BGA Component Rework Profile Development, Standard Method

BGA Component Rework Profile Development, Standard Method

TOOLS & MATERIALS Ball Mills
BGA Specific nozzle
Cleaner
Cleaning Wipes
Flux, Liquid
Microscope
Hot Air Rework Station
Hand held Digital Thermometer
Oven
Precision Drill Press
Scrap development Assembly
Scrap BGA Components
Tape, Kapton
Temperature recorder, and Analysis Software
Thermal Adhesive


CAUTION - Operator Safety A thorough review of the equipment manual and comprehensive training are mandatory. Daily maintenance is essential. Consult the equipment manual for more information.

CAUTION - Component SensitivityThis method may subject the component to extreme temperatures. Evaluate the component's tolerance to heat prior to using this method. Plastic BGA's are especially sensitive to moisture absorption. Carefully evaluate pre bake requirements.

CAUTION - Circuit Board SensitivityPC Boards are made from a great variety of materials. When subjected to the high temperatures they are susceptible to the following types of damage:
1. Layer delamination.
2. Copper delamination, separation of pads, barrels of inner layers.
3. Burns and solder mask chipping.
4. Warp.
Each circuit board must be treated individually and scrutinized carefully for its reaction to heat. If a series of circuit boards are to be reworked, the first several should be handled with extreme care until a reliable procedure is established.
PROCEDURE
GENERAL INSTRUCTIONS

1. Figure 1: Flux manufacturers reflow profile.

   NOTEYour solder paste manufacturer will have a recommended time/temperature curve for these applications. It is recommended as guidance for profile development. (See Figure 1)
   
2. One fully assembled development board is required.
   
3. 4 to 6 sample BGA components for development may be required.
   
4. A steady state operating temperature or threshold must be established prior to launching the reflow cycle. A consistent temperature starting point is necessary for repeatable results. Between 110ฐ C and 140ฐ C is recommended.
   BGA SOLDERING PROFILE INSTRUCTIONS
   NOTE The BGA replacement profile may require more time and even more heat (usually not more than 20 seconds time) to not only reflow all joints but to properly evacuate flux gases and create uniform joints across the package.

   1. Figure 2: Precision drill press.

      Using a precision drill press (See figure 2), drill through the bottom of the board as follows: One hole in the center of the part, preferably into one of the center balls, if available. One hole into the corner of the part, preferably into one of the balls in the outside row. One hole through the opposite corner of the part into one of the balls. Another hole may be also drilled or substituted for this last one, this being one drilled on the side of the component in an outside row. (See Figure 3).
      
      NOTE
      If the BGA component is susceptible to high temperatures, additional thermocouples may be placed into the device itself to measure the vertical temperature differential (Z- axis).
      

   2. Figure 3: Thermocouple locations.

      Insert thermocouples into the holes. Ideally the thermocouple is secured in place by the solder of the pad that the thermocouple is placed in. (See Figure 4) Otherwise secure in place with Kapton tape, and thermal adhesive. Additional thermocouples will also be placed on the top of the circuit board, about two inches from the site and on the bottom side under the site. These thermocouples may be merely taped in place.
      
   3. Select an appropriate nozzle and install. Be mindful of the component height and clearance area around the component.
      
   4. Secure the board in place with appropriate tooling, clamps, and pins. The board should be secure but allowed to move with thermal expansion. Antistatic solder wave fixtures may be used to prevent warp.
      

   5. Figure 4: Ideal thermocouple placement.

      Connect the thermocouples (drilled locations) to a temperature recorder. Connect other monitoring thermocouples to hand held digital thermometers as required. Temperature recorder should present graphic display as per Figure 5.
      
   6. Select soldering process parameters from similar existing profiles. If none are available, contact the manufacturer of the rework station.
      
      NOTE
      It may be advisable to shut off all vacuum commands when developing a removal profile to prevent inadvertent component removal.
      
      NOTE
      Pre bake the board to drive out accumulated moisture. The length of pre bake will be affected by the board's environmental exposure. A pre bake temperature of 75 ฐC to 100 ฐC is recommended.
      
   7. Place a pre baked board onto the fixture.
      

   8. Figure 5: Temperature data using analysis software.

      Establish a bottom side, under part threshold temperature from which to begin the reflow ramp. 140ฐC underneath the part should correspond to approximately 90ฐC at 2" from the nozzle on the board's top side. Choosing a starting point in this approximate temperature range will help to reduce localized warping during BGA ball reflow.
      
   9. Run the process and monitor the temperature of the bottom and top of the board, next to and under the component.
   10. Disconnect the thermocouples and download recorded data.
       
   11. Analyze the data and optimize parameters for reliable rework as follows:
       1. Maximum temperature at the solder ball / board pad interface should be 205ฐC.
       2. Minimize temperature differential to less than 15ฐC for internal thermocouples measuring the various solder ball / board pad interface.
       3. Time above 183ฐC should be between 30ฐ and 60ฐ seconds
       4. Temperature rise and fall should not exceed 3ฐC/ Sec ramp
       
       
   12. Make changes to process settings as applicable.
       
   13. Run the process and return to step 13.
       
       
   14. Examine the surface under the component for warp.
       
       NOTE
       Excessive localized warp may be reduced by increasing the pre ramp temperature threshold. A convection oven can be used to decrease the thermally induced stress caused by the process. Even heating across the whole board may be required.
   DEVELOP REMOVAL PROCESS
   
   NOTE The component will typically release from its pads when two of the thermocouples measuring joint locations pass the 183ฐ C mark. In order to reduce mask, part or board damage due to excessive heat, the removal cycle is typically shorter than the replacement cycle. Reflow the joints just enough to effect removal.
   1. Copy the parameters of the soldering profile to develop the removal profile.
      
   2. Change process parameters as needed.
      
   3. Connect the thermocouples (drilled locations) to the temperature recorder. Connect monitoring thermocouples to the hand held digital thermometers.
      
   4. Run the process and monitor externally connected thermocouples.
      
   5. Disconnect the thermocouples and download recorded data.
      
   6. Analyze the data and optimize parameters for reliable rework as follows:
      1. Maximum temperature at any location should be 210ฐC.
      2. Minimize temperature differential to less than 15ฐC for internal thermocouples.
      3. Time above 183ฐ C should be between 30 - 60 seconds.
      
   7. Adjust process parameters as needed.
      
   8. Determine the time in the process when all ball locations reach 183ฐC. Note the bottom side monitoring thermocouple temperature.
   9. Reconnect thermocouples.
      
   10. Inject flux under the component.
       
   11. Run process and lift nozzle three seconds after reflow has been reached.
       
   12. Remove the component using vacuum tool.
       
       NOTE
       If software controls the vacuum, select the vacuum on command for all events. This will lift the component as soon as reflow temperatures have been reached.
       
   13. If the component can not be removed, analyze the temperature data and adjust the parameters. Return to step 10.
       
   14. Inspect the BGA footprint area for signs of unexpected overheating, solder mask or pad damage.