Hi Joe!
I still think best rule of thumb is symmetry. Copy what the BGA manufacturer did, if possible. But be warned, how you design the pad and how the pcb fab builds it tend to be very different things!
Wayne Thayer
________________________________
From: TechNet on behalf of Joe Macko
Sent: Thu 3/20/2008 7:00 PM
To: [log in to unmask]
Subject: Re: [TN] Solder Mask Defined Pad BGA Reliability
All,
It seems like many BGA manufacturers recommend "NSMD" pad sizes on the
PCB vs. SMD pads for obvious reason. However, some board designs use SMD
while others use NSMD pad designs for the same BGA component because of
VIPs, design constraints, etc.. So, I am wondering if there is some type
of diametrical adjustment factor that should be included when changing
from a NSMD pad to a SMD pad (e.g., should a SMD pad be slightly larger
in diameter than a NSMD to adjust for say the lack of solder wrap around
the circumference like you would see on a NSMD pad design). Just a
thought.
Also, can anyone direct me to a source of information on the use of SMD
designs to improve smaller pad adhesion that was mentioned below? Sounds
interesting.
Best Regards,
-joe
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Tuesday, March 18, 2008 8:32 AM
To: [log in to unmask]
Subject: Re: [TN] Solder Mask Defined Pad BGA Reliability
I thought the topic was stressors on the sj causing a crack, assuming
the pad is anchored to the PWB.
But Wayne, you are absolutely correct that pad adhesion becomes a big
factor as the pad diameter is reduced, and I can tell you that below
.022" diameter the pad separation becomes a significantly greater issue,
but I do not know why. I can only assume that as the pad diameter gets
smaller there is proportionatly less pad adhesion, especially during
reflow. So if you are dealing with pad diameters less than .022", then a
SMD pad may work better, because as Werner pointed out, when they get
that small the stressors are redundant.
I should note that there are particular circuit board substrate
materials that have proven to be a problem in the industry in that the
copper pads do not bond as well to them as others. So much so that I
have seen the pads float up off of the board into the BGA solder joints
during standard reflow. You should see this in an X-Ray; a whole bunch
of solder balls with a single straight side, no order to which side the
straight side is. It is quite a sight. I hope none of you ever see it on
your hardware.
And as far as mask registration, every board house has its bad days. I
have yet to see a pwb house provide SMD masking registration dead on,
day in and day out. It is an issue.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Thayer, Wayne
Sent: Tuesday, March 18, 2008 10:03 AM
To: [log in to unmask]
Subject: Re: [TN] Solder Mask Defined Pad BGA Reliability
Hi Richard!
I knew my comments would stir things up!
I was talking about the weak link being the pad adhesion to the
underlying laminate, not solder to the pad.
On the studies you did of mask defined vs. non-mask defined, I
understand that on your pcb, you varied this parameter. But what about
the pad on the BGA itself? As I stated previously, I have yet to see
anything other than mask defined pads. If that's the case, perhaps your
problems with mask defined pads have all been due to a poor masking
process which left contamination and tapered crack-initiating residue
around the edges of the pads.
I have huge problems, especially with 0.5mm pitch devices, with non-mask
defined pads because the board fabs typically over-etch the pads. Sure,
the mask on a mask defined pad can be poorly done too, but at least I
have something left to solder to. Unfortunately, I am forced into this
condition for extremely high density designs--I have to route between
the rows of the 0.5mm pitch BGA anyway, so the question becomes
moot--the pad and mask have to be just about perfect.
Wayne
________________________________
From: TechNet on behalf of Stadem, Richard D.
Sent: Tue 3/18/2008 10:29 AM
To: [log in to unmask]
Subject: Re: [TN] Solder Mask Defined Pad BGA Reliability
I hate to disagree with you, Wayne.
Soldermask-defined pads typically have the soldermask up on the pads.
Therefore they reduce the amount of pad area for the solder to form a
good IMF because the design seldom allows for a larger pad surface area
to compensate for the maskant reduction. They also preclude solder
wetting down around the sides of the pads. Having solder totally cover
the pads including the sides provides a much stronger (mechanically)
solder joint. I do know that on boards with soldermask on top of the
pads, there is often a very thin layer of the maskant over the whole
pad. I have yet to see that happen with non-soldermask-defined pads.
The stresses during cycling are concentrated around the area where the
soldermask comes in contact with the solder joint. I have seen the same
boards with and without soldermask defined pads temperature cycled and
the soldermask-defined boards had daisy-chain failures as much as 50%
fewer cycles than non-soldermask defined pads. Same was true of BGAs
with soldermask-defined pads versus non.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Igoshev, Vladimir
Sent: Tuesday, March 18, 2008 8:18 AM
To: [log in to unmask]
Subject: Re: [TN] Solder Mask Defined Pad BGA Reliability
There is a very short answer: Fracture Mechanics :-)
When we have an SMD pad, there is an unavoidable stress concentration in
the vicinity of the triple junction "solder/solder pad/solder mask".
Non-SMD pads are free from that kind of stress concentration, as they
are "wrapped up" with solder.
Regards,
Vladimir
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Kirk, Graham (GE
EntSol, Intelligent Platforms)
Sent: Tuesday, March 18, 2008 8:06 AM
To: [log in to unmask]
Subject: Re: [TN] Solder Mask Defined Pad BGA Reliability
Hi Joyce,
I realize that CTE mismatch is responsible for the fatigue, I am trying
to understand why the solder mask has so much effect on the solder
joint.
Graham Kirk
Senior Technologist
Radstone Embedded Computing
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Joyce Koo
Sent: 18 March 2008 11:07
To: [log in to unmask]
Subject: Re: [TN] Solder Mask Defined Pad BGA Reliability
Fatique, due to CTE mismatch or poor design. My 1st guess.
Jk
--------------------------
Sent using BlackBerry
----- Original Message -----
From: TechNet <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Sent: Tue Mar 18 06:18:08 2008
Subject: [TN] Solder Mask Defined Pad BGA Reliability
Hi All,
Can anybody tell me what the actual mechanism is that propagates cracks
in the device to ball joint on plastic BGA's (Leaded balls) with SMD
pads under thermal cycling?
Graham Kirk
Senior Technologist
GE Fanuc Intelligent Platforms Ltd
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