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How to solder BGAs with reflow oven, and what footprint to use?

M

Mike Noone

Jan 1, 1970
0
Hi - In the near future it looks like I'm going to be needing to make a
couple boards that have BGA components. I've worked hard to avoid BGA
for a long time now - but it looks like I just can't escape it any
longer.

I have found that at my school (I am a senior EE student) there is a
reflow oven that I can use. I spoke with the guy in charge of it and
and he has fairly minimal experience with BGA, though others have used
the oven for that purpose. What do I need to know? First of all - what
should the footprint look like? One of the parts I may be using is the
Analog ADXRS401
(http://www.analog.com/en/prod/0,2877,ADXRS401,00.html)

It does not give a recommended footprint, unfortunately. I'm assuming I
should just use round pads centered beneath each ball? How large should
they be? If I want a via connected to a ball - can I put it on the pad
itself, or does it need to be away from the pads?

Now - once I have my board made - how do I solder it? The guy I spoke
with said that I should add some solder paste to the part. Does this
make sense? I had always thought that the ball of solder was enough,
though he said the paste would help make sure the part does not move as
it is going through the reflow machine. If I do need to use paste -
Would I need to use a stencil for this, or could I just dab small drops
of solder paste on each pad?

Thanks,

-Mike
 
Mike said:
Hi - In the near future it looks like I'm going to be needing to make a
couple boards that have BGA components. I've worked hard to avoid BGA
for a long time now - but it looks like I just can't escape it any
longer.

I have found that at my school (I am a senior EE student) there is a
reflow oven that I can use. I spoke with the guy in charge of it and
and he has fairly minimal experience with BGA, though others have used
the oven for that purpose. What do I need to know? First of all - what
should the footprint look like? One of the parts I may be using is the
Analog ADXRS401
(http://www.analog.com/en/prod/0,2877,ADXRS401,00.html)

It does not give a recommended footprint, unfortunately. I'm assuming I
should just use round pads centered beneath each ball? How large should
they be? If I want a via connected to a ball - can I put it on the pad
itself, or does it need to be away from the pads?

Now - once I have my board made - how do I solder it? The guy I spoke
with said that I should add some solder paste to the part. Does this
make sense? I had always thought that the ball of solder was enough,
though he said the paste would help make sure the part does not move as
it is going through the reflow machine. If I do need to use paste -
Would I need to use a stencil for this, or could I just dab small drops
of solder paste on each pad?

Thanks,

-Mike

I am not sure about how to get the footprint for this chip but I would
look around for a similar pitch/size BGA and see if someone provided
the fp for it. Also, most commercial layout packages, such as Eagle
have footprint libraries that can choose the footprint for you. I do
not think that there is anything special to the pad layout (other than
the inevitable signal integrity issues), so you can put a via under a
pad.

Finally, I saw with my very own eyes how a BGA was soldered on in a
toaster oven controlled with a laptop (it is important to follow the
temp curve recommended by the manufacturer). Surface tension
essentially positions the chip where it has to go. Check out this guy's
page where he describes a similar procedure:

http://www.dlharmon.com/

Hopefully, more knowledgeable people in this group will provide more
insight for you.

Good luck!

Alex
 
J

Jamie Morken

Jan 1, 1970
0
Hi Mike,

Mike said:
Hi - In the near future it looks like I'm going to be needing to make a
couple boards that have BGA components. I've worked hard to avoid BGA
for a long time now - but it looks like I just can't escape it any
longer.

I have found that at my school (I am a senior EE student) there is a
reflow oven that I can use. I spoke with the guy in charge of it and
and he has fairly minimal experience with BGA, though others have used
the oven for that purpose. What do I need to know? First of all - what
should the footprint look like? One of the parts I may be using is the
Analog ADXRS401
(http://www.analog.com/en/prod/0,2877,ADXRS401,00.html)

It does not give a recommended footprint, unfortunately. I'm assuming I
should just use round pads centered beneath each ball? How large should
they be? If I want a via connected to a ball - can I put it on the pad
itself, or does it need to be away from the pads?

Yes round pads are typically used. I ended up using 0.52mm diameter
circles for the adxrs BGA pads (just under the average 0.55mm ball size)
Although I think some people use 75% or less for the pad size.

For vias on the pads, the via's will suck solder into them unless they
are filled. They can be filled by you manually or by using a board with
buried/blind vias. If you don't fill them before putting on the gyro,
you may lose a connection as a solderball may get "sucked down the
drain". I think this is why people usually put vias in between the
pads, but this requires small drilling to make the vias small enough
so they don't short out to the BGA pads.

I've used a toaster oven to solder these gyro's and it works fine.
I have eagle cad files for the PCB footprints that I used here:
http://mav.rocketresearch.org/ (look under electrical)
Now - once I have my board made - how do I solder it? The guy I spoke
with said that I should add some solder paste to the part. Does this
make sense? I had always thought that the ball of solder was enough,
though he said the paste would help make sure the part does not move as
it is going through the reflow machine. If I do need to use paste -
Would I need to use a stencil for this, or could I just dab small drops
of solder paste on each pad?

