At the EMPF, the potential rework of an expensive PCB assembly or component is carefully considered. For SMT production, specific attention is paid to board mass and area, so that the proper heating cycle and times can be reasonably estimated for proper solder joint formation over the entire area of the circuit board.

In the rework arena, attention to the same detail must be considered, although in a slightly different manner. In general, the objective is to follow a typical or “golden” heating profile at the site of the repair, without overly stressing the PCB by heating it too quickly or at too high a temperature. The higher the mass of the heater, or the greater caloric output in a given time cycle, the more control you can exert over the mass of the PCB and mounted parts.

Following a typical production reflow profile leads us to the following methodology:

Step 1: Ramp and soak. Preheat for zones 1 and 2 respectively. Warm up the board (or preheat) to a temperature slightly higher than the “transition point” of the board materials (the point where the PCB has reached a temperature which has allowed maximum expansion of the material – usually around 90°C/200°F). During this heating cycle, the PCB will also be “shedding” the heat input at a fixed rate. The task is to place an amount of energy into the PCB which is equal to the amount of heat being shed, plus enough to bring the PCB up to the desired temperature. This will stabilize the board at 90°C/200°F.

Step 2: Ramp to flux activation and soak. Flux activation in Zones 3 and 4 respectively. The board is heated to a point where flux activation takes place. Fluxes of different manufactures will activate at different temperatures, around 150°C/302°F in general. Again, the task is to place an amount of energy into the PCB which is equal to the amount of heat being shed, plus enough to bring the PCB up to the desired temperature prior to flux activation.

Step 3: Ramp to reflow temperature. Soak to allow for time above reflow. Following flux activation, the solder joint area needs to be brought up to reflow temperature. Typically SN63/37 solder needs to be heated to a peak of about 205-210°C/400-410°F and spend a total time of 40 to 60 seconds above the melting point of 183°C/361°F. Once again, place an equal amount of energy into the PCB to the amount of heat being shed from the area of repair, plus enough to bring the solder joint to the desired temperature. Follow with a cooling cycle that terminates the time above reflow at the required 60 seconds.

With this technique, a PCB is assigned a Board Mass Index number (BMI) which will help to determine the proper
temperature profile settings for use with various styles of 110v BGA rework equipment from APE South. For these examples, we are using the Flo-Master, with 1200 watts topside reflow, combined with 600 watts bottom side heating. For each given step in the methodology above, the machines will have a setting where the input of energy balances with the temperature requirement for the solder connection.

Higher BMI numbers are associated with increased shedding of heat energy from the PCB. Higher BMI numbers also show a lessening of peak temperature achieved at the solder joint for a given temperature profile.

With Examples 1 and 2 above, the indication is to raise the machine temperature settings to increase the peak solder joint temperature and/or increase the dwell or soak times in Zones 2, 3, and 4.

If Example 3 is considered “golden”, then for Examples 1 and 2, an estimated temperature increase can be calculated as follows:

Example 1: Peak 185°C; begin by increasing temperature in all zones by 20°C; peak air temperature: 245°C

Example 2: Peak 190°C; begin by increasing temperature in all zones by 15°C; Peak air temperature: 240°C

In both of the above examples, peak air temperatures have not yet reached 250°C, a common peak exposure reading of components.

Rework stations with lower wattage heaters or lower energy ratings will require a higher temperature setting to stabilize the PCB at any given, desired temperature. These temperatures will usually exceed the maximum rating of most components used today, especially plastic BGA packages. The ability to reduce rework machine temperature settings to levels closer to production settings has become a critical advantage. The best repairs will result from temperature profiles which model the original reflow environment. The further a rework profile moves away from this safety envelope, the more risk for damage to the assembly.

Rework can be well managed and costs kept minimal by using equipment which can more closely duplicate the original production environment. This will provide a fixed cost for a component rework task, consisting of replacement component, operator time investment per cycle, and materials and energy investment per cycle. This allows accurate projection of increased machine needs and calculation of realistic maximum repair throughput based on available machines.

The EMPF utilizes the APE Sniper and Intruder. If you would like a demonstration of this system, please contact Jeff Stong at (610) 362-1200, extension 224 or jstong@aciusa.org.