Electronics Forum / CAD / February 2005

Ivan,
   Take a lot of the information you gain here with a grain of
salt. I don't know where Robert gained his experience but by his
comment on allowing for meniscus he must have been dealing with
3-6mil high parts/leads. 1 - 3 mils heel length is totally
inadequate for a normal real world part. He was kinda right about
the IPC land pattern standard, one cannot design land patterns
allowing for the wide variation of manufacturers dimensional
allowances as IPC suggests (i.e. an SOIC lead foot length of
14mil - 50mil, use 32mil as the real length +/- 18mil but forget
the tolerance when designing the land pattern. Do you ever see a
SOIC foot vary by more than 3 times the minimum length in the
real world?). Their system does work pretty good if you design
for the median part dimensions and drop the min and max cases.
SMT Plus' system says the same thing, forget designing for the
max and mins.

   I have to think that Robert was talking about a SOIC type
lead but he doesn't state so. Besides SOIC leads there are a
large and varied number of pad requirements for various devices.
Some of those requirements are for soldering, some are for test
and some are for reworkability, good pad designs are a blending
of all three. By the way, for a standard common SOIC lead I would
use 20mils on the toe and heel of the lead, the heal for solder
joint strength (meniscus) and the toe for somewhere to get your
heat into the pad for rework (also works well for a test probe
point). (i.e. a 12mil wide SOIC lead with 22mils of foot, a pad
14 - 16mils wide and 62 mils in length, centered on the
centerpoint of the lead's foot. On small pitch SO devices (also
QFPs, LCCs, TSSOPs and all other small pitch devices) the pad
width will approach the same width as the lead, i.e. 8mil lead
width, pad - 8-9mil width. Sometimes the gap between pads can
take priority over the pad width and drive the pad width right
down to matching the lead width.)

   There are too many issues to discuss in this forum but I
would suggest that you look at SMT Plus' literature. I know that
he had two books that he used to sell that discuss land patterns
for SMT devices and runs through a lot of the rules to designing
good SMT land patterns. I have used SMT Plus' system for about 12
years now, it has never let me down through many companies and
across numerous assembly shops.

   As for your question about Protel patterns and where to find
them, I don't know because I never use canned libraries. On that
note be warned that Protel libraries do have errors in them. Some
errors are significant, not just borderline issues. Protel does
have some sort of a library listing on their website if I
remember correctly and it is still there. It might be an
interactive utility or spreadsheet.