Rework repair consists of several procedures that must be followed carefully. Rework profile development is among the most difficult and time consuming part of the process. Profiles are used in both reflowing and reballing of the BGA or SMD chip. An SMT rework profile is typically shorter in time than that of BGA rework profiles and that is simply because of the heat required to reach a liquidus temperature. In order to properly develop a rework profile the following are needed.
- Scrap boards
- Thermocouples K Type
- Kapton Tape
Scrap boards are used so that you dont risk damaging a repairable board. Rework stations include temperature sensors connected to the controllers but it is best to have a few extras to monitor different parts of the board. Thermocouples, we recommend Omega they will connect to the thermometers. Lastly Kapton tape is simply used to tape the sensors on to the board. With all of these items you are ready to start the process of rework profile development. We will refer to thermocouples as TC from now on. Start by placing a TC next to the BGA or underneath the chip if the bead is thin enough TC A. Place a TC on top of the chip but not on the die TC B. Another will be placed underneath the board directly underneath the BGA TC C. The industry standard for to develop a rework profile is to drill a hole in the bottom of the board and insert the TC, this ensure the most accurate temperature display but for the purposes of this guide we simply use extra TCs and calculate the difference in temperature. The last TC will be placed away from the chip and about an inch or 2 away from the edge of the board TC D.
- TC A = Solder ball temp
- TC B = Package temp
- TC C = Component temp
- TC D = Board level temp
The goal is to have TC A & B as close as possible to each other. Modern BGA/ SMD packages can typically tolerate temperatures of up to 250c, we want to get the solder ball temperature above liquidus but monitor the package to ensure it does not reach its threshold. TC C will typically be about 20c lower the solder ball temp with a properly developed profile. TC D is to make sure that the board reaches an adequate temperature to prevent board flex and warpage. Essentially by not bringing the board to an appropriate temperature you will have a hot spot on a cold board. By properly placing the TC sensors you can evaluate and control the variables that will result in improper wetting, unreliable joints, popcorn, delamination and component damage.
It is widely known that 30/70 rule in rework profile development provides the proper amount of heating to the respective sides of the board. This involves choosing a target temperature for the package to be reworked. With this chosen temperature the bottom heater setting is adjusted to perform 70% of the work. The top heater swoops in and provides the final 30% to bring the solder balls to a liquid state.
Ramp rate is the speed at which the temperature will rise, in the initial stages of the profile a slow and steady pace is required to avoid thermal shock and allow for the heat to be absorbed throughout the board. The final stages of the rework profile typically call for a fast ramp rate in order to bring the solder balls to a liquid state at which point a steady temperature is maintained. If we were to reverse the ramp rates and use a fast heating cycle it would most certainly result in bumps on both the chip and the board. This is because of the moisture trapped inside the board is not expelled in the early stages and is simply forced out because of the extreme temperature. Taking a slow approach towards the end of the rework profile could potentially cause a longer than needed TAL or time at liquidus and while the rework job may end up successful the reliability of the joints is of concern and can only be determined by the use of an inspection system. Inspection systems consist of either scopes or X Ray units.
Article collaboration with XModdz