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ES Solder Void Inspection in Power Modules
Solder voids slow heat flow from the die to the heatsink. This guide compares 2D X-ray, 3D X-ray, acoustic imaging and thermography, explains how void limits are set, and lists what to ask a module supplier.
A power module can pass every electrical test at room temperature and still carry a thermal weakness inside. Solder voids, small pockets of gas trapped in the die attach or substrate attach layers, do not change a datasheet value on day one. They change how heat leaves the chip, and that shows up later as higher junction temperature and shorter life under load.
This article explains where voids form, how they are inspected, why acceptance limits vary, and which questions are worth asking when you qualify an IGBT or SiC module supplier. HIITIO Semi lists “100% X-ray void inspection” among the 23 quality control checkpoints on its About Us page, so we also look at what that kind of control does and does not tell a buyer.
Why Voids Matter in Power Modules
In a typical module, heat travels from the die through a solder or sinter layer into a ceramic substrate, then through a second attach layer into the baseplate or cooler. Voids in these layers hinder that path because the gas inside a void conducts heat far worse than solder. Kokott, writing in Electronic Design, notes that large solder joints (up to 25 cm²) make it hard to avoid enclosed gas, and that voids can cause a module to malfunction or fail in operation.
Not every void is equally harmful. A finite element study by Bhatti and co-authors in Microelectronics Reliability found that, for the same total void percentage, large coalesced voids raise thermal resistance more than small distributed ones. Voids spanning the full thickness of the solder layer, or sitting close to the heat-generating area of the chip, were also more damaging. The authors attribute voids mainly to outgassing during reflow and defective metallisation.
Three void characteristics therefore matter in practice:
- Total area: the sum of all voids in a layer.
- Largest single void: one large void is often worse than many small ones.
- Location: voids under the die center or active area affect junction temperature more than voids near an edge.
Where Voids Form: Two Layers, Two Risks
| Layer | Typical function | Why voids occur | Main concern |
|---|---|---|---|
| Die attach (die to substrate) | First thermal path from the chip | Flux or gas outgassing during reflow, metallisation defects | Local hotspots under the die, higher junction temperature |
| Substrate attach (substrate to baseplate) | Spreads heat into the baseplate or cooler | Large joint area traps gas; warpage and wetting variation | Higher module-level thermal resistance; poor lateral heat spreading |
The two layers behave differently. Die attach voids sit directly under a heat source, so even a modest void can matter. Substrate attach voids cover a much larger area, so the question is often how many large voids exist and where they sit relative to the dies above them.
Inspection Methods Compared
No single method finds every defect. The right choice depends on what you need to detect, how many parts you inspect, and where in the process you inspect.
| Method | What it detects well | Limits | Typical use |
|---|---|---|---|
| 2D / 2.5D X-ray | Void area and distribution; fast projection image | Cannot easily assign a void to a specific solder layer; stacked layers overlap | In-line screening after soldering |
| 3D X-ray (laminography / CT) | Voids separated by layer | Slower and more costly; artifacts from heat sink structures need adapted reconstruction | Layer-resolved inspection, failure analysis |
| Acoustic microscopy (SAM / AMI) | Voids, non-bonds, delamination and cracks at bonded interfaces | Needs water coupling; image time can limit throughput | Layer-gated inspection of die attach and substrate attach |
| Lock-in thermography | Thermally relevant defects, including adhesion problems | Less established for routine production | Research and comparison studies |
Kokott’s 2011 article argues that standard 2D and 2.5D X-ray cannot assign voids to a specific layer, which risks fault escapes or excessive false rejects, and presents 3D X-ray with adapted reconstruction as the viable route for 100% inspection. Acoustic methods have also matured for production use. Adams, in a Bodo’s Power Systems article hosted on EEPower, describes an automated acoustic micro imaging system that scans the module from below through a water column and gates the solder layer and the die attach layer in a single scan. He also states that X-ray will not catch all of these defects, while ultrasound will.
One boundary is worth remembering. Dreher and co-authors, comparing X-ray, scanning acoustic microscopy and lock-in thermography on Ag-sinter die attach in Microelectronics Reliability, note that X-ray can determine the void rate but that pure adhesion problems cannot be detected by X-ray. A module can look clean in an X-ray image and still have a poorly bonded interface.
How Acceptance Limits Are Set
There is no single universal void limit for power modules. Ottinger and co-authors, in a HiTEN 2022 paper on location-dependent void down-rating for automotive power modules, describe the uniform limit used in industry as typically 5–10% of the die connection area. Their simulation work suggests that a position-dependent factor could allow larger voids where they are less thermally harmful and require smaller ones near the die center.
