Reading time ( words)
The IPC-7711/21 Rework of Electronic Assemblies/Repair and Modification of Printed Boards and Electronic Assemblies is being “upreved” from version “B” to version “C” and will soon be released to the industry. There are a couple of notable changes that strengthened, modernized and brought together changes from the previous “B” version, which was approved in 2007.
Several of the changes from the previous version deal with the manner in which leadless devices are being reworked. Leadless devices have become the most widely placed in terms of units worldwide. In addition to the methods previously described when the initial version of the IPC7711/21 “B” came out, the new “C” version includes three new methods, summarized below:
- Method 18.104.22.168: Prebumping the leadless device using a polyimide stencil
- Method 22.214.171.124: Prebumping the device using polyimide stencils with the “bumped” component then fitting in to stay-in-place capturing stencil on the rework location on the PCB
- Method 126.96.36.199: Hand bumping center ground and hand soldering the IO connection method
Another area in which some newer rework techniques are memorialized in the new “C” version are the various pad repair techniques including:
- Method 4.7.4: Land repair land with integral via repair film method which leans on the previous repair film method for pad repair
- Method 4.7.5: SMT/BGA land repair with integral via repair circuit extension film adhesive method
Other areas of change for this “C” version include:
Method 188.8.131.52 focused IR for BGA rework and Method 5.7.12 focused IR placement of BGAs. These changes mirror the inroads that IR rework have made since the B standard as the focused heating is important for ever higher board part densities and the continuing proliferation of underfilled devices, very small devices and highly temperature sensitive devices finding their way onto PCBs.
In addition to the above procedure upgrades there are a variety of other changes due to operations running in a mixed alloy environment of both lead-free and tin-lead solders. One of the changes with the IPC 7711/21 has to do with the segregation of the hand soldering equipment limiting cross-contamination. These changes are due to there being significant rework differences between the processing of these two alloy types. The major differences in processing these alloys which impact rework operations are the fluxes being used, the cleaning materials being used and the longer soldering dwell times which may or may not require the addition of the nitrogen in to the rework/repair operations. The higher reflow temperatures of the lead-free solders requires that the device in proximity to the rework location needs to be protected from thermal damage.
There are also a couple of newer proposed methods up for consideration which did not make this version of the standard and will have to wait for Version “D” or beyond. These include:
- Flex circuit repair
- New micro blasting method for coating removal
- New method for solder mask replacement
Keep a watch out on the IPC website as this revised standard makes its way to being available or speak with your training services provider.
To read this entire article, which appeared in the December 2016 issue of SMT Magazine, click here.