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Lead Free BGA Rework Process | Lead Free BGA Repair
Segregation of materials
There are several ways to effectively control the potential for
cross-contaminating lead-bearing and lead free soldering materials in the BGA
rework area. One approach includes the complete physical separation of both
space and equipment between the lead-free and tin-lead reworks areas. All
processing materials including solder paste, fluxes and wire braid as well as
equipment such as soldering irons and convective reflow equipment are completed
dedicated to either the lead-free or tin-lead rework areas. This approach, while
adequate for large assemblers, is costly and consumes floor space. Another
approach is to strictly control all materials on a per job basis. Using this
approach, each job has materials which are “checked out” from a controlled
material control area. If a removal and replacement of a BGA is required then
the appropriate paste flux, solder paste and solder wick are released from the
material control area for that specific job. In terms of hand soldering
equipment, operators are trained to “rinse” their soldering iron tips (see
http://www.solder.net/technical/rinsing.asp) prior to working on any assembly in
order to prevent cross-contamination. BEST utilizes a visual indicator system
(Figure G) which includes rework technician into the type of solder alloy
required for the work. “Lead is Red and Green is Clean” has been communicated to
each of the BEST operators for several years. Finally, an ongoing continuous
training and retraining in procedures and internal processes will make sure that
the importance of material segregation in the transition period is well
understood by personnel.
Figure G- Color-coded visual indicator cards can be used as a reminder to
technicians as to which materials are to be used with which alloys
Mixed Alloy BGAs
An understanding of the potential consequences in mixed alloy assemblies needs
to be considered. Using tin-lead solder paste when assembling lead free alloy
BGA components is possible, but there is the potential for poor alignment and
open solder joints. Since the usual SnPb reflow profile will not exceed the
melting point of the BGAs’ lead-free solder balls, the SnPb solder paste melts
but the lead free solder balls do not reach a liquidus temperature. The lack of
ball collapse may cause a lack of contact between the solder paste and the
solder ball. In these instances, one needs to use a reflow profile which ensures
the lead free solder balls collapse, and the lead from the molten SnPb solder
paste mixes thoroughly. In addition care needs to be taken to make sure at this
elevated temperature that both the PCB and the parts can withstand these higher
processing temperatures. It always safest to use similar alloys when reworking
BGAs. However, after making sure the part and board temperatures can withstand
the reflow temperatures of the process, mixed alloy systems can work for
prototypes and engineering evaluation samples. However if considering a mixed
alloy soldering attachment method in production or critical use applications
should not be done unless the associated reliability testing for use is
completed.
Lead Free BGA Rework
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