• Both of these devices used the same stencil and reflow profile but different solder pastes
• Solder paste behavior and reduction is to some extent dependant on solder paste rheology and wetting

NOTE: if you are using a top side heatsink, you may have problems mounting heatsinks to tilted devices.

If the machine is at fault then the devices are likely to tilt in the same direction relative to the component orientation. Other devices are also likely to be affected.

If it is board related then devices are likely to vary across the board. Look to see if:

• Boards are bowed prior to an assembly
• Boards are supported in pick & place machine (additional supports under the centre of the panel may be required if board is very flexible)
• Board is not deflection due to excessive over-travel

• Slight twist may occur on some devices
• If the pad is to IR’s design and the device sits within the pad perimeter then no problems should occur

• The effect is rare
• Tombstoning is caused by differential surface tensions across the device, uneven heating rates are a likely cause

“…shorted devices on server boards assembled for end customer.”

There were two boards designs: A1 and A2

Only one board (A1) was exhibiting this problem

A1 had been designed by one location, a second location designed A2

Images from A1

MT

MQ

• While the report contained a reasonable amount of background information, there was not enough to draw full conclusions
• The two images of the board suggested board and stencil design as possible causes of the problems

• An x-ray image of the high side FET showed that the solder pads were too big on the PCB
• The solder can be clearly seen at the edge of the die, which will result in shorting.

• White areas are where customer's and IR’s recommended designs match
• Light green areas are where customer’s design was larger than IR’s
• Pink areas are where IR’s design was larger than customer’s

• The A1 board and stencils were redesigned to match our recommended footprints and the issues of shorting disappeared