by Lee Whiteman
Electronics manufacturers are taking the initiative to take Pb out of electronics manufacturing. Because of this change, it is inevitable that commercial or military hardware built with tin lead (SnPb) solders will have to undergo a rework or repair operation in order to be sustained. While there is plenty of data with respect to performing rework and repair operations with SnPb, there has been minimum documentation on rework and repair operations with Pb free solders. Rework and repair operations are critical to maintaining operation availability in the field, as part of a program's sustainment. The military must be aware of the challenges faced when rework ing or repairing its electronic hardware in a Pb free environment.

To following tips demonstrate how Pb free rework and repair has been successfully accomplished at the EMPF. The EMPF-009 board (Figure 1) was the test vehicle for this demonstration. Concentrating on the PBGA-169 package and the 80 pin QFP, the following tips are provided to account for using Pb free solders instead of SnPb solders.

Removing the PBGA-169 package was done using a standard rework and repair process. The rework station was programmed to a top side peak temperature of 240oC to 260oC, which allowed the component to be exposed to a peak of 210oC. Upon the component's removal, the board's pads were cleaned, and had the Pb free solder, Tin (Sn), Silver (Ag), Copper (Cu), applied. SnAgCu has a melting temperature of 221oC, as opposed to the SnPb 183 oC melting temperature. This required the rework station to be programmed to a top side peak temperature of 300oC. This high temperature was needed to support the SnAgCu higher melting temperature. The EMPF-009 board had a ground plane under portions of the PBGA, which increases the hardware's thermal mass, forcing the use of higher processing temperatures.

The resulting solder joints on the PBGA without the ground plane were acceptable. The Pb free solder joints had a dull grainy appearance, when compared
to their SnPb counterparts. However, the PBGA
package, soldered on the ground plane, produced unacceptable solder joints (Figure 2, Figure 3). Also, due to the higher temperatures used, the PBGA package warped.


TECH TIP #1: The top side and bottom side of the board must be raised to an adequate temperature to produce an acceptable solder joint without damaging the component or board. Too much of a thermal gradient between the top side and bottom side can warp the component and/or the board.

TECH TIP #2: Both the boards and components must be baked, prior to having the rework operations performed. This will reduce the probability of component and board delamination due to the higher temperatures associated with Pb free solders.

TECH TIP #3: The area to be soldered must have all solder removed from the pads, to avoid Pb contamination to the Pb free solder joint.

TECH TIP #4: Because the solder tip will be set at a higher temperature, the tip is more susceptible to oxidation and contamination. Therefore, the tip must be covered with fresh solder and cleaned more often than with SnPb solders.

ACI’s EMPF offers a course on Pb free soldering, that provides detailed instructions on performing rework and repair operations. For more information, contact the Helpline at 610-362-1320.