>> Hi Steve,
>> Your posting really caught my attention. I have a product that has the
>> concave termination R-packs; however, I don't see this self-centering
>> mechanism you are speaking of. The problem I have over here are open-solder
>> joints do to skewing/misregistered components. The placement looks slightly
>> of the mark prior to entering the reflow oven and it comes out of the oven
>> looking even worse. My initial impression was that the packages are
>> unstable wrt. self- centering. Am I missing something here?
>> Would appreciate any more comments you may have.
>> Thanks,
>> Robert Borenstein Phone:(416) 448-5607 Dept 110,
>> Celestica Fax: (416) 448-4736 Process Engineering
Internet: [log in to unmask]
Good morning Robert!
(I hope you don't mind, but I'm gonna' post your question and my response
to the list...your question is too good to not share, okay?)
Hmmmm...I was having the same open solder/skewing problem for a while after
I had gotten a few reels of the Concave part to verify what the Phillips people
had told me. I discovered that my problem was being caused by a combination of
the footprint that was being used on the PCB, and the modifications of my
stencil openings I had done to try and take care of the bridging problem with
the Convex parts.
I guess it's best to explain how things began. The original footprint that
was being used on the PCB had the gap between the two rows of pads too close
together, also adding to the problem was that there was no mask between the
pads. There's also another little problem with the convex parts that you don't
see unless you look at one real close under a microscope, see my crude drawing
below:
\ \\\\\\\\\\\ \
\ \\\\\\\\\\\ \ Shaded area
\ \__\\\\\\\\\\\__\ |||--indicates Plated
\ | ||||||||||| | termination
\ | ||||||||||| |
\ | ||||||||||| |
\| ||||||||||| |
----------------
One chip-RPACK termination
This is supposed to be just one of the eight terminations on a 4-resistor RPACK.
When you look at the drawings the different vendors have in their catalogs for a
convex part, it'll show that the termination or solderable surfaces are at the
top, front face, and bottom of each resistor termination. The surfaces in
between each resistor on the RPACK are shown in the catalogs to be part of the
resistor material, and not have any termination metalization...but a portion of
it does when you look closely under at it under a microscope...and that's part
of why they bridge easier and don't self-center as well as the Concave part. The
bit of metal that winds-up on the inside surfaces between each resistor (I call
it "termination flash" for lack of a better term) is not too consistent either,
on some RPACK's it'll cover quite a bit of area, on others it's not there.
So what I did with my stencil openings to try and reduce the bridging and
skewing was to micromodify them by reducing the length of each pad opening by
.010" on the end towards the center of the RPACK footprint. The stencil image
was also compensated for etch factor to industry standards. Now I had a stencil
that printed the paste for the RPACK terminations out more towards the outside
at a reduced volume, which did improve things a bit.
But when I was trying the reels of Concave parts out, I was having the same
thing happen as you, opens and skewing. Then I learned how much difference
having the correct footprint and pad geometry makes...but that's 'nuthing new,
is it? See below:
__ _______ _______ _______ _______
/|\| | | | | | | |
| | | | | | | | |
| | | | | | | | |
| | | | | | | | |
| | | | | | | | |
| | | | | | | | |
| | | | + | | + | | |
| |_______| |___|___| |___|___| |_______|
| /|\ | |
| |<--- P --->|
|
F A
|
|
| __\|/__ _______ _______ _______
| | | | | | | | |
| | | | | | | | |
| | | | | | | | |
| | | | | | | | |
| | | | | | | | |
| | | | | | | | |
| | | | | | | | |
\|/|_______| |_______| |_______| |_______|
~ |<--B-->|
Recommended Footprint for Concave Chip-resistor RPACK's
A= .8mm (+\-.1mm)
B= .45mm (+\-.05mm)
F= 3.1mm (+\-.5mm)
P= .8mm
We used that footprint, I had my stencil remade and with just a etch factor
compensation (it was 6-mils thick BTW), and now life is grand! No 'mo bridgin'
or skewin'! YEE-HAW!!
Sorry this was so long...
__\/__
. / ^ _ \ .
|\| (o)(o) |/|
#------.OOOo----oo----oOOO.-----#
# Steve Gregory #
# [log in to unmask] #
# #
#________________Oooo.__________#
.oooO ( )
( ) ) /
\ ( (_/
\_)
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