I wouldn't recommend using solder paste for the adxrs parts, just put a
very light and smooth coating of flux on the PCB before you set the gyro
on and then bake it. For other BGA parts you may or may not want to use
solder flux, depending on if the part has solder balls or non-melting
balls. (I think most parts have melting solder balls though).

If you do want to put paste on the pads, it is much easier to use a
stencil than to manually place small drops of solder by hand.

Also maybe check out other newer gyro's like this one:
http://www.sparkfun.com/commerce/product_info.php?products_id=700

cheers,
Jamie
 
D

DJ Delorie

Jan 1, 1970
0
so you can put a via under a pad.

Only if you plan for solder wicking into the via during reflow, which
would starve the solder joint. I've seen recommendations to use
tented vias (on the other side), or fill them with conductive polymer.
Perhaps applying paste and reflowing the bare board (to pre-fill the
vias with solder) beforehand would help?

Random thought: If you had a piece of glass the size of the BGA, you
could rest it over the paste when pre-filling the vias, to help get a
nice coplanar surface to put the BGA on.
(it is important to follow the temp curve recommended by the
manufacturer).

I wonder if it would be better to use a hotplate for BGAs, as the
solder would melt before the chip itself got cooked, since the heat
comes up through the board. Not that my hotplate is controlled in any
way (it heats at 0.8°C/sec, regardless of the knob setting), but it
did well with non-bga parts. Mostly, I'm curious about which part of
the assembly is most sensitive to the profile, and why; perhaps a
change in procedure could make it less sensitive and thus easier to do
for hobbiests?

More likely, I just got lucky and would crack the BGAs due to thermal
stress :p The chips I used so far were SOJ and TSOP styles, more flex
in the pins.
 
DJ said:
Only if you plan for solder wicking into the via during reflow, which
would starve the solder joint. I've seen recommendations to use
tented vias (on the other side), or fill them with conductive polymer.
Perhaps applying paste and reflowing the bare board (to pre-fill the
vias with solder) beforehand would help?

Good point, prefilling is probably the best course of action if that
via is desired under a pad,
I stand corrected.
Random thought: If you had a piece of glass the size of the BGA, you
could rest it over the paste when pre-filling the vias, to help get a
nice coplanar surface to put the BGA on.

This is also a great idea. I saw people using superglued `guides' made
out of smd resistors to hold the chip in place but a nice flat surface
would make it easier to position.
I wonder if it would be better to use a hotplate for BGAs, as the ....
More likely, I just got lucky and would crack the BGAs due to thermal
stress :p The chips I used so far were SOJ and TSOP styles, more flex
in the pins.

I would be more worried about the traces coming off on the side of the
PCB that is in direct contact with the hotplate. Also, when you heat
with a hotplate, the solder balls on the chip that have poor contact
with the PCB below will melt last and might make a poor connection.
Another possibility is that surface tension will pull the chip at one
end and lift the opposite end without creating a soldered connection.
Your `flattening off' idea might solve this problem though. Thanks for
good ideas, in any case!

Alex
 
D

DJ Delorie

Jan 1, 1970
0
I would be more worried about the traces coming off on the side of the
PCB that is in direct contact with the hotplate.

The three boards I've done so far didn't have any problems with that.
The board is only solder-hot for a few seconds, there wasn't even any
discoloration. Ovens get both sides of the board that hot, and they
don't melt the copper off.

I've put a thermometer right on the hotplate before, the plate itself
is not much hotter than the board, at least the solder melts at about
the temperature I'd expect. It's not like it's 50 degrees hotter on
that side.

The trick is to move the board around so it heats evenly, otherwise it
burns in one spot before it's hot enough to melt in some other spot.
Also, when you heat with a hotplate, the solder balls on the chip
that have poor contact with the PCB below will melt last and might
make a poor connection.

I'd probably want to put solder paste on the pads. They'll help wick
heat and ensure that they all make contact, plus the flux would help a
little too, both soldering and sticking the part down.
Another possibility is that surface tension will pull the chip at
one end and lift the opposite end without creating a soldered
connection.

From what I've read, that's a problem with the smaller parts, like
0204's, because the tension on the *side* of the part pulls it
upright. For something like a BGA or QFP, the weight of the chip
keeps it down well enough, and the middle pads soon melt and pull the
chip to the board.
 
B

Ben Jackson

Jan 1, 1970
0
If I want a via connected to a ball - can I put it on the pad
itself, or does it need to be away from the pads?

Does the gyro part require any pins to escape? The one I worked with
was in a BGA package but only the outer row of pins was unique. The
internal pins were duplicates.
 
M

Mike Noone

Jan 1, 1970
0
Ben said:
Does the gyro part require any pins to escape? The one I worked with
was in a BGA package but only the outer row of pins was unique. The
internal pins were duplicates.

I believe that is the case with the Analog gyros - but I'd also like to
be able to do other chips as well. I mean if I can get a reliable
process for soldering BGAs it would open up a whole world of chips that
I hadn't been able to use before (like alot of high end ARM parts).

-Mike
 
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