Rework and Repair
Rework and repair are required to support first piece production. In addition, rework and repair is required to support equipment in the field during its life cycle. Lead Free soldering complicates the rework and repair process. In a “pure” production environment, Tin Lead (SnPb) hardware would be used only on Tin Lead (SnPb) finished hardware. Lead Free solders would be used on Lead Free finished hardware.
Unfortunately, as illustrated in Table 2, there will be a period of transition, where Tin Lead (SnPb) solders will be used to solder Lead Free finished components. It is conceivable that Lead Free solders will be used to solder Tin Lead (SnPb) finished components.
|
Tin Lead (SnPb) Processes |
Lead Free Processes |
Production |
Tin Lead (SnPb) Solder
Tin Lead (SnPb) Finished Boards
Tin Lead (SnPb) Finished Components |
Lead Free Solder
Lead Free Finished Boards
Lead Free Finished Components |
Transition
and
Sustainment |
Tin Lead (SnPb) Solder
Tin Lead (SnPb) Finished Boards
Lead Free Finished Components |
Lead Free Solder
Tin Lead (SnPb) Finished Boards
Lead Free Finished Components |
Tin Lead (SnPb) Solder
Lead Free Finished Boards
Tin Lead (SnPb) Finished Components |
Lead Free Solder
Lead Free Finished Boards
Tin Lead (SnPb) Finished Components |
Tin Lead (SnPb) Solder
Lead Free Finished Boards
Lead Free Finished Components |
Lead Free Solder
Tin Lead (SnPb) Finished Boards
Tin Lead (SnPb) Finished Components |
Table 2. Production vs. Transition and Sustainment Matrix for
Tin Lead (SnPb) and Lead Free Processes
As previously mentioned, Operators and Inspectors will have to be recalibrated to the nuances Lead Free Soldering presents to the Rework and Repair operations.
Hand Soldering Rework and Repair
As with first piece production, hand soldering rework and repair have the same ground rules:
Lead Free Solders will require a longer dwell time – the time the soldering iron was in contact with the hardware – was required to promote adequate heat transfer during the soldering process. Due to the higher soldering temperatures, the soldering iron had to be removed quicker for Lead Free than for Tin Lead (SnPb).
The higher soldering temperature requires that the soldering iron must remain clean and coated with the solder alloy. Lead Free solders are more sensitive to the effects of a dirty soldering iron. The higher soldering temperatures can result in the soldering iron tip becoming oxidized if not cleaned and coated. Do not use solder tips that have soldered Tin Lead (SnPb) solders. The Lead (Pb) in the solder will contaminate the Lead Free Solder joint.
Here are some suggestions for using Lead Free Solders in a rework and repair mode, as shown in Figure 15:
- The preheat temperatures will be higher. With SMT rework and repair operations, the ramp times will be longer.
- For hand soldering, SnPb solder tips are set ~ 280oC. For Lead Free Solders, the solder tip can be set as high as 350oC.
- Due to the higher solder tip temperatures, the solder tip has to remain on the solder joint longer to promote heat transfer. However, the solder tip must not remain on the pad to avoid
- As previously noted, the operator must remove the solder tip quickly after soldering to avoid damaging the solder joint and the board pads. The solder pads are more sensitive to heat with Lead Free Solders.
- For leaded devices, techniques like mini wave hand soldering may have to be modified for Lead Free solders due to the higher temperatures and solderability differences.
- Work closely with your solder and solder paste manufacturers. For the same Lead Free solder alloy, there will be differences between solder manufacturers.
- More active fluxes should be used to improve solderability. However, this may result in more aggressive cleaning operations.
- Tip tinning and cleaning becomes more important due to the higher Lead Free Soldering temperatures.
- Choose the proper solder tip size carefully.
- Lead Free Solders requires an operator to be more careful and patient with the process.
Figure 15. Example of Operator performing Hand Soldering Rework and Repair
3.6.2 SMT Rework and Repair
For Lead Free Solder rework and repair processes, the thermal profile is more critical than for Tin Lead (SnPb), due to the higher reflow soldering processes. The higher processes may damage the components and boards.
Multiple thermal profiles may be required. For example, one thermal profile may be required to remove a component with Tin Lead (SnPb) solder, and another thermal profile may be required to solder that same component with a Lead Free solder as depicted in Figure 16.
|
|
Tin Lead (SnPb) Thermal Profile |
Lead Free Thermal Profile |
Figure 16. Examples of Rework and Repair Thermal Profiles
It is recommended not to use the higher temperature thermal profile to remove the component, for risking damaging the assembly via thermal shock.
Pad dressing, the cleaning of the pads prior to soldering the new component, is an important step. It is critical to remove all Tin Lead (SnPb) solder from the pads, to prevent Lead (Pb) contamination.
The Lead Free Soldering rework equipment should be capable of reaching the soldering temperatures for the various Lead Free solders. Remember, for Tin Lead (SnPb) solders, the peak reflow soldering temperature is between 220oC and 230oC. For some Lead Free Solder alloys, the peak reflow soldering temperature can be as high as 240oC to 260oC, depending upon the alloy and the solder paste manufacturer. As Table 3 depicts, the rework and repair processing parameters are significantly different between Tin Lead (SnPb) and Lead Free solders.
|
Tin Lead (SnPb)
Soldering Parameters |
Lead Free Soldering Parameters |
Preheat |
90 seconds to 150oC |
Top Heater: 160oC
Bottom Heater: 183oC
Air Flow: Medium |
110oC in 110 seconds |
Top Heater: 110oC
Bottom Heater: 150oC
Air Flow: Medium |
50 seconds to 150oC |
Top Heater: 140oC
Bottom Heater: 200oC
Air Flow: High |
Soak |
150oC to 180oC in 90 seconds |
Top Heater: 160oC
Bottom Heater: 183oC
Air Flow: Medium |
150oC to 200oC in 90 seconds |
Top Heater: 180oC
Bottom Heater: 250oC
Air Flow: High |
Reflow |
Peak Temperature at 225oC |
Top Heater: 230oC
Bottom Heater: 180oC
Air Flow: High |
Peak Temperature at 243oC |
Top Heater: 243oC
Bottom Heater: 243oC
Air Flow: Medium |
Table 3. Example of the Rework and Repair processing parameters for removing a 169 I/O BGA
Process parameters will change, depending upon the hardware and equipment capabilities.
There are several dangers this process presents to hardware quality:
- Too high a ramp could thermally shocking the board. This could cause the board to measle and become delaminated.
- Too high a ramp rate could damage the components. Moisture trapped in the component may delaminate Too high a ramp rate could
damage the components. Moisture trapped in the component may delaminate “popcorn”
- Excessive heat could damage the solder joints next to the component being worked on.
- Components may be exposed to excessive heat, which could result in electrical damage
