Hello everyone,
In response to the posts about the chip resistor networks, I'd like to
share something I've learned. These are great little parts, they provide for
very efficient use of PCB real estate, and for efficient machine utilization
too. You're placing multiple resistors in one machine placement cycle instead of
just one resistor per cycle.
The ones I've been using recently are 4-resistor RPACKS that basically are
four 0603 resisistors grouped together into one package...each resistor is on a
.031" pitch from the other. We were putting 16-per of these on the backside of
some Level-2 "Pipeline Burst" Cache' memory modules and double-side reflowing
them.
When we first started using them we were experiencing a lot of bridging and
mis-alignment with these little puppies. I double-checked placement accuracy,
tweeked the reflow profile, and also modified the stencil openings to cut down
on some of the paste I was depositing to try and resolve this problem. All that
helped somewhat, but didn't completely resolve the issues.
Then I discovered that there are two different termination styles. They're
called different things depending on who you talk to; Convex and Concave, Leaded
and castellated, and one of my favorites; "Outie" and "innie". But basically,
they're like this:
_______________
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|__ __| _) (_
__|---------|__ |_______________|
| | |_ _|
|__ __| _) (_
__|---------|__ |_______________|
| | |_ _|
|__ __| _) (_
__|---------|__ |_______________|
| | |_ _|
| | _) (_
|_______________| |_______________|
Convex, Leaded, or "outie" Concave, Castellated, or "innie"
The above is a crude illustration of a 4-resistor RPACK and shows basically
how the network is arranged, it also shows the two types of terminations. With
the Convex RPACK's, the solder fillet is formed at the end of each resistor. On
the Concave RPACK's the fillet is formed in the small indentation at the middle
of each resistor...kinda' like how LCCC's are. The point of all this is that I
met with some gentlemen from Phillips (who also make these things) and they
shared with me a report from a study that their Product Application Group did to
see if there were any valid reasons as far as performance, reliability, or
manufacturability to make two different termination styles.
What they discovered was that with RPACK's with Concave terminations had
much, much better yeilds during assembly than did Convex styles. This was due to
the Concave style's ability to "self-center" itself during reflow if placement
was less than perfect...and from experience, they are "absotively" right!
I've got a copy of the report and some beautiful color pictures of before
and after reflow that shows the difference between each style RPACK. The report
also has recommended footprints and pad geometry's for the components too. I
know that the IPC-782 doesn't have a footprint out for this part yet, so if
anyone would like it, jes' ask!
Ya'll know how ta' get in touch wif' me...
__\/__
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# Steve Gregory #
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