Hi Jim,

First, I believe that tracking process performance prior to the touch-up
operation is very important.  Mainly because ruducing the amount of
touch-up, reduces the amount of handling and manual operations that
assemblies go through.  In order to do this you must record the data that
is available in your post-process inspection.

An effective way to track these defects is with a ratio of defects to the
number of opportunities to produce defects.  A very simple example would be
an assembly with 400 solder connections that you were producing a quantity
of 100.  During post-wave inspection for example,  you find a total of 240
areas that require "touch-up" (defects).  This comes out to an average of
2.4 defects per board, and likely a yield of 0%-10%.  Obviously the yield
info tells you very little about the actual frequency of defects the
process produces.  So a better way to look at it is to multiply 400
opportunities to produce defects, times the number of boards built (100) =
40,000 opportunites.  The result is 240 defects produced out of 40,000
opportunities.  The result is 240 divided by 40,000 or .006 DPO (defects
per opportunity).  In the industry, most companies base their performance
goals on the number of defects per million opportunities.  So now take .006
times 1million to get 6000 DPM (Defects per million opportunies).  Some
also use PPM to describe the same ratio.  From here you can set new goals
for your "process" and not just individual assemblies.  For now you have a
number that represents your "rate" of producing defects.  Now try to bring
that number down.

Formula summary:

A= Number of defects found in a "Lot"
B= Number of Opportunites To Produce Defects Per Assembly (Often the number
of individual solder connections, yep ya got go count em)
C= Number of Assemblies per "Lot"

DPM=A/(B*C)*1000000

Now that you have a way to track current performance and progress towards
goals, the most important thing is HOW you will reach those goals.  This is
where commitment to make quality products comes in.  It is beneficial to
now make Paretos of the defect types and defect locations.  Shorts and
Opens are likely to vague as descriptions.  You will want to learn to
identify Insufficient solder, Cold Solder,  Open due to misalignment etc,
etc.  also track the locations of these defects.
For Example:
Insufficient solder at U1-7 (Pin 7 of IC U1)

A matrix of check marks can be an easy way for the post-process inspectors
to record this data.  As they put in arrow on the Assembly for touch-up,
they can use a matrix worksheet to record the defect type and location.
Here is a crude sample

                    Post Wave Inspection Log

Assembly Number______
Lot Quantity             ______


     Insufficient Solder Cold Solder     Missing Part   Solder Bridge
Damaged Part

R1   XX                       X
R2
R3                                       XXX
C1
C2
C3
U1
U2
U3                  6 (means pin 6)

Hope this helps,
Sean

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