In a message dated 12/11/98 11:37:45 AM Pacific Standard Time, [log in to unmask]
writes:
<<
Has anyone else had severe tombstoning problems placing chip resistor arrays.
All our other components reflow just fine, but we're getting 25 to 30% failure
on the chip networks. We can't figure the cause or solution. We're using
convection reflow.
Thanks in advance. >>
Hello Kyriakos,
I've not experienced tombstoning with chip-networks before, but I have
had problems with solder shorts, and the part pulling to one side and giving
open solder on the other...in my case, I found the key is having the right
footprint, and using the right style of chip-network.
I learned there are two styles of terminations that chip-networks can
come in; Concave, and Convex. Sometimes called castellated, and leaded, or
what I like to call them; "Innie's and outtie's...like belly
buttons...hehehe....and I like my chip-networks like I like my belly buttons:
Innie's... B-)~
The bugger's are the outtie's, the convex, or leaded ones. They bridge
very easily, and they don't self-center as well as the innnie's do. The reason
is because of the termination area. The innie's have a more precise,
consistantly sized termination and seems to be more suited to this type of
package that does the outtie style. Wetting forces seem more balanced with
innie's...
I was given some papers from a Phillips components rep back a couple of
years ago that I still have that show some really good pictures of the
differences between the two styles, as well as some before and after reflow
pictures that illustrate how each performs during soldering, the fillets that
form, and how well each part self-centers during reflow. These papers were
from a study that engineers from Phillips PEBEI RSMD Product Application Group
(Whatever that means). They spec out three different footprints, Standard, Low
Limit (when you're hurting for real estate and need things small), and High
Limit when you've a football field to play with. The recommended footprints
are as follows:
(dimensions in mm)
STANDARD
pad width: .45mm
pad length: 1.15mm
pad pitch: .8mm
gap between: each row of pads: .8mm
LOW LIMIT
pad width: .4mm
pad length: .85mm
pad pitch: .8mm
gap between each row of pads: .9mm
HIGH LIMIT
pad width: .5mm
pad length: 1.45mm
pad pitch: .8
gap between each row of pads: .7mm
They used a 8-mil thick stencil and a alpha metals no clean paste (SM35-10
RMA9083) for the parts they took pictures of. They also have a reflow plot
that shows a 229 degree C. peak temp., liquidous for about a minute, and
preheat of about 140 degrees C. for around 2.5-3.0 minutes.
I hope this'll help ya' a lil' bit...
-Steve Gregory-
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