowing break-out.
4. Are there other alternatives (not so expensive) to improve
hole
reliability?
I would appreciate any feedback.
Thanks,
[log in to unmask]
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Please visit IPC web site (http://jefry.ipc.org/forum.htm) for additional information.
For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311
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