Lead Free Rework and Repair of Ball Grid Arrays
By Joe Kelly
Rework and repair of lead free ball grid arrays (BGAs) will present new challenges. Because of the higher temperatures required for rework and repair of lead free BGAs, new defect issues may arise. The following Tech Tips will help with the rework and repair of lead free BGA components.
1. The most important step in removing a lead free BGA from a printed circuit board (PCB) is obtaining an accurate thermal profile. Check the profile with thermal couples. Because the profile can be tracked with a one- or two-second lag time, the profile can be adjusted.
2. When using a convection rework station, place one thermal couple under the BGA if possible. Otherwise, place it directly next to the component using Kapton® tape. Place a second thermal couple on the bottom side of the board, beneath or as close as possible to the BGA being removed. Ramp the temperature from room temperature to 150°C in 60-90 seconds. The bottom temperature should be as close to the top temperature as possible without taking it through the “soak zone,” which is typically 150-170°C for 30-60 seconds. This allows the board to heat up evenly. As the heat spikes to the reflow zone (which is approximately 225°C at 60-90 seconds for tin lead and 235-245°C at 30-60 seconds for lead free, depending on the alloys used). The solder source-side should provide the heat to reflow the component. While the top heat is increasing, the bottom heat should maintain the “soak zone” temperature. This will help prevent warping and blistering of the board. The solder destination-side temperature will evenly warm the board substrate. If the bottom heat is too high, the substrate will warp and/or delaminate. Typically, the bottom should be about 70% of the top heat.
3. An infrared-based system may only have a remote sensing thermal couple. If using such a system, keep in mind that the measuring beam may be blocked by other components on the surface. When this occurs, use a portable thermal couple sensor to measure the temperature as set forth in item 1, above. Some systems have a video camera which will allow you to watch the solder reflow and calibrate your profile accordingly. Typically, the bottom should be about 70% of the top heater’s temperature in this case, also.
4. Follow the recommended profile times for the solder paste. The lead free time above liquidus (218°C) is approximately 30 seconds shorter (30 +/- 15 seconds) than tin lead time above liquidus (183°C; 60 +/- 15 seconds), because of the higher temperature. It is important to stay below the maximum allowable temperature to minimize the heat cycle on the board substrate and the internals of the component. If the machine has nitrogen capabilities, use them in the reflow zone to reduce voiding and improve wetting. Nitrogen eliminates the oxygen in the atmosphere and allows the flux to clean the pads more effectively, thus providing a more solderable surface.
5. Consider the type of board substrate being used. The most common is FR4 (fire retardant fiberglass and epoxy); however, there are many other substrates and mixed laminates available for the more exotic radiofrequency (RF) applications (such as ceramic, Teflon, and poly type materials). Because the surface adhesion between the substrate and the pads may be very sensitive to the higher temperature, the adhesive properties of each material should be evaluated.
6. When removing the component, ensure that the component is in full reflow. If the component is removed before reflow is complete, you will lift the pads. This is common with rework stations that have automatic pick up features. The head will pull the component prematurely and remove pads in the process. Some alternate techniques would be to use a spring-loaded pick up head with minimal force. If the pads are removed and the profile seems good, inspect the component solder joints on the BGA. If the pads are on the board-interface side of the joint, flat and on the bottom, it is likely that the profile was not hot enough to remove the component yet. If the pads are in the ball area and have floated, the problem may be from poor surface adhesion to the substrate or component.
7. Placement is also a critical step. The closer to thecenter of the pad that the component is placed, the better the joint will appear. It is critical to ensure that the placement system is calibrated to the center of the component pick up head. This will ensure that the placement is accurate. With tin lead solders, if you are off by 25% on the split vision system, you may be okay due to the surface tension of the solder being able to bring the BGA into alignment; however, with lead free solders, the heat is high enough that it may let the solder tension lift the pads. This is a major defect. Use an X-ray inspection system or an endoscope to inspect the component joints for these defects.
The rework and repair of BGA devices is becoming a common area of concern based on the varying factors for lead free processes. The tips above should help ensure a consistent removal and placement of a BGA device. For any further information on lead free rework and repair of BGAs, please call the EMPF Helpline at (610) 362-1320.
