Some soldermask issues with BGA include ...
1. Mask defined pads are more difficult to cover in one pass during
horizontal HASL; a function of surface tension of solder and aperture
size.
2. Mask defined pads can lead to long term reliability problems. After
reflow the BGA ball conforms to the mask around the pad. A paper by Mike
Petrucci (et al) of Compaq showed cracks can form between the BGA ball and
PCB pad on mask defined pads. However, their data showed no significant
difference in reliability when compared to non-soldermask defined pads
(etch defined). Another paper by Dr. Abbas I. Attarwala (et al) of TI
showed mask defined pads on the device can lead to failures between the
ball and pad. Both situations are caused by the mismatch in Cte between
the mask and solder.
3. Etch defined pads with a mask clearance of approximately .003" allow
for more solder paste to be printed and possibly a more reliable joint.
4. The clearance between the mask and pad on etch defined pads provides a
place for no-clean paste residues to collect after reflow instead of on
the PCB surface.
5. Maximize the amount of soldermask between the BGA pad and the
connecting via. At BGA rework the mask separating these features can
become damaged. If completely removed, reassembly will be difficult since
any new paste will run down the via instead of staying on the BGA pad.
Regards,
D. Mitchell
______________________________ Reply Separator _________________________________
Subject: Re[2]: BGA
Author: [log in to unmask] at corp
Date: 1/11/96 1:32 PM
Doug,
Dennis is absolutely right! I've read that anything with
I/O >200 is a prime candidate for BGA. Also, the BGA pkg is durable.
If accidentally dropped, it can easily be assembled without any
potential problem due to the way it's made. (As opposed to a fine
pitch/TAB device)
I too have seen multiple finish requirements on the same
outer surfaces. (As many as 3) Not to mention other temporary
protective coatings on top of it. Also the Soldermask clearance
on the BGA sites has raised some questions. Some designers are
overlapping the pads and others are including clearance and others
are 1:1. For the fabricator, it raises a lot of questions. The
DFM guidelines for these should be discussed up front to be
clearly defined as to the end users requirements. The DFM guidelines
for these sometimes goes against what has been used in the past.
I know I'm learning, and I understand that the clearance issue
is still being evaluated. (From an industry stand point/long
term reliability)
Groovy
______________________________ Reply Separator _________________________________
Subject: Re: BGA
Author: [log in to unmask] at SMTPLINK-HADCO
Date: 1/11/96 10:07 AM
Doug,
First and foremost, get directly involved with your customer at the PCB design
stage. The type and quantity of BGA devices will affect layer count, internal
and external routing, test-ability, and soldermask apertures.
I have seen all types of surface finishes specified; HASL, OSP, hard and soft
Ni/Au, and solid solder deposits. I know of one fabricator that supplies BGA
product with solder bumps on the all SMD sites but the BGA with the BGA and
PTH sites OSP. The assembler does not print any paste, just sticky flux for
the SMD's. They're using BGA's with 63/37 balls which supply the solder to
the joints.
Probably the most significant impact on the PCB fabricator is at electrical
test. Depending on your technology level of fixturing software, fixturing,
and test equipment, you can easily get into multiple test fixtures that take
many hours to solve. Getting involved early in the PCB design can prevent
having 4-625 pin CBGA's in a 3"x3" area.
Regards,
Dennis Mitchell
Zycon Corp.
[log in to unmask]
______________________________ Reply Separator _________________________________
Subject: BGA
Author: [log in to unmask] at corp
Date: 1/10/96 5:32 PM
-- [ From: Doug Jeffery * EMC.Ver #2.10P ] --
Question?
What does BGA technology mean to board fabricators? Do we have to
become solder paste experts and reflow solder again.. I thought we got
rid of that problem.....:)....
No Seriously, what are the responsibilities at fabrication for
creating boards to accept BGA's?
Thanks?
Doug Jeffery
Electrotek..
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