Dennis,
The relation of bump volume to solder paste volume will be roughly
half. This is because solder paste is comprised of tiny solder spheres (the
powder). The spheres stack together much like square blocks and the gaps
between spheres are filled with flux. Mathematically, you can calculate the
volume occupied by the sphere, compared to the volume occupied by the
imaginary box it is in. The relation is roughly 50%, give or take a couple
of percent by variations in stacking.
This means you can then calculate how big you aperture opening
should be, given the thickness of the stencil, and the ultimate size
(volume) of solder bump desired.
Keep in mind, you will never get all the tiny solder powders out of
the stencil opening, so the ultimate volume of solder deposited through the
screen print process will be less than the theoretical calculation. How
much you will deposit is heavily dependent on how the screen printer is set
up, and somewhat dependent on the specific manufacture of the solder paste.
You can minimize the effect of solder powder sticking inside the
aperture by using a laser cut, electropolished nickel plated stencil, or one
that is electroformed. However, solder powder will still stick to the side
walls of the stencil. You will have to actually measure exactly how much
solder paste is transferred from the stencil to your substrate to correct
for the amount of solder ultimately desired. For a rough starting point, I
would assume 70% of the solder powder actually transfers. My own studies
have found that it can vary from 50% to 90% (never 100%).
This ultimately affects the repeatability of the size of your solder
bumps. To minimize the affect of some apertures retaining more solder
powder than others, make the aperture opening as wide as reasonable, and
adjust the stencil thickness to thinner values to compensate. Keep in mind,
thinner stencils are more delicate, and will wear out faster. (A small
price to pay for a big benefit). The smaller the side wall surface area of
each aperture opening in relation to the volume of paste in that aperture,
the less solder paste sticking to the walls of the aperture will affect the
total volume of solder paste transferred.
There will be a normal distribution of solder volume printed for
each aperture size. So when you measure the solder paste deposits, look to
optimize the screen print process to make that distribution as narrow as
possible.
As far as size of the solder bump is concerned, bigger is better. A
bigger bump will give a larger standoff, and allow for more compliance in
the joint.
Thanks
Ryan Grant
Advanced Technology Engineer
MCMS
(208) 898-1145
[log in to unmask]
> -----Original Message-----
> From: Dennis Fall [SMTP:[log in to unmask]]
> Sent: Wednesday, May 24, 2000 2:54 PM
> To: [log in to unmask]
> Subject: [TN] solder bumping
>
> I am looking for any information I can get about solder bumping with
> solder
> paste. For example:
>
> How large should the stencil opening be compared to the pad the bump is
> going to be placed onto?
> Are there any general "rules of thumb" regarding the process?
> What can I expect the bump volume to be compared to the volume of the
> stencil opening?
> How repeatable can I expect my deposit to be?
> Is there a standard size for bumps (height, diameter, etc.)?
> Are there any major concerns with the process?
>
> I have gone through the information supplied to me by my paste vendor, but
> I am still a bit uncomfortable. We are looking to use solder bumps as
> termination for one of our new products. It will have four bumps on the
> bottom. The chip is 2.5x3.0 mm in size. Is four solder bumps going to
> provide enough rigidity?
>
>
> Any info would help.
>
> Thank You,
> Dennis Fall
> Thin Film Technology Corp.
> N. Mankato, MN
>
> P: 507-625-8445 x17
>
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