CAF vs Electrochemical Migration
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CAF vs Electrochemical Migration – Two Hidden Threats to PCB Insulation Resistance
CAF and ECM – two failures, one root cause, different appearances
A telecom backplane and a consumer power supply both failed due to conductive bridging between adjacent conductors. The backplane failure was CAF (conductive anodic filament) – a filament grew through the glass/epoxy interface along the glass fibers after 18 months in a humid central office. The power supply failure was ECM (electrochemical migration) – tin‑based dendrites grew across the solder mask surface after 6 months in a high‑humidity consumer environment. Both passed standard insulation resistance tests initially. The lesson: CAF is a bulk material/interface phenomenon; ECM is a surface contamination phenomenon. They require different test methods and mitigation strategies – but both demand clean, high‑quality materials and processes.
CAF vs ECM – Different Mechanisms, Different Test Methods
Although both result in reduced insulation resistance and electrical shorts, the physical mechanisms are distinct.
| Parameter | CAF (Conductive Anodic Filament) | ECM (Electrochemical Migration) |
|---|---|---|
| Growth medium | Within PCB laminate (along glass fibers, through cracks) | On PCB surface (solder mask, exposed metal, or contamination layer) |
| Driving force | Electric field between biased conductors (e.g., adjacent vias) | Electric field + moisture + ionic contamination on surface |
| Typical metal involved | Copper (anode dissolves, deposits as Cu²⁺ near cathode) | Tin, silver, or copper (from solder, pads, or finishes) |
| Visual appearance (microsection) | Thread‑like filament along glass fibers, often branched near cathode | Dendritic, fern‑like crystals bridging between conductors on surface |
| Test method (IPC‑TM‑650) | 2.6.25 (CAF test) or modified 2.6.3.7 with bias | 2.6.3.7 (SIR) – surface insulation resistance |
| Typical conditions | 85°C/85% RH, 50‑100V bias, 500‑1000 hours, spacing ≤0.5mm | 85°C/85% RH, 5‑50V bias, 168‑500 hours, any spacing |
| Mitigation strategy | High‑CAF resin, clean laminate, thick dielectric, tight registration | Ionic cleanliness, conformal coating, improved solder mask, no exposed metal |
Key insight: CAF is a material quality and lamination issue. ECM is a surface cleanliness and assembly process issue. You can have a board with excellent CAF resistance but still fail due to ECM if left contaminated.
CAF – Conductive Anodic Filament (IPC‑TM‑650 2.6.25)
CAF occurs when copper from an anode dissolves into the laminate under bias and humidity, migrates along glass fiber bundles, and deposits at the cathode. It is a subsurface phenomenon.
- Critical spacing – Typically ≤0.5 mm between biased conductors (e.g., via‑to‑via, via‑to‑trace).
- Acceleration factors – Higher voltage, higher humidity, higher temperature, and acidic residues accelerate growth.
- Susceptible materials – Standard FR‑4 with low resin content, poor glass fiber wetting, or delamination.
- Test method – IPC‑TM‑650 2.6.25: biased comb pattern at 85