The solder joint is a connection point that is both conductive and resistant, and almost all of its true quality lies in the invisible part: below the component base, inside the tin block, at the interface with the pad. Cross-sections are the only way to read those.
This article explains three characteristics to read on a solder joint cross-section — wetting angle, fillet penetration, and intermetallic compound (IMC) layer — and four common errors and limitations when drawing conclusions from cross-section images.
1. Why must solder joint joints be evaluated by cross-section?
Three main reasons:
- The most important part is not visible: The bond between tin and the metal surface is hidden under the component base.
- Appearance can be misleading: A solder joint that looks good from the outside may still be hollow on the inside or not completely wet.
- It is necessary to distinguish process errors from design errors: only the internal structure tells which one is the cause.
2. Three characteristics to read on the cross-section
| Characteristics | Meaning | Normal signs | Signs to pay attention to |
|---|---|---|---|
| Wetting angle | Indicates whether tin “eats” into the metal surface or not | Smooth concave surface, continuous transition from pad to component pin | Convex surface, broken angle, clear boundary streaks |
| Fillet height | Indicates the amount of tin adhered to the component pins | The tin rises evenly along the foot, with a stable thickness | Too little tin adheres or flows into a block on one side |
| Intermetallic layer | Is evidence of real metallic bonding | Thin strip evenly along the interface | No band is seen, or the band is too thick, or the band is not continuous |
These three characteristics complement each other. Missing one of the three, the conclusion about the solder joint is still not well-founded.
3. Wetting angle: how to read?
The wetting angle is the angle formed by the tin surface with the base metal surface at the interface. How to read on the cross-section:
- Small angle, concave surface: Tin spreads evenly and bonds well.
- Large angle, convex surface: Tin shrinks into a sphere, with poor bonding ability.
- Clear broken boundary: signs of surface contamination or oxidized coating.
One point that is easily overlooked: for the same solder joint, the wetting angle on the pad side and the component leg side may be different. When reading, you should clearly state which side you are evaluating.
4. Foot absorption: landmark determines the result
| How to calculate | How to do it | Risk |
|---|---|---|
| Calculated from the pad surface | Measure the tin height from the pad surface up to the feet | Ignore the tin located at the foot |
| Calculated according to solder joint thickness | Compare the tin adhesion height to the total pin thickness in the solder joint | It is necessary to clearly identify the foot landmark |
| Calculated according to component thickness | Compared to the thickness of the component body | Not suitable for components with long legs |
Because there are many calculation methods, the report must clearly state the method used. In quality disputes, most disagreements about “enough tin or not enough tin” are actually disagreements about quality.
5. Intermetallic layer: evidence of metallic bonding
When molten tin comes into contact with copper, an intermetallic alloy layer is formed at the interface. This is evidence that the bond has formed at the metal level, not just surface adhesion.
| Condition of the intermetallic compound (IMC) layer | Implication | Risk |
|---|---|---|
| Very thin or invisible | Maybe the heat is not enough or the surface is not clean | Weak bond, easy to separate when subjected to force |
| Thin and continuous | Desired state | No worries |
| Abnormally thick | The temperature or holding time is too high, or the solder joint has been worked at high temperature for a long time | Brittle, easy to crack when subjected to vibration or thermal shock |
| Discontinuous, interrupted | Unlinked region | Crack initiation point |
Note: the intermetallic compound (IMC) layer continues to thicken over time at high temperatures, even after the product has passed the output test. Therefore, evaluating the intermetallic compound (IMC) layer needs to be placed in the context of the actual working conditions of the product.
6. Four common errors on the solder joint cross-section
| Error | Marks on the cross-section | Common causes |
|---|---|---|
| Local tin deficiency | The tin block is thin, does not cover all the components | The amount of solder paste is not enough, the pad design is skewed |
| Crack at the heel of the solder joint | The crack originates from the outer edge of the solder joint | Mechanical stress when bending the circuit board, difference in thermal expansion |
| Void concentration | Large voids in the load bearing area or heat conduction area | The air cannot escape in time, the solder paste is damp, and the heat profile is not optimal |
| Not wet | Broken boundary between tin and surface, with gaps | Surface contaminated, oxidized, poor quality coating |
7. Limitations when drawing conclusions from cross-sectional images
- Only see one plane: voids or cracks outside the section will not appear in the image.
- Small sample, limited quantity: A successful solder joint does not prove that the entire circuit board is successful.
- Influence of sample preparation: Too much grinding can erase the intermetallic compound (IMC) layer or create fake marks.
- Does not reflect the entire sample history: A solder joint that has been heated many times has a different structure than a solder joint that has been heated only once.
8. Minimum information in the report
- Position of cut solder joint and cutting direction.
- The photo is magnified enough to see both the component pins and the pad, with a scale.
- Separate magnified image for the area of interest (wetting angle, intermetallic compound (IMC) layer, void).
- Conclusion for each feature, separate from the description.
- Standards and product classes apply for evaluation.
9. Frequently asked questions
Does a solder joint that looks good on the outside need to be cut and inspected?
With first batches, new products or when there are questions about the procedure, inspection should be done. External appearance does not prove internal connection.
Is there a way to measure the intermetallic compound (IMC) layer without destroying the sample?
There is no non-destructive method that gives equivalent results at the microscopic scale. Ultrasound and X-ray can suggest but cannot measure the thickness of the intermetallic compound (IMC) layer.
What percentage of Void is considered an error?
The threshold depends on the applicable standard, component type and product class. In addition to ratio, it is necessary to consider the size of each void and their position in the solder joint.
Which magnification should I choose for shooting?
Take two levels: one wide enough to see the entire solder joint and its relationship to the pad, one high enough to see the intermetallic compound (IMC) layer boundary.
What if both the pad and foot are good but there are still errors?
At that time, factors other than the solder joint should be checked: stress from the circuit board, assembly force, or thermal conditions during use.
10. Conclusion
The solder joint cross-section answers three questions: is the tin wetted properly, is there enough tin on the pin, and is a metallic bond formed. These three answers combined allow a conclusion about the solder joint.
Four things to do: clearly state the infiltration benchmark in the report; Shoot at two magnification levels; always have a control sample; and clearly state the limitation of conclusions due to the method observing only one cutting plane.
References
- IPC-A-610 — Electronic assembly acceptance criteria, solder joints with pins.
- J-STD-001 — Requirements for electrical soldering and electronic assembly.
- IPC-TM-650 Method 2.1.1 — Microsectioning.
- IPC-7095 — BGA design and assembly, solder joint defect assessment section.
Related articles
- Why do solder joints crack after thermal shock test? Read the marks on the cross-section
- Cross-section BGA: void measurement and head-in-pillow detection
- Cross-section QFN/DFN: void under the heat sink and solder pins
- Measuring dimensions on cross-sectional images: how to set the ruler to minimize the error?
- 10 circuit board defects can only be detected by cross-section
- Set of standards governing cross-section: IPC-A-600, IPC-6012, IPC-TM-650 2.1.1, J-STD-001 and IPC-A-610
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Disclaimer
This article is an interpretive content compiled by us; not legal advice. Enterprises need to compare relevant documents/standards verbatim before applying them to specific products.
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