Bob,

  This is copy / paste from draft version of IPC 7095 ( issued may 1999 )
...

7.3 Assembly accept/reject criteria
7.3.1 Voids in solder joint
a. Sources of Voids
There can be voids in solder balls, or at the solder joints to the BGA, or
at the solder joints to
the PCB. Various sources or reasons can be responsible for these voids.
Voids can be carried over from original voids in solder balls. These voids
could be the result
of ball manufacture process or materials used. Voids can be induced into the
reflowed solder
joint by either the voids in the original component solder ball, or during
the reflow attachment
process. These voids could be the result of ball manufacture process or
attach process
parameters, or board design (e.g., via in pad), or materials used.
Voids can also form near the PCB-Ball interface during BGA attach to PCB.
These voids are
typically induced during the assembly process, which is typically flux that
has been vaporized
during the reflow process, and entrapped during the solidification of the
molten solder. The
source of vaporized flux can be either from flux itself (typically rework),
or flux which is one
of the constituents of the solder paste used in the reflow assembly process.
Voids can also be formed via expanded air from plugged vias (via in pad
consideration) in
the PCB. Expanding air from plugged via under pad may also create a void in
molten ball.
Typically most voids are detected in the middle to top (ball / BGA
interface) of the reflowed
solder joint. This is expected because the entrapped air bubble and the
vaporized flux,
which is applied to the PCB BGA pads, rises during the reflow profile. This
occurs when the
applied solder paste and the BGA's collapsible eutectic solder ball(s) melt
together during the
reflow profile (typically 235 to 240 *C peak temperature). If the reflow
profile cycle doesn't
allow sufficient time for either the entrapped air or vaporized flux to
escape, a void is formed
as the molten solder solidifies in the cool down area of the reflow profile.
Therefore, the
development of the reflow profile is extremely important as a contributor to
the formation of
voids. BGA components having non-collapsible balls (high temperature solder
(90% Pb 10%
Sn, with a melting point of 302 *C) typically will have little or none
induced voids because the
ball solder metallurgy never melts during the reflow profile. Voids in
solder joints are not new
because of the use of collapsible BGAs. Voids can be detected under leaded
components
when using x-ray equipment. However leaded component solder joints are
usually visually
inspected, not x-rayed, and therefore never detected.
b. Impact of Voids
The questions are how many and how large are these voids allowable in the
product that will
impact the product's required reliability. Another question is where in the
"Z-axis" in
conjunction with the void size is also allowable without impacting the
required reliability.
Voids at either interface, solder ball to BGA or BGA to PCB, can have
quality and reliability
implications depending on their size, number, and location. Gases entrapped
in the voids
may give rise to stress while expanding and contracting during heat
excursions. They can
serve as stress initiation (and in some cases, stress absorption points).
They can start (and
in some cases, terminate) a stress crack. Their elimination or, at least,
substantial reduction
is preferred.
80
Large voids are more detrimental but small pre-existing voids can merge
during reflow to
create large voids.
Voids reduce the mechanical strength of the interface by reducing the
interface area. The
impact of their presence is a function of the properties (such as CTEs) of
the materials
surrounding the solder and their dimensional location, shapes and
relationships.
c. Inspection of Assemblies for Voids
Inspection of BGA joints using an x-ray can detect these voids. A sample
in-coming x-ray
inspection should bring the issue to light.
X-ray analysis is therefore required for the inspection and the detection of
voids in BGA
solder joints. Unlike a leaded component, BGAs have solder joints that are
not only on the
component's periphery, but have internal solder joints that are not
inspectable by normal
visual techniques. X-ray equipment is required for inspection or detection
of voids in BGA's
solder joints. This is probably one of the two new pieces of equipment (the
other could be hot
gas rework equipment) that's required for the qualification of a SMT BGA
assembly line. X-ray
equipment can range from $30K to $500K. The lower cost equipment is
transmission x-ray,
whereas the higher cost equipment is x-ray laminography. The difference
between the
two pieces of analytical equipment is that the transmission x-ray can only
detect a void, but
can not determine where the void exist in the "Z-axis" (bottom, middle, or
top of the solder
joint). The x-ray laminography via programming can take slices of the solder
joint in the "Z-axis",
and determine the location of the void(s). Typically because of the high
cost of x-ray
laminography equipment, only larger or high volume assembly shops can afford
this
equipment. One lower cost scenario for achieving either sampling or 100%
x-ray inspection
of BGAs would be to have a transmission x-ray in each SMT line. If a
suspicious void is
detected, that the suspicious assembly is taken off line for finer analysis
and defect
determination on off line x-ray laminography equipment.
d. Elimination of Voids
