Hello,
There are a couple of ways to look at x-outs. I have seen one application
where the impact on yield is calculated by using the binomial probability
distribution. You know the single-image yield and the number of boards in
the array. From statistics, you can then calculate the overall yield as a
function of maximum x-outs allowed.
I think this way considerably overstates the yield impact. For example,
allowing a few x-outs max on a 20-image array where the single-image yield
is 90% would drop the overall yield to a very low number using binomial
probability distribution. It is unlikely that would really happen.
At the other extreme, the 20-image array could be treated as one large
board. How many board fabricators figure in the size of the board when
determining yields for pricing?
These are the two extremes; reality is somewhere in between. What's
reasonable depends on the difficulty and volume of the board. For an array
of 20 images, 5% is only one image, which may be a bit restrictive for the
fabricator. I personally think the greater of 20% or 1 image is reasonable
unless there are circumstances that require more restrictive
specifications. As an additional spec, we sometimes specify the max percent
of the arrays in a shipment that can have any x-outs. For example, on an
array with five images, we might allow one x-out on 20% of the arrays in a
shipment (the remainder would have to be 100% good). This is usually the
spec we give our fabricators and I have not seen price increases because of
it.
Regards,
Denis Mori, Materials Engineer
Hewlett-Packard Company
Roseville, CA
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______________________________ Reply Separator _________________________________
Subject: X-outs On Multiple Board Arrays
Author: Non-HP-patb
(rosemail.rose.hp.com!relay.hp.com!simon.ipc.org!uunet.uu.net!zytec!owl!patb) at
HP-Roseville,mimegw3
Date: 12/13/96 1:04 PM
I was wondering what policies others have regarding accepting x-outs on
multiple board arrays. We currently allow 50%, with the intent being we
would use a two up array with one x-out, but don't want to see 10 x-outs on
a 20 up array. We are contemplating going to something different, say 5%
or 1, whichever is greater. Or possibly no x-outs, but I'm concerned about
the cost ramifications of this (cost is a function of yield, right?). I
would appreciate any input from both fabricators and assemblers.
Regards,
Pat Bailey
Zytec Corporation
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