|
Lead Free BGA Rework Process | Lead Free BGA Repair
Reworking of Lead Free Area Array Devices
There are several issues faced when transitioning to a lead free BGA rework
process. The greatest issues include:
• Slight geometry changes in rework stencils when working with lead free solder
paste printing
• Tighter control of temperature deltas between the board and the part make for
a tighter process window
• Greater reflow temperatures increase the risk of damaging nearby parts,
sending adjacent parts into reflow or inadvertently melting or discoloring
components (See CoolCap Thermal Protectors)
• Slightly tighter control in the rework process temperature caused by this
narrowed process window
• Slight changes in the inspection criteria to confirm that the BGA has been
reflowed properly
MSD
The challenges of process control in lead-free BGA rework have been exacerbated
by the increased moisture sensitivity of the devices. As the BGA rework
processing temperatures have increased, so have the pressures on the package
seals. These increased pressures (approximately double that compared to tin lead
device processing temperatures) will increase the corresponding frequency and
magnitude of moisture-related damage. Most BGAs have a 2 level increase in their
MSD levels compared to their tin lead counterparts. Therefore, it is even more
critical to make sure that parts are packaged and handled and marked properly in
accordance with the JEDEC 033 guidelines. It is important part of the lead free
rework process to take care in that parts are properly baked out, stored in a
desiccators or a dry box when not being processed and reballed parts are
properly sealed in a bag along with a moisture indicator card and desiccant for
later re-use.
Stencil Printing
Due to the wetting characteristics of lead free solder vis-à-vis traditional
tin-lead based solders rework stencils need to be designed to more closely fit
the land patterns.
Figure H- Comparison of lead-free and tin-lead solders post reflow with
purposefully misprinted solder paste patterns
The reduced spread characteristics of the lead free solder paste require more
accuracy in alignment of the apertures for rework stencils, as the lead-free
solder paste, once reflowed, does not spread as readily as to tin-lead solder
counterparts.
In order to open up the lead-free BGA rework process window it is recommended that
a greater volume of solder paste is applied to either the device or the PCB land
patterns. This is especially important as increased rework temperatures have the
associated increased risks of board and part warpage. Greater solder paste
volumes compensate for these increased co planarity discrepancies assuring a
higher first pass rework yield, especially when compared to flux only
attachment. With solder paste printing used as the attachment method for lead
free BGAs you should expect to be in the high 90th percentile with respect to
first pass rework yields.
Greater solder paste volumes can be assured with the “stay in place” stencil
(See Figure I) printing technique first developed by BEST Inc. These stay in
place type of stencils are typically sized 60% greater volume then the typical
“print and release” type as the aspect ratios of these “stay in place” type do
not have to be maintained. (See a complete report on the reliability of these
type of stencils here)
Figure I- Stay in place stencil compensates for greater co planarity
discrepancies found in lead free BGA rework
Profiles and Part Deltas
The thermal profile for the most common lead free BGA solder ball alloy, SAC305
differs significantly from that of its tin-lead counterpart. Table “A” compares
the typical temperatures measured on an average board during the ramp-spike-cooldown
profiles for both types of alloys. Note that the temperatures for the “preheat”
part of the cycle are higher for the lead free alloy. This longer preheat
temperature, generated by the underside heating element of the BGA rework system
causes the overall cycle time to be extended. Also note that a “ramp” time
period has been added to the typical lead free profile as the part being
reworked needs to get up to reflow temperature. Again this adds time to reflow
profile. The addition of the “soak” cycle and the higher processing temperatures
cause the overall cycle time of the “typical” BGA rework profile to be extended
a few minutes (this assumes that the equipment is capable of reaching these
temperatures) for lead free alloys. This extended profile time was confirmed by
the NEMI task group as they recently assessed the rework process required for
lead free alloys. As seen in the
iNEMI rework study, the "thermal profile time length" of the test boards
averaged a few more minutes in length for the rework process compared to its
tin-lead counterpart.
Table A- Comparison of lead-free and tin lead reflow temperature profiles
Higher processing temperatures will lead to a few consequences of both the
device and board being reworked. The first of these is that the top of the
component body temperature is very close to the rated maximum temperature of the
device. Most “lead free” devices have been qualified to withstand between
250-260°C. In most cases the top component temperature will be very close to
this range, so insuring that this limit is not exceeded is key to not damaging
the component. Figure J shows pictorially how close these temperatures are to
one another. These elevated temperatures will have side effects for the parts on
the underside of and nearby to the device being reworked. These side effects
include: nearby parts being skewed (see Figure K), neighboring parts being
discolored or destroyed (see Figure L) or backside parts falling off.
Figure J- Tight temperature control during lead free rework is critical
Figure K- Skewed components are a result of nearby heating causing components to
go into reflow.
Figure L-Higher lead free BGA rework peak temperatures will destroy both boards
and components if the process is not closely monitored
Machine considerations
The increased demands of lead free rework require that BGA rework systems be
capable via their feature set to process lead free BGAs. The first consideration
of a “lead free capable” BGA rework system is to make sure that the machine is
capable of reaching the higher lead free processing temperatures. In most cases,
the machines possessing adequate preheater sections are capable of providing
enough thermal energy to get the solder joint temperatures to a reflow stage,
but for boards of a larger thermal mass this may not always be the case.
Adequate underside board heating elements need to be in place in order to get
the underside of the PCB to 150°C. Some more advanced machines allow for
multiple zone underside heating where the part area underneath the device to be
reworked is selectively heated to a temperature closer to the lead free liquidus
point. Not only should both the bottomside and topside heating elements be able
to reach the temperatures required for lead free processing, but these heating
elements need to be efficient enough in order to reach these temperatures in a
reasonable period of time so as to not unduly extend the rework process time.
Nozzle designs should also be checked as to their applicability for processing
lead free BGAs. Since the lead free BGA rework process window has shrunk in that
the temperature differential across is reduced the nozzle efficiency needs to be
considered in qualifying a machine for lead free BGA rework.
Lead Free BGA Rework
|
