pad cratering

hi,

the last post on pad cratering dates from 2012. We have (likely similar) problems with a RoHS 783 solder ball BGA size 29x29mm^2 (organic package BT), pitch is 1mm and solder ball diam is 0.5mm

the field failure is after 12 months, almost exclusive in asia region. The application is powered on/off 5 to 10 times per day and is mostly switched off, so we suspect some form of stress/strain during power cycling. The chip delta T is below 40degC above ambient, ambient is supposed to be about 25-30degC

spare parts experience the same problem as BGAs that ship (from europe to asia) in the application, so an issue with transport/storage is unlikely

we have prelim analysis of two returned boards that show pad cratering and the onset of pad cratering and much more (but indirect) evidence of pad cratering. A local reflow on the BGA removes the problem temporary for 6 months The solder balls are SAC305, the analysis judges the soldering very good, no indications of solder ball creep.

field data also indicates that the problems started with the transition to RoHS

there is a heat sink assembly with solder anchors and a standard spring with force < 2.3kf, but the board is relative stiff due to the presence of a memory socket, so currently we think that the strain during assembly and ICT is OK (but maybe we should check this)

the PCBA has double reflow (top-bottom) and selective soldering for the memory socket, the reflow soldering process is no-clean, max number of repair cycles is 3 in our specs

the IPC class of the laminates is IPC4101/99

what are our alternatives: -1- corner bonding or underfill? -2- other laminate IPC class? we could switch to 101 -3- outer prepreg without glass fiber? seems exotic and not all PCBA manufacturers can support it

Suggestions and advice are appreciated

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