Introduction
Ball grid array (BGA) packages are increasingly used for integrated circuits and CPUs due to their high density interconnect capability. However, the underneath grid of solder ball connections also makes BGA chips very difficult to rework when faults occur. Proper BGA rework technique is essential to safely remove and replace BGAs without damage. This guide covers BGA rework fundamentals, the necessary tools, and step-by-step process instructions.
Overview of BGA Packages
BGA packages connect the IC to a PCB through an array of solder balls on the bottom of the package:
- Dense grid arrays up to 35×35 mm package size
- Pitch between balls as small as 0.5 mm
- I/O counts over 1500 pins possible
- Allows high speed routing on PCB top layer
However this interconnect method poses challenges for rework:
- Solder balls are not easily accessible
- Heating the entire BGA evenly is difficult
- Mechanical shear force risks pad damage on removal
- Precise alignment needed for replacement
Special BGA rework equipment and processes are required to address these challenges and successfully replace BGAs.
BGA Rework Equipment
The key pieces of equipment for professional BGA rework include:
BGA Rework Station
A dedicated rework station provides fine temperature control and tools for accessing underneath BGAs. Features include:
- Bottom pre-heater to evenly heat the board from below
- Top infrared heater and nozzle for local chip heating
- Microscope for viewing the ball grid array during operation
- Tweezers, spatulas, and pick-place tools for alignment
Soldering Station
A temperature-controlled soldering iron between 350-450°C may be required for reballing and local touchup of solder joints. Fine tip sizes under 1 mm are preferable.
Solder Paste
SAC305 lead-free solder paste matched to the solder alloy on the board is required for reballing BGA pads.
Solder Balls
New solder balls with diameter and alloy matched to the BGA are necessary for reballing the package.
X-Ray Inspection
Post-rework x-ray imaging validates proper solder joint formation under the BGA.
BGA Rework Process Overview
The BGA rework process involves the following primary steps:
- Site Preparation – Cleaning, ESD protection, flux application
- Removal – Heating to reflow all solder joints, then lifting BGA
- Site Redressing – Cleaning pads, removing residue
- Reballing – Applying new solder balls using solder paste
- Replacement – Aligning and placing new BGA, reflowing in place
- Inspection – Visual and x-ray inspection of solder joints
- Testing – Electrical testing to validate proper function
Each phase requires care and the proper equipment to avoid damaging PCB pads, traces, or the replacement component.
Process Step 1: Site Preparation
Prior to removal, the site must be prepared:
- Ensure ESD-safe environment – use wrist strap grounded to station.
- Clean board thoroughly around the BGA area using isopropyl alcohol.
- Apply liquid flux around the perimeter of the BGA onto the wetting surfaces. This aids heat transfer during reflow.
- Secure board in rework station and calibrate optics.
Process Step 2: Removal
With flux applied, the BGA can be heated and removed:
- Position preheating nozzle under board aligned with BGA center.
- Set bottom side pre-heater to 150°C and heat soak for 1-2 minutes to slowly warm board.
- Position top side IR heating nozzle aligned to BGA center, approximately 1 mm above package.
- Set top side heater temperature to 350°C.
- Close top heater nozzle and flood BGA with IR heat. Monitor temperature.
- Keep heating until solder reflow temperature ~250°C is achieved.
- Hold at reflow temperature for 20-40 seconds for full joint liquefaction.
- Raise top heating nozzle and use pick tool to slide BGA off site with gentle shear force.
Go slow during heating and lifting to prevent pad damage. Any remaining solder should stay on pads.
Process Step 3: Site Redressing
Once BGA is removed, the site must be cleaned for rework:
- Use solder wick braid and flux to thoroughly clean pads, removing any solder residue.
- Inspect pads under microscope to ensure no damage or lifting occurred.
- If needed, use soldering iron at 350°C to re-wet any lifted pads.
- Clean off all remaining flux using isopropyl alcohol.
