Scanning Acoustic Microscopy for Package Delamination
Some of the most damaging defects in a plastic package are invisible from the outside and completely transparent to X-ray. A thin layer of air between the mould compound and the die pad, or between a solder mask and a copper plane, changes almost nothing about how the part looks or how it conducts. It only changes how long the part lasts. Scanning acoustic microscopy exists to find exactly that class of defect, and it does so without cutting the sample apart.
What Scanning Acoustic Microscopy Detects
An acoustic microscope sends a short pulse of ultrasound into a sample and listens for reflections. Every time the pulse meets a change in acoustic impedance — a boundary between two materials — part of the energy bounces back. A solid, well-bonded interface returns a specific reflection; an air gap returns an almost complete reflection with inverted phase. Delamination is therefore not inferred, it is measured directly.
The technique is sensitive to what X-ray misses. X-rays are absorbed by dense material, so they excel at showing solder voids and wire position. Sound travels differently through solids, so it excels at showing where two solids have come apart. The two methods are complementary rather than competing, which is why qualified packages are often checked with both.
Why Package Delamination Matters in Service
A delaminated interface is a crack waiting to grow. Under thermal cycling the materials on either side expand at different rates, and a void provides the free surface a crack needs to start. Once it propagates to a bond wire or a solder ball, the failure becomes electrical and the product fails in the field rather than on the line.
Moisture makes the situation worse. Vapour collects in the gap during storage, then expands violently during reflow and pushes the layers further apart — the mechanism behind popcorn cracking. Because of this, delamination found after assembly is often the visible end of a problem that began with incoming material stored in the wrong conditions.
How the Transducer and Coupling Work
Ultrasound does not travel through air, so the sample must be coupled to the transducer. Most systems immerse the part in a tank of deionised water, with the transducer scanning above it. Water is convenient, cheap and acoustically well matched, but it means the sample must tolerate immersion, which rules out parts that cannot be dried afterwards, a constraint worth planning around in the PCB production flow.
Transducer frequency sets the trade-off between resolution and penetration. A high-frequency transducer resolves fine features near the surface but struggles to reach the deeper interfaces of a thick package. Lower frequencies penetrate further at the cost of blur. For a typical plastic ball grid array, the practical answer is usually a mid-range probe focused on the specific interface of interest.
Reflection Modes: A-Scan, B-Scan and C-Scan
An A-scan is a single point measurement plotted as amplitude against time, which is the raw data from one position. A B-scan sweeps the transducer along one line and displays the result as a cross-sectional image. A C-scan sweeps in two dimensions and produces a plan view, which is the format most people picture when they talk about acoustic imaging.
Gating is what turns the raw data into an image. The operator defines a time window corresponding to a particular depth, and the system colours each pixel by the amplitude of the reflection inside that window. Move the gate and a different interface appears. This is powerful but also dangerous, because an image produced with the wrong gate can look convincing while showing the wrong layer entirely.
Moisture Absorption and Popcorn Cracking
Plastic mould compounds absorb moisture from the air. The amount depends on the material, the relative humidity and the time spent in that environment, and it is quantified by a moisture sensitivity level. Parts that exceed their floor life must be baked before reflow, or the absorbed water will flash to steam at reflow temperature.
Scanning acoustic microscopy is the standard way to verify that a bake worked and that a package survived reflow without internal separation. A common qualification sequence is to scan the part dry, expose it to a defined humidity soak, run it through a simulated reflow, and scan again. Any interface that was bonded before and separated after has been identified unambiguously.
Sample Handling and Preparation
Immersion requires care. Parts that have already absorbed moisture will release it into the water and the coupling will degrade, so the scan should be done promptly. Components mounted on a board present additional challenges, because the board itself reflects sound and can obscure the interface being examined.
Where a board must be inspected, a focused transducer with a short working distance is used to image through the package from above while the board sits on a flat support. Where the interface of interest faces downward, the part may need to be removed first. Documenting how the sample was mounted is essential, because positioning errors produce artefacts that look exactly like real defects.
Reading Acoustic Images: Grey Levels and Artifacts
In a typical C-scan, bright regions mean a strong reflection, which at a bonded interface indicates separation. Dark regions mean the sound passed through, which indicates a good bond. Most standards therefore define a delamination as a contiguous area above a chosen grey level covering more than a specified percentage of the interface.
Artefacts are common and must be recognised. A tilted sample produces a gradient across the image that can be mistaken for progressive delamination. Reflections from an internal via or a die edge appear as bright shapes that are not defects at all. Analysts normally compare the image against a reference part of the same construction, which makes genuine anomalies stand out.
Comparing Acoustic Results with X-ray and Cross Sections
Acoustic imaging answers questions about interfaces; X-ray inspection answers questions about solder and internal metal. Neither tells the full story of a package failure, and a finding from one should be confirmed by the other wherever possible. When a delamination is flagged near a ball, an X-ray of the same region will show whether the solder was affected.
Destructive cross sectioning remains the arbiter of last resort. It is slow, it consumes the sample and it can itself introduce the separation it is meant to prove, so it is used to confirm a specific hypothesis rather than to survey a lot. Together the three techniques give a coherent account of what happened and when. Material-level context in this laminate properties guide helps explain why some constructions tolerate thermal stress better than others.
Setting Acceptance Criteria and Reporting
An inspection result is meaningless without a criterion. The usual approach is to define, for each critical interface, the maximum allowable delaminated area, whether voids may connect to the die surface, and whether any crack may reach a bond wire. These thresholds come from the relevant industry standard plus the product’s own qualification testing.
Reports should include the transducer frequency, the gate settings, the reference used, and a map of where each image was taken. Without those details a second laboratory cannot reproduce the result, and a supplier dispute becomes an argument about pictures rather than facts. Traceable measurement is what converts a striking image into usable engineering evidence.
FAQ
Is acoustic microscopy destructive? The measurement itself is non-destructive, but it requires immersion in water, which some assemblies cannot tolerate because residual moisture may later cause corrosion or popcorning. Parts that are scanned must normally be baked and dried afterwards, so the practical answer depends on how the result will be used and whether the sample can be safely dried.
Can it find delamination under a solder ball? Yes, provided the transducer can be focused on that interface and the package geometry allows sound to reach it. Dense solder reflects strongly, so the image around a ball can be noisier than a plain laminate region. In practice operators scan the whole package, flag suspicious areas, and confirm the critical ones with X-ray or a cross section.
How often should production parts be scanned? Routine scanning is usually reserved for qualification, process change verification and failure investigation rather than every lot, because each scan takes time and consumes a sample. Builds that use a new mould compound, a new substrate supplier or an adjusted reflow profile should always be scanned before release.
