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Manufacturing question

M

Michael

Jan 1, 1970
0
Does anybody know how well a SMT component position is defined relative
to PCB pads?
We are using Hall sensor in SOT23 package. Considering that the sensor
floats on solder, is there any way to make its location on the PCB
repeatable down to 20 mils?
Ten mils?
Five??
Two???
X-axis? Y-axis??
Any insight would be greatly appreciated.
 
J

Joerg

Jan 1, 1970
0
Hallo Michael,

Does anybody know how well a SMT component position is defined relative
to PCB pads?
We are using Hall sensor in SOT23 package. Considering that the sensor
floats on solder, is there any way to make its location on the PCB
repeatable down to 20 mils?
Ten mils?
Five??
Two???
X-axis? Y-axis??
Any insight would be greatly appreciated.

Might want to consider a sacrifical holder that keeps it lined up until
after the reflow.
 
K

Keyser Soze

Jan 1, 1970
0
Joerg said:
Hallo Michael,



Might want to consider a sacrifical holder that keeps it lined up until after the reflow.
You should look at the accuracy of the Hall sensor as well.

It does not help to position a sensor to plus or minus 2 mills
when the sensor is only accurate to plus of minus 20 mills.

If mechanical position is so critical try:

1 - use a through hole part.
2 - use an SMT package that has a reference stud or pin.
3 - route a hole in the PCB for the SOT23 package and mount it inverted.
4 - use high temp glue to hold the SOT23 where the pick and place puts it.
 
M

Michael

Jan 1, 1970
0
1 - use a through hole part.
2 - use an SMT package that has a reference stud or pin.
Not an option - no room, the part is selected
3 - route a hole in the PCB for the SOT23 package and mount it inverted.
I am not sure the mechanical designer will be happy with it
4 - use high temp glue to hold the SOT23 where the pick and place puts it.
This should work.
Thanks!!!!
 
C

Chris Jones

Jan 1, 1970
0
Michael said:
Not an option - no room, the part is selected
I am not sure the mechanical designer will be happy with it
This should work.
Thanks!!!!

As the other guy said, glue is probably the answer.
Quite a lot of SMT boards (especially older ones) have the parts glued down,
there must be an existing glue product (and dispensing system) for doing
this.
Chris
 
Michael said:
This should work.

It isn't totally guaranteed, though; the package can soften, and
the glue can soften,
and the surface-tension forces of the cooling solder can be
significant. Elastic
glues shrink when heated.

Hall sensors are sensitive to stress, too, so having three solder pads
pulling on it
might give different performance than three solder pads and a glue dot.

One relatively safe approach would be to solder it up, then align the
circuit board on
the sensitive chip, before drilling the board for its mount/alignment
pins (or are you
doing that anyway?). On second thought, your fab line probably uses
machine vision
and printed targets on the PCB for alignment, thus the original
question 'how closely
aligned to the printed pads'.
 
Chris said:
As the other guy said, glue is probably the answer.
Quite a lot of SMT boards (especially older ones) have the parts glued down,
there must be an existing glue product (and dispensing system) for doing
this.

And you need to be sure that there is enough solder on the pads to
bridge the gaps that the glue will enforce.

In fact, when the SMT was floating on mollten solder, its position
could be tightly defined by the surface tension in the molten solder.
You would need good control of the volume of solder on each pad to get
precise positioning out of this and you'd have to be careful about the
geometry of your pads, but this could be a neat way to go.
 
Michael said:
Does anybody know how well a SMT component position is defined relative
to PCB pads?
We are using Hall sensor in SOT23 package. Considering that the sensor
floats on solder, is there any way to make its location on the PCB
repeatable down to 20 mils?
Ten mils?
Five??
Two???
X-axis? Y-axis??
Any insight would be greatly appreciated.

I don't know, but even 0.5mm pitch packages seem to end up in the
right place most of the time....

-Lasse
 
J

Jeff L

Jan 1, 1970
0
Design the pads so that they are the same width as the leads on the IC -
the solder surface tension will pull the leads to the middle of the pad
during reflow! This will keep the theta under control, along with one axis.
 
E

Eeyore

Jan 1, 1970
0
In fact, when the SMT was floating on mollten solder, its position
could be tightly defined by the surface tension in the molten solder.

That might be a problem with lead-free.

Graham
 
E

Eeyore

Jan 1, 1970
0
Jeff said:
Design the pads so that they are the same width as the leads on the IC -
the solder surface tension will pull the leads to the middle of the pad
during reflow! This will keep the theta under control, along with one axis.

Doesn't work well with lead-free AIUI. No wetting ability.

Graham
 
M

Michael

Jan 1, 1970
0
Thanks a lot to everybody!
I will do this:
"Let it go" as is (no fancy tricks) and see how repeatable the location
will be. My guts feeling is that surface tension will do the trick (I
do not know how much difference lead free solder will make).
If our mechanical guy finds that position variation is still too high
we'll try the glue or positioning jig. Punching the hole in the board
and pressing the part in (up side down) is the last resort as it will
require mechanical changes beyond the pcb.
:eek:)
 
P

Phil Hobbs

Jan 1, 1970
0
And you need to be sure that there is enough solder on the pads to
bridge the gaps that the glue will enforce.

In fact, when the SMT was floating on mollten solder, its position
could be tightly defined by the surface tension in the molten solder.
You would need good control of the volume of solder on each pad to get
precise positioning out of this and you'd have to be careful about the
geometry of your pads, but this could be a neat way to go.

If you make the pad pattern a little bigger than the pin pattern, so
that the solder has to stretch sideways just a little to reach, you get
much better self-centering.

Cheers,

Phil Hobbs
 
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