Process Step 4: Reballing
Next solder balls are replaced onto BGA pads:
- Apply small dab of solder paste onto center of each pad using mini stencil or dispenser.
- Use tweezers to place a new solder ball onto each paste deposit. Follow BGA ball layout pattern.
- Ensure balls sit flat on paste and not on neighboring pads.
- Once all sites reballed, use heating nozzle above 150°C to gently tack balls in place without reflowing fully.
Process Step 5: Replacement
The new BGA can now be aligned and soldered:
- Position BGA above site using pick tool and align fiducials.
- Lower BGA onto pads slowly allowing self-centering with gentle downward force.
- Inspect under microscope to confirm all solder balls making contact with paste deposits.
- Apply flux around perimeter of BGA onto any visible solder.
- Use heating nozzle to reflow attach the component using thermal profile up to 250°C peak temperature.
- Maintain heat until solder fully wets pads and reflow is complete.
- Allow assembly to cool down before further handling.
Process Step 6: Inspection
Verify joint quality under BGA after replacement:
- Visually inspect perimeter joints and look for proper fillet formation.
- Use x-ray imaging to view hidden solder joints under package.
- Confirm all joints are formed without shorts or opens.
- Take high magnification photos documenting rework process if desired.
Process Step 7: Testing
Functionally test board after BGA rework:
- Perform in-circuit testing if fixture available.
- Power up and exercise system functionality.
- Run board self-test, diagnostics, or functional test routines if exist.
- Retest any parameters out of specification before rework.
- Document electrical test results.
Thorough inspection and testing is crucial to confirm a successful BGA rework.
BGA Rework Challenges
While following the fundamental rework process, there are some common challenges:
Residue Cleaning – Failed joints or charred flux residue under BGAs requires effort to clean entirely before reballing.
Pad Damage – Lifting BGAs risks tearing pads or traces, requiring soldering repairs.
Reballing Small Units – Precise paste deposition and ball placement is difficult for fine pitch < 0.8 mm BGAs.
Voiding – Entrapped air in solder joints under package due to incomplete wetting or flux boiloff.
Thermal Stress – Repeated high temperature exposures during removal and replacement can overheat board or adjacent components.
Proper training, fine tip tools, and extensive practice helps mitigate these challenges when reworking BGA packages.
Conclusion
Reworking BGA packages requires specialty heating equipment, precise soldering technique, and training to master the process. With practice, the majority of BGAs with 600 balls or less can be successfully reworked using suitable tools and diligence following the steps outlined here. Larger high-density BGAs may require outsourcing to an advanced rework facility. But for most common applications, understanding proper in-house BGA rework technique enables repair of these challenging packages.
Frequently Asked Questions
What key tools are required for BGA rework?
The essential tools include:
- BGA rework station with split top/bottom heaters
- Temperature-controlled soldering iron
- Solder paste and solder balls matched to BGA
- Solder wick, flux, picks, tweezers
- Microscope for working under BGA
- X-ray imaging capability
What size BGAs can be reasonably reworked by hand?
BGAs up to about 600 balls with 0.8 mm pitch can be reworked with practice using manual tools and optical magnification. Above ~35 x 35 mm size and 0.5 mm pitch requires more advanced rework equipment.
How can I avoid damaging pads when removing BGAs?
- Keep board as cool as possible during BGA removal
- Avoid excessive shear force when sliding off component
- Use generous flux underneath to lower surface tension
- Consider hot air tools instead of force to lift large BGAs
What is the most common defect seen in BGA rework?
Insufficient solder paste application prior to reballing typically results in unattached balls and unreliable solder joints after replacement BGA attachment. Generous paste volumes are key.
How can I validate successful BGA rework?
- Detailed visual inspection around all ball joints
- X-ray imaging to view hidden solder connections
- Testing board electrically before and after rework
- Running system diagnostics and functionality check
