Some more thoughts on defect counting...

.....for a DPMO basis
Classic WS-6536 and subsequent MIL-SPEC systems used the following:
For ASSY DPMO, 1 per solder joint/lead, 1 per part, plus one for the PWB.  I
believe that leadless SMT parts get NO COUNT for their "leadless solder
joint", only a part count.
For SOLDER DPMO, 1 per solder joint.  (top & bottom count as one)  When
counting defects, count each joint to be reworked as one, with a minimum of
one.  A solder bridge affecting 4 solder joints is 4, a bridge between a via
and a lead is 1, a bridge between two traces is 1.

However, let me add the following idea to trash about.  What are you going
to do with this wonderful DPMO.  Are you going to evaluate changes in your
process with this, such as testing new fluxes or comparing future equipment
to existing processes.  Will you use it to determine the optimum profile
parameters for the soldering process????

PLEASE DON'T.  This is the rationale...

Your company exists for one sole reason, to make money.  Not all defects
have the same cost.  Some defects are easy to correct.  Some may be MRB
actions with repair or scrap costs associated.  Some have greater
reliability impact than others.  Consider assessing defect types with a
rework cost, and measure your factory performance based on your rework,
repair, scrap and warranty costs.  Some of this will be guestimates, but
they can be developed over time.  If you want your management to understand
your achievements, tell them in DOLLARS.

It is my personal experience that in many cases, the lowest cost to rework,
with lowest risk of scrap or repair was achieved using a soldering process
that yielded a higher defect rate than other parameter settings.  This is
because we delibertly chose the defect types we were willing to accept
(solder peaks) for rework and drove the others into extinction (insufficient
solder topside where components obscure the joint or heavy ground planes
affect IC and capacitor topside fillets).  The same concept applies when
comparing assembly defects to solder defects.  Post-soldering identified
assembly defects almost always involve desoldering two or more GOOD solder
joints.  In the case of a 256 lead Quad, a misaligned part results in many
reworked, and therefore less reliable, solder joints.

Although this has been a personal philosophy of mine for the 15 years I've
been the solder guy at my company, I recently had the joy of finding someone
who had written a book that closely follows my ideas.  It may provide a
fresh approach and help you appreciate the difference between success and a
low DPMO.  Look up "Optimizing Quality in Electronics Assembly, A Heretical
Approach" by James Allen Smith and Frank B. Whitehall from McGraw-Hill.

Thanks for listening
Steve Mikell
[log in to unmask]

***************************************************************************
* TechNet mail list is provided as a service by IPC using SmartList v3.05 *
***************************************************************************
* To subscribe/unsubscribe send a message <to: [log in to unmask]>   *
* with <subject: subscribe/unsubscribe> and no text in the body.          *
***************************************************************************
* If you are having a problem with the IPC TechNet forum please contact   *
* Dmitriy Sklyar at 847-509-9700 ext. 311 or email at [log in to unmask]      *
***************************************************************************