The assemblers can work with their suppliers to eliminate voids in the
incoming BGA solder
balls.
The manufacturers can adjust their process and/or materials to eliminate
these voids. The
user can work with the supplier to eliminate voids in incoming BGAs.
Recently and typically,
little or no voids are detected in the incoming BGA solder joints.
Reflow time-temperature profile, flux amount, type and properties should be
investigated for
improvement. Such voids can again be eliminated through material and/or
process
adjustment and optimization.
e. Accept/reject criteria for voids in solder balls:
With constantly decreasing BGA pad sizes, solder ball sizes and pad pitches,
the product
dimensional parameters will be continually changing on the production floor.
Materials and
processes will be changing to attain pricing, throughput, quality and
reliability goals. There
will be a continuous need for process development for changing
configurations.
81
Voids and other defects will be encountered in various stages of product's
developmental
and manufacturing ramp up life. It will be necessary to maintain a minimum
acceptable
standard so that the product is manufacturable, meets customer expectations,
has a useful
working life and meets product reliability requirements.
The manufacturers need to be encouraged to use process control for
continuous product
improvement. Voids are an anomaly. Manufacturer should work towards their
elimination
using available statistical process control and process improvement tools.
A general presence of voids should indicate a need to control and/or to
improve process and
materials. An accept/reject limit can be used to help the manufacturer
realize the need to
eliminate voids. As an example, a reject limit of no more than 5% solder
balls with voids set
by a customer can alert the manufacturer that presence of voids is not
acceptable. A
profusion of voids all over the part or a general presence of voids should
not be considered
an acceptable norm.
Voids, when present, will normally have a distribution of sizes. The size of
the largest
probable void can be estimated from measurements performed on a sample. A
largest
probable size detrimental to the life of the contact should not be
acceptable. The
unacceptable void size may vary from one BGA design to another.
A void larger than 25% of solder ball contact area diameter (~6% of contact
area) can be
rendered unacceptable by a customer, again to encourage elimination of
voids. (see Figure
36) Any such criterion will be used in conjunction with the reject criteria
for the proportion of
voided acceptable solder balls.
When there are more than one voids per solder ball, then the dimensions of
the voids will be
added to calculate the total voiding in that solder ball.
Solder outline
Void outline
d
0.25 d
Figure 36 Example of voided area at land and board interface
Defective determination is made by the product's reliability requirements.
Typically the
maximum allowable size void is 30% of the solder ball diameter (equivalent
to 11% of area).
This can be either one void, or the summation of many voids. Unfortunately
the currently
algorithms for x-ray laminography doesn't perform the summation of the
voids. X-ray
laminography via programming can determine a defect greater than the
pre-determined size.
Therefore if the diameter of a reflowed solder joint is .030", the maximum
size of the void in
the middle of the joint would be .009". If the .009" void was detected at
either the top (BGA
interface) or bottom (PCB interface) that joint would be rejected because
the diameter of that
joint at that slice would be less than .030", and the void would be greater
than 30% of the
diameter.
The theory is believed that the voids at either interface can be more of a
reliability problem.
Greater stress is achieved at the interfaces as the distance from the
neutral point grows, and
the component substrate and PCB are of different materials. This causes
greater stress
because of the possible mismatch of CTEs. However stress testing / thermal
cycling with
voids at 30% have not failed medical class III criteria. Smaller voids are
believed by some to
be more favorable than no voids at all. The reason being as a crack is
propagated, upon
reaching a small void, the void stops the crack propagation. Voids have been
either reduced
or eliminated with the use of a nitrogen blanket during the reflow process.
Accept/reject criteria for the number and size of voids, on the one hand
discourages a
general presence of voids which indicates an out of control process and on
the other hand
encourages the manufacturer to use necessary tools for process and material
improvement.

> -----Original Message-----
> From: Bob Perkins [SMTP:[log in to unmask]]
> Sent: 20 de Março de 2000 18:01
> To:   [log in to unmask]
> Subject:      [TN] Voids in BGA Balls after reflow
>
> Hello Technetters
>
>         Recently with the help of a high end piece of x-ray equipment we
> have noticed voids in the spheres of the bga after reflow, "see
> attachment-file".  Has anyone seen this before, or has any idea how the
> voids are created?  Also does anyone know if this is a bad or good thing?
> Does anyone have any recommendations to the profile?
>
>
> Thanks
>         Bob Perkins
>         Automation Technician/Manufacturing Engineer
>         Aimtronics
>         [log in to unmask]
>
>
>  <<wboard6.jpg>> << File: wboard6.jpg >>

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