Adams gives a different example: a reject criterion of total void area above 2% of the raft area, noting that criteria are user-defined and can weigh defect location, such as voids under a die.
These figures are examples, not standards you can copy into a purchase order. A practical acceptance specification usually defines:
- Maximum total void area per layer, as a percentage of the bonded area.
- Maximum single void size.
- Stricter zones, for example directly under the die.
- The inspection method and its detection threshold, since the same part can show different void percentages in 2D and 3D images.
Reading a Supplier’s Void Control Claim
A statement such as “100% X-ray void inspection” is useful, but it leaves open several details. HIITIO’s About Us page describes a process that includes die attach, wire bonding, HTRB testing and final product testing, and lists X-ray void inspection among its 23 quality control checkpoints. The page does not publish void acceptance thresholds, so these are details a buyer should request in writing during qualification.
| Question to ask | Why it matters |
|---|---|
| Which layers are inspected (die attach, substrate attach, or both)? | 2D X-ray alone may not separate the layers |
| Is inspection 100% or sampled, and at which process step? | Rework is far easier before encapsulation |
| What are the void limits, and how are they defined (total, single, zone)? | Limits without a definition cannot be compared between suppliers |
| Is acoustic imaging used for adhesion or delamination checks? | X-ray does not detect pure adhesion defects |
| Are images stored and linked to lot or serial number? | Needed for failure analysis and traceability |
For the traceability side, see our article on single lot traceability for high-reliability power modules. For how void control fits a wider validation plan, see power semiconductor module PPAP and production validation and reliability testing of power modules.
Void Inspection at the Buyer’s End
Most buyers will not run X-ray or acoustic inspection on every incoming module, and a sealed module is hard to inspect in any case. Incoming checks usually focus on electrical and mechanical parameters, covered in our guide to incoming inspection tests for power modules. Void control is therefore mainly a supplier-side process control that you verify through documentation, audits and sample evaluation.
Thermal behavior in your own application is also worth checking. Our article on thermal design and cooling solutions for new energy inverters covers how module-level thermal resistance interacts with the cooler and mounting.
Modules Where Thermal Path Quality Matters
Thermal path quality is relevant across the 62mm, Econo and Easy families. Two examples from the HIITIO range:
- HCS200FH120A2C1 is a 62mm SiC half-bridge module rated 1200 V and 200 A with a maximum junction temperature of 175 °C. Its product page lists a Si₃N₄ AMB substrate with low thermal resistance, without giving a numeric value. See the HCS200FH120A2C1 product page.
- HCG200FH120A2D is a 62mm IGBT half-bridge module rated 1200 V and 200 A with a maximum junction temperature of 150 °C, aimed at motor drive inverters, servo drives and UPS. See the HCG200FH120A2D product page.
Neither product page states void limits or Rth(j-c) values, so request the full datasheet and the supplier’s void acceptance criteria when you evaluate them. Browse the wider range in the SiC module and IGBT module categories.
Request Void Control Documentation From HIITIO Semi
Choosing a power module is a thermal and reliability decision as much as an electrical one. If you are qualifying a 62mm, Econo or Easy module for a drive, inverter, charger or storage converter, HIITIO Semi can share product datasheets and discuss quality documentation as part of your evaluation. Tell us your target application, voltage class and cooling method, and we can suggest suitable IGBT or SiC modules and matching gate drivers from the IGBT and SiC driver range. When you contact our team, include any internal void acceptance limits you already use so that inspection scope and reporting format can be reviewed against your requirements early, before samples are built into your system.
FAQ
What is an acceptable solder void percentage in a power module?
There is no single universal value. Published examples include a typical industry limit of 5–10% of the die connection area and an example reject criterion of 2% of the raft area. The right limit depends on layer, void location, current density and the supplier’s qualification data, so define it in your specification.
Can X-ray inspection find all solder voids?
X-ray is the established method for measuring void rate, but 2D images cannot easily assign a void to a specific layer, and X-ray does not detect pure adhesion defects. Acoustic imaging or 3D X-ray is often used to complement it.
Are small voids or large voids worse?
For the same total void percentage, simulation studies indicate that large coalesced voids raise thermal resistance more than small distributed voids. Voids near the die center or spanning the full layer thickness are also more harmful.
Can a buyer inspect voids in a finished module?
Usually not in routine incoming inspection, because the module is sealed and the equipment is specialized. Buyers normally rely on supplier inspection records, audits, sample evaluation and thermal testing in the target application.
Do voids affect SiC modules differently from IGBT modules?
SiC devices are smaller for the same current rating and can have higher heat flux, so the thermal path may be more sensitive to attach quality. This depends on the module design, so compare Rth(j-c) data and inspection criteria rather than assuming.
