Learning objectives
- Explain why hidden solder joints need X ray inspection
- Read voiding, bridging and head in pillow faults on an X ray image
- Measure voiding per ball or per pad against the agreed product criteria
- Follow the radiation safety rules that govern X ray equipment
Lesson
Some of the most important joints on a modern board cannot be seen at all. Ball grid arrays hide every joint under the package. QFN and DFN devices solder to a large thermal pad on their underside. LGA parts sit flat with their joints underneath. Cameras and microscopes stop at the package edge, so this stage inspects with X rays. Two dimensional systems image the joint field directly, tilted views reveal more, and three dimensional systems using CT or laminography slice the joint into layers so a single ball can be judged on its own.
The image tells a clear story once an operator learns to read it. Healthy balls appear round, evenly collapsed and aligned to their pads. Voids show as lighter circles inside the ball, trapped gas from flux and profile behaviour frozen into the joint. Head in pillow shows a ball resting on the paste deposit without ever merging into it, a joint that can pass a gentle test and fail in the field. Bridging appears where neighbouring balls have flowed together, non wet opens show as gaps, and missing balls and popcorning damage from moisture loaded packages complete the gallery of faults this stage exists to catch.
Void measurement carries the key professional lesson of this stage. Voiding is measured as a percentage of the ball or pad area and compared against the criteria agreed for that product, with IPC 7095 as the usual guidance reference for BGA processes. Published limits differ between product classes and customers, so a competent platform and a competent operator never quote one universal number. The skill is to measure honestly, compare against the criteria that govern the job in hand, and escalate when the trend worsens. Many shops run X ray manually or on samples, while high reliability and high volume lines run it inline. Either way, the equipment is interlocked, radiation controlled and used only by trained operators. James Onsomu closes his PCB quality chapter on thorough inspection and testing to verify functionality and reliability. X ray is that same discipline, extended to the joints no eye can reach.
Key numbers
- Voiding: measured as a percentage of each ball or pad and judged against the criteria agreed for the product
- Guidance reference: IPC 7095 is the usual reference for BGA process guidance
- Universal void limit: none exists, limits differ by product class and customer
- Access rule: X ray systems are interlocked and operated only by trained, authorised operators
Defects to spot
- Voids: gas from flux behaviour and profile trapped inside the solidified joint
- Head in pillow: the ball rests on the paste without merging, from warpage or a weak profile
- Non wet opens: joints that never wetted to the pad
- Bridging under the device: neighbouring balls flowed together during reflow
- Missing balls: balls absent before placement or lost in handling
- Popcorning damage: moisture loaded packages cracked by reflow heat
Stage check, self test
Answer each question, then open the answer to check yourself. Passing standard: 80 percent, at least 4 of 5 correct.
1. Why do BGA joints need X ray inspection?
A. The joints are hidden under the package where cameras and eyes cannot see them
B. X ray makes the joints stronger
C. The packages are too small to hold
D. Customers request pretty pictures
Show answer
Answer: A
2. On an X ray image, what does head in pillow look like?
A. A ball that has melted into a perfect sphere twice its size
B. A ball resting on the paste deposit without merging into it
C. A missing package
D. A ball brighter than the board itself
Show answer
Answer: B
3. A trainee quotes one universal void percentage limit for every product. What is the correct teaching?
A. The trainee is right
B. Voiding is never measured
C. Limits differ by product class and customer, so measure and compare against the criteria for the job in hand
D. Any void at all means scrap, always
Show answer
Answer: C
4. Which reference is the usual guidance for BGA processes and voiding?
A. The packing manual
B. IPC 7095
C. The feeder list
D. The stencil drawing
Show answer
Answer: B
5. What is the correct rule for operating X ray equipment?
A. Anyone may use it if they are careful
B. The interlocks may be bypassed for speed
C. Only trained and authorised operators use it, with its radiation safety controls respected
D. It should run with the cabinet open for a better view
Show answer
Answer: C
Previous: Stage 7: Automated optical inspection