The IC component removal and resolder operations generally do not subject
the packaged device to as benign (?) an environment as provided by an inline
reflow oven. It remains quite common for devices to be removed and
resoldered with very high heating rates and excessive temperatures, despite
the excellent rework systems available. This certainly can degrade the long
term reliability of the part, or even kill it on the spot.
If the assembled board sits in an ambient atmosphere for several days, care
must be taken to carefully bake out the part before undertaking the rework.
Massive PCB assemblies and bake out ovens with poor air flow do not make it
easy to get good bake out on the device to be reworked. Properly baking out
a flip chip device prior to removal may be even more difficult since it is
critical to remove moisture from the underfill which is masked on one side
by the impermeable IC, and on the other by a substrate that may be
impermeable ceramic, or may be a multilayer organic substrate with almost
solid pwr/gnd planes. Bake out is capable of damaging the unit if performed
too aggressively. One also has to consider that removed BGAs are often
re-balled, so the part is subjected to another reflow operation not
reflected in the descriptions below, so a single rework will subject this
BGA to three additional reflows.
IC components are qualified using JEDEC guidelines for Moisture Stress Level
determination, which stipulates the use of 3 reflow cycles before moisture
preconditioning and subsequent reliability testing. These three reflows can
reasonably approximate the thermal stress seen by ICs on Side One of a
double sided board (Side one reflow, Side two reflow, wave solder). In the
case of the reworked and replaced BGA, by the time this unit is replaced on
the PCB, it is likely to have seen the side one and two reflows, plus a wave
solder exposure, the rework removal step, the reball reflow, and the final
reattachment reflow. This is 6 reflows. This is already extreme. Without
careful verification of component integrity and functionality, more than one
rework on the same device seems to be a bad thing to do. Any rework and
replacement of the reworked device needs to be very carefully managed.
Leo
------------------------------------
Leo M. Higgins III, Ph.D.
Director of Applications Engineering / ASAT Inc.
[log in to unmask]
3755 Capital of Texas Hwy-So
Suite 100
Austin, TX USA 78726
tel: 512-383-4593
fax: 512-383-1590
mobile: 512-423-2002
------------------------------------
-----Original Message-----
From: Phil Bavaro [mailto:[log in to unmask]]
Sent: Monday, August 09, 2004 2:43 PM
To: [log in to unmask]
Subject: [TN] Complications re: Rework Cycles
We are discussing the maximum number of rework cycles that an assembly can
withstand before being considered a reliability risk.
Historically we have always used five rework cycles as our guideline which
permitted the part to be taken off, and put on twice during rework.
1. Reflow oven exposure
2. Removal cycle
3. Resolder cycle
4. Removal cycle
5. Resolder cycle.
The first question is where does this requirement actually come from and
the answer usually is give based on the standard coupon float testing.
But in today world, with one piece shields incorporated, such a simple
approach is usually not the case.
1. Reflow first side oven exposure
2. Reflow second side oven exposure
3. Remove shield cycle
4. Remove part cycle
5. Resolder part cycle
6. Resolder shield cycle
7. Remove shield cycle
8. Remove part cycle
9. Resolder part cycle
10.Resolder shield cycle
Comments anyone?
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