Fillet Lifting

07 February 2006

Fillet lifting, a fairly new term in the industry, is specifically associated with lead-free soldering processes. Originally the phenomenon was highlighted by research engineers during the early introduction of lead-free alloys. Basically during the soldering process a plated through hole board will expand in the Z-axis and then contract during cooling. Lead contamination was thought to affect the rate of solidification of the joint at different interfaces. In practice the solder solidifies with the printed board still contracting, leaving the solder fillet tilting against the edge of the plated through hole. This does not happen on all through-hole joints but can be seen during optical or x-ray inspection.

The examples illustrate the classic fillet lifting which has often been reported on lead-free soldering trials when joints have been produced with alloys that may have become contaminated with lead. The joints were produced with tin/silver/copper and clearly show the solder fillet lifting from the pad. Often the pad can be lifted from the laminate as well, referred to as pad lifting. There was no lead contamination in the solder alloy or at the lead/pad interface in these examples which suggests that there is still research necessary to determine the cause of this lead-free technology process defect. This type of defect can be found on wave, selective and intrusive reflow of through-hole terminations. Normally it is only visible on the top-side for wave and selective soldering, it can be seen on both sides of the board for intrusive reflow. This is because the heating and cooling is different on these soldering processes.

It has been seen with all popular lead-free solder alloys like, tin/silver/copper, tin/copper, tin/copper + nickel. Experience shows that testing of joints like these does not lead to joint failure. IPC 610 also allows this as a process indicator, however the criteria included in the document is not totally correct and hopefully will be corrected in the next revision

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