Phil,
What you are seeing actually makes sense. Consider the sum of forces acting
on the bubble. If the void forms when solder surrounds it, only the
pressure difference between the top and bottom of the bubble will cause it
to move. This will push it to the top of the joint, but it may stop half
way if the solder returns to solidus before the bubble gets to the top.
However, when the bubble forms on a surface, the pressure difference will
depend both on its geometry, or size. Geometry being how far developed it
is from a half sphere to a full sphere. You have probably noticed that the
bubbles in you soda drink tend to cling to the surfaces of the container.
The same holds true for a solder joint void. The second force for surface
voids is the interface tension between the solid and vapor. Traditional
wetting formulas assume constant and negligible vapor pressure. That no
longer holds true inside a liquid. The increase in pressure tends to hinder
wetting.
The Casper the Ghost phenomenon is curious; I have never seen it. However
it kind of makes sense. Pictures would be nice! I'm guessing, so let me
know if I'm right or wrong. In a V shaped hole, the void will likely be a
semicircle shape at the solder interface and may increase significantly in
volume before the solder undercuts the arc. In an A shaped hole, as the
void gets larger, it would quickly make a second bubble like blowing bubbles
with bubble gum. If your mouth is the bubble in the via, the bubble gum
would form the second bubble provided your lips form a diameter smaller than
the diameter of the inside of your mouth.
BTW, I have since cross-sectioned several microvias since I wrote the e-mail
below. 100% have had a void that consumed the entire via. I'm suspecting
that voids in microvias are left over from air entrapment during screen
printing.
Kind Regards
Ryan Grant
Advanced Technology Engineer
MCMS
(208) 898-1145
[log in to unmask]
> -----Original Message-----
> From: Phil Bavaro [SMTP:[log in to unmask]]
> Sent: Tuesday, May 29, 2001 1:58 PM
> To: [log in to unmask]
> Subject: [TN] Pad in Via for BGA's - Part II
>
> We have recently begun to see that the voids beneath our larger pitch
> (1.27 mm) BGAs which are on pads containing the larger size microvias
> consistently are exhibiting voiding which gets partially trapped in
> the microvia but stretches up into the ball as well. It looks kind
> of like Casper the friendly ghost when microsectioned.
>
> The typical location of these voids historically has always been near
> the top of the bga ball and never at the pad or sub-pad level so this
> is a new development for us. Same oven profile temps and time, just
> different microvias creating different final resting spots for voids
> which result.
>
> It is hard to believe that the shape of the microvia construction
> could cause the voids to not rise normally but that is my gut feeling
> after looking at the microsections. These microvia are smaller at
> the pad level than the bottom of the microvia. This ususally happens
> with .004" thick dielectric material and becomes exaggerated when
> .006" material is used.
>
> Once in a while, the entire void makes it into the ball area and
> enlarges the total ball diameter, but this is unusual.
>
> Anyone else exhibiting this type of phenomena?
>
> BTW, the neighboring solderball connections are void free, which
> eliminates the paste suspicion as to root cause of the voids.
>
>
> At 12:54 PM -0700 1/27/00, Ryan Grant wrote:
> >Jim,
> > I specifically looked for voids in microBGA's that had microvias
> in
> >the pad. I found voids in every pad with micro vias. However the voids
> >were very small. I'm not entirely convinced the voids I saw were not an
> >illusion from looking into a via; although I did view the voids and via
> at
> >an angle. (I guess if I really cared, I could cross section a joint and
> >know for sure).
> >
> > Voids in BGA balls can be caused by many factors. I think
> microvias
> >will sometimes get a bum rap because large voids are assumed to come from
> >the microvia; when in fact the large void may be caused by the paste,
> reflow
> >profile or whatever else. While it may or may not be true that micro
> vias
> >cause voids, they are definitely here to stay.
> >
> >> -----Original Message-----
> >> From: Jim Kittel [SMTP:[log in to unmask]]
> >> Sent: Wednesday, January 19, 2000 1:11 PM
> >> To: [log in to unmask]
> >> Subject: [TN] Pad in Via for Micro BGA's
> >>
> >> Tech Netters
> >>
> >> I have a question for you on micro BGA's and I don't want this to be
> >> confused with the recent discussion on via-in-pad which may still be
> in
> >> process.
> >>
> >> The working draft copy I have of "Design and Assembly Process
> >> Implementation
> >> for BGAs" states on page 63 that a 0.1 mm micro via may be placed in
> the
> >> BGA
> >> solder land to allow more space for routing of traces. They say the
> only
> >> noticeable affect is a small dimple. I seem to remember articles in
> >> literature about outgassing and large voids in the BGA solder ball
> >> centered
> >> over the embedded via. Could anyone comment on their experience
> placing
> >> microvias in BGA solder lands and if they agree with this comment in
> the
> >> design guide?
> >>
> >> Jim Kittel
> >>
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>
> Phillip A. Bavaro
> QUALCO/\/\/\/\ Incorporated
> Manufacturing Engineer, Staff
> [log in to unmask]
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>
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