James,

You are correct to assume that the members of the committee included
idividuals from most product categories. The participants are listed inside
the front cover of the standard. I also have listened to those same
arguments, which are valid issues.

The first thing that must be understood are the rules that the committee had
to meet. Most designers today do not know what soldering process will be
chosen for a particular assembly. If the soldering process is at a captive
facility, then the decision is relatively simple. However, many products are
outsourced, making the designer's choices very difficult. Therefore, the
committee was tasked to recommend land pattern design principles that are
process independent. The result is land patterns that may just be larger than
those used in a captive assembly facility. The product may not pass through
the soldering line as fast as the assembler wishes, but the result WILL be
high yield and high reliability.  

The second thing to recognize is that the formulae ask for specific
information about component sizes, tolerances, fabrication allowance and
assembly accuracy. The user may input any numbers they choose. I'm positive
that when your manufacturing people provided their input to the land
patterns, you design, they took license to certain criteria. It's possible
that they purchase tightly controlled components that never exhibit the
tolerance extents shown on an EIA specification drawing. Therefore, they use
a reduced number in their calculations. Maybe the pick and place accuracy is
better than what the industry considers normal. These are just a couple of
examples that would  tend to reduce the size of a land pattern.

The original IPC-SM-782 gave recommendations, for specific families of
components. The user community requested that the principles be specified so
that they may apply them to new components that are introduced. There were
others that still wanted to know what the rest of the industry was
recommending. Therefore, the IPC-SM-782A contains the formulas, explanations,
and anaysis for the components that were available at it's writing and the
tables with industry assumptions and recommendations. 

Lastly, you will find that if the same assumptions were applied to the
formulae, as were applied to the home grown land pattern, the resulting land
pattern sizes would probably be very close to the IPC landpattern. 

Most of the feedback has been positive, with size being the number one
complaint. Very few, if any, complaints about yield or reliability.

Regards,

Gary Ferrari
IPC Staff


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