Having done my share of direct, hands-on BGA and CSP rework and BGA
manufacturing (thousands), I can tell you this:
Within the IPC specs (4621B or 6012?) for the PWBs there is a
peel-strength requirement. IPC-TM-650 defines the peel strength testing.
However, nearly all circuit boards can meet this peel-strength
requirement, when it comes time to heat up the PWB and the solder goes
liquidus as part of the rework procedure, some laminate types exhibit
much better copper peel strength than others. This is because of the
porosity (or lack thereof) of the particular laminate material that the
resin between the copper and the laminate is attempting to adhere to.
And within a given laminate type, the peel strength under hot conditions
is much better for some prepreg suppliers than others. When the resin is
applied, if it is too thin or not cured correctly, or for whatever other
reason, small portions of the copper can peel much more easily under hot
conditions.
The smaller the pad diameter, the less resin, and the easier it is to
inadvertently remove pads that are not connected to vias or traces.
Several times I have been called in to look at a problem where the
client reported BGA pads floating up off of the board during the initial
reflow of the components. Typically this occurs only on smaller BGA pads
(.020" diameter or less).
This problem will often go away and return from lot to lot of the PWBs.
I once traced the same problem on two different company's totally
different assemblies to the same lot of prepreg purchased by different
fabricators of the two different PWBs. It just happened that the CEM I
worked for at the time was assembling and soldering the two different
products. When I checked with the PWB fabricators, they both came back
with the same lot number of prepreg from the same vendor, and the same
explanation.
In every case I have seen, the root cause has always been the prepreg
lamination process. Sometimes it can be compounded by rework
temperatures that are too hot. My experience has been that it manifests
itself within a reflow or rework process that had been humming along for
some time without any changes made or problems being seen, and then,
whammo.
It can often be detected at X-ray after reflow. Many of the solder balls
will have a straight side. This is the pad frozen into the solder ball.
The straight sides have a random pattern.
On the other hand, lifted traces do not usually occur from this problem,
only unused (unconnected) pads, in my experience.
Lifted traces are usually caused by attempting to lift the component
while the solder is in its solid stage. I have seen this happen with
Pb-free solders because of a delay of only 2-3 seconds after the hot air
shuts off and the component is lifted. The profile should keep the hot
nitrogen or air flowing until after the component is lifted.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ray, Carl (GE
Indust, GE Fanuc)
Sent: Tuesday, August 07, 2007 3:14 PM
To: [log in to unmask]
Subject: Re: [TN] Pads removed during BGA Repair
total adhesive separation of the pad and laminate substrate
________________________________
From: TechNet on behalf of Whittaker, Dewey (EHCOE)
Sent: Tue 8/7/2007 3:10 PM
To: [log in to unmask]
Subject: Re: [TN] Pads removed during BGA Repair
When you say pads are lifting, do you mean a partial or total adhesive
separation of the pad and laminate substrate or a cohesive failure of
the laminate itself under the pad?
Dewey
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ray, Carl (GE
Indust, GE Fanuc)
Sent: Tuesday, August 07, 2007 1:01 PM
To: [log in to unmask]
Subject: [TN] Pads removed during BGA Repair
All,
Got a question or two for the group. We have a situation where we are
experiencing pads and traces being lifted during BGA removal. We use an
Summit 1800 Machine and have verified calibration and thermal profiles
(even used an external device, ECD Mole to verify the profiles and
temps). The products range in sizes as well as the BGAs. The surface
finish is ENIG and the board venders are the same for all the products.
Some of the products are Pb Free and others are standard lead products.
What types of testing can be done to verify tensile strength and copper
adhesion? What other actions would you recommend?
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