BGA Rework Reflow
One of the most critical processes for BGA rework is the process of reflow. The reflow process occurs after the previous device has been removed and the site prepped. The replacement device is replaced using either flux or paste attachment. The reflow process in BGA rework should emulate the manufacturing process as closely as possible. Given the thermal mass of the board in and around the BGA the profile should match that of the replacement solder balls (if the device has been reballed and will be used a the replacement device) or match as closely as possible the profile of the device vendor's data sheet of the solder paste printed.
There are several “rules of thumb” when zeroing in the process of reflow profile. It is good to learn about the thermal characteristics of the PCB when trying to dial in a reflow process profile. One of the best ways to “learn” about the thermal characteristics of the PCB when there is only one PCB and there is not a profiling board available, is to use what is learned during the removal process to help “dial in” a reflow process . Many times a BGA rework technician will use a standard profile in order to remove the device, tweaking or adjusting the profile based on the results achieved. If there is the availability of a solder sample which allows the BGA technician to embed thermocouples into the solder balls (a corner, and 1 or 2 other places depending on the size of the package), into the die, around the BGA, near other components etc. all in conjunction with learning about the thermal characteristics of the device and board during the reflow process. The proper method for embedding these thermocouples is high-temperature epoxy for their attachment to the device or internal to the PCB. Another “rule of thumb” for the reflow profile is making sure that for lead-free profiles the solder joint, as seen by the temperature in both corner and other balls, is above liquidus for a period of 60-90 seconds. Tin-lead solder should be above liquidus for a period of 30-45 seconds.
Not only should the profile be correct and be confirmed through temperature measurements but the components in and around the BGA being reflowed need to be protected. This is especially tried when using a hot air source and for the device which is heat-sensitive including but not limited to ceramic capacitors, plastic connectors, batteries, and MELFs. In addition devices with underfill or components with TRV or glue around them should be watched and protected as these materials will become soft and potentially run all of the board making a large mess. Protection from the heat sources, especially when using a hot air reflow source comes in many different flavors. One of the most common-used but least-effective types of protection used is the Kapton ™ tape found in many areas of the SMT process. This has been shown in several studies on this topic to be LEASE effective type of heat shielding material. Other more effective sources include a water-soluble gel or a ceramic-based nonwoven material. Whatever the type of heat shielding material used to protect neighboring devices during reflow, their use is important to protecting devices from excessive heat which damage the devices.
In order to run a complete profile, the PCB should be adequately supported. This is especially true of there are “imbalanced” copper section of the board or in cases where very thin.032 “thick boards are being reflowed. Without adequate board support, there may be board warping which may damage inner layers or cause the board to be badly deformed making placement of difficult components or have a reliability problem with respect to the solder joints. There are a variety of board support systems on the market with most higher end rework systems offering a flexible board mounting and support system design.
Not only is adequate board support required but proper bottom side side heating of the boards will help ensure minimal differences in temperature across the board and a lesser propensity for board warpage. Modern BGA rework systems are equipped with sophisticated bottom side heaters. Advancements for making sure the process of reflow is optimized include multizone bottom side heaters. These heaters allow the user to have the rework area at a higher temperature than the remainder of the PCB thus reducing the likelihood of board warpage during the process of reflow.
A typical lead-free, hot air source thermal profile is seen below. First, the bottom side heating begins to warm up the board with one temperature (typically 160 or 170C) being in the rework location and another, typically 150 C, being at other areas of the board. During the time this temperature is applied to the bottom side of the board the nozzle temperature begins to climb during the “ramp” period of the reflow profile. Too fast a ramp may damage bound components or the laminate. Then a “soak” phase, which lowers the ramp rate and starts to activate the flux, begins. After this phase, the liquidus temperature, somewhere between 205-220C is reached. This begins the reflow zone. In this, the maximum temperature is reached and the rework location “sees” a temperature which is above the liquidus temperature for a minimum of 60 and as long as 90 seconds. The reflow profile ends with a cool down zone. The cool down zone cannot be so extreme so as to cool the board where the negative temperature gradient may cause brittleness in the solder joint at the end of the process of reflow.