[log in to unmask] on 01/29/98 04:57:08 AM
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cc: (bcc: Brad Kendall/Hella North America Inc.)
Subject: Re: [TN] Use of Solder Mask Defined pads for BGA ?
Hi,
My problem with NSMD pads is the limitation on the pad size for 50 thou
pitch components. I have decided on dimensions for pad / via combination as
follows. 30 thou pad with solder resist window of 24 thou and via pad of 28
thou with 10 thou via and a trace of 28 thou joining. 30 thou pads are as
large as I can allow for 50 thou pitch and the 3 thou tolerance on solder
resist gives a 24 thou window.
24 thou might be fine if 1) eutectic solder spheres (collapsing) are used,
2) the spheres are around 23 to 30 thou diameter. The JEDEC standard for
BGA (collapsable)spheres is 23 to 36 thou. This is an huge tolerance
(anybody have any insight as to why JEDEC settled for such a wide range?),
and it's hard to imagine 24 thou pads with the 36 thou spheres plus solder
paste. (Although after collapsing it is going to change shape).
Has anybody experience of these dimensions in practice?
The other thing is, how narrow can you risk with the solder resist border
on top of the pad? Can you use a 2 thou border giving a 26 thou window? Or
even 1.5 thou border?
Obviously with non-collapsing spheres you have to put down alot more solder
paste. 30 to 32 thou aperture size is what I would recommend. If you have
24 thou windows obviously you lead into the reason for your posting
initially.
I would say that printing solder paste onto resist is undesirable. It is
dependant on the flux activity as to whether it wicks back onto the
joint/pad without leaving balls. It is also dependant on the process
(profile) as to whether it leaves solder balls behind. Especially in the
case of no-clean pastes.
It may be that for some people no problems occur printing onto the resist,
but for others who have a no-clean with a lower flux activity and a profile
which is on the edge of it's process window, it may not be so good.
Can anybody else share their NSMD BGA dimensions with us?
Regards
We use 26 thou pads with a 5 mil annular ring. I do not recall the via
size but they are tented.
Brad Kendall
At 11:30 28/01/98 -0500, you wrote:
> Reply to: RE>[TN] Use of Solder Mask Defined pads for BGA ?
>
>Hi Johannes,
>There is some debate about Solder Mask Defined (SMD) pads for BGAs. Some
>papers indicate that Non Solder Mask Defined (NSMD) pads increase the
fatigue >life of the solder joint (SMD pads cause stress concentration
point in solder >joint). However, it is interesting to note that many of
the BGA component >suppliers actually use SMD pads on the BGA substrate.
So, although you can >choose SMD vs NSMD for your board, the supplier
dictates the substrate design >(typically SMD).
>
>To address your overprinting question (printing paste on solder mask), we
have >been using overprinting for Ceramic BGAs successfully for the past
couple of >years without problems. The paste will pull back to the solder
joint. You >may want to verify on your particular assembly (X-sections,
etc.).
>
>Hope this helps.
>Charles Elliott
>Newbridge Networks
>
>--------------------------------------
>
>The pads of PBGA`s that we use are defined by solder mask on our
>multilayer FR4 pcb`s
>The diameter of stensil aperture is bigger than the pad diameter , so we
>are actually printing
>( no-clean ) paste on the soldermask.
>Is this OK, or should it be avoided ? I presume that the adhesion between
>stensil
>aperture-paste is bigger than the other one between soldermask and paste,
>resulting
>to increased risk to solder balls, which could be fatal under PBGA ( pitch
>.05" )
>
>Stencil width is 0.15 mm ( 6 Mils ), laser cut
>Stensil diameter .56 mm ( 0.22" )
>Pad diameter 0.72 mm ( 0.28" ) max
>Solder Mask diameter 0.52 mm ( 0.20" )
>
>Any opinions?
>
>Brs, Johannes
>
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