Cross-sectioning cuts the sample in a planned direction, mounts it, polishes it and examines it under a microscope, revealing interface information that X-ray cannot show. It is the core method for determining the root cause of solder joint cracking.
What cross-sectioning can show
Failure interface: whether the crack runs along the pad interface, through the solder, or through the PCB substrate.
Intermetallic compound (IMC) thickness: too thin gives insufficient interfacial bonding, too thick gives brittle cracking.
Kirkendall voids: brittle phases in the IMC initiate micro-cracks under thermal stress, a common cracking origin.
Plating thickness and density: measurement of copper plating, nickel plating and gold fingers.
Solder paste or flux residue: affects wetting and interfacial bonding.
How IMC relates to failure
IMC too thin (for example below 3 um) → insufficient bonded interface area, so it peels under mechanical stress.
IMC too thick → a higher proportion of brittle phases, producing intergranular cracking under stress.
Serrated IMC (Kirkendall voids) → thin brittle phases become crack initiation sites under stress.
Generally, solder composition, soldering temperature and high-temperature dwell time together determine IMC thickness.
Sampling and preparation essentials
The sampling location must be precise: the cut path has to align with the failure location indicated by X-ray.
Mounting must prevent resin stress from affecting soft solder; a low-hardness resin is recommended.
Polishing must avoid introducing artificial cracks or smearing; the final step uses an etchant briefly to reveal the interface.
Observe several sections through the same failure, to avoid judging from a single point.
Measurement and reporting
IMC thickness: measure on the pad side and the solder side, recording each separately.
Crack width and propagation path: measured and recorded with a microscope scale.
Void ratio: measure the solder area proportion on the section and cross-check against the X-ray result.
The report must state sampling location, preparation method and measurement tools, so that results can be reviewed independently.
Frequently asked questions
Does cross-sectioning destroy the sample?
Yes, it is irreversible destructive analysis. The workflow must therefore start with non-destructive methods (X-ray, microscopy) to confirm the location before cutting. Cutting first risks cutting the wrong place, which fails the analysis and loses the sample.
What IMC thickness is appropriate?
There is no absolute value; it depends on the solder system and the application. A range of 3-8 um is generally considered reasonable, but the solder composition matters. High-silver solder paste grows IMC faster and needs tight control of high-temperature dwell time, while lead-free pastes show more pronounced brittleness. Assess together with the solder system and the actual failure behaviour.
Service notes
Our laboratory holds both CMA and CNAS accreditation, covers about 10,000 m2 and runs more than 500 sets of equipment. All testing is carried out in our own laboratories — nothing is subcontracted. Plant visits and on-site witnessed testing are welcome.
Scheduling follows the actual project pipeline. You can state your preferred lead time when you submit samples; we will confirm a feasible plan against current capacity and keep you updated at each milestone. Once the report is issued we send the electronic copy; corrective-action advice can be arranged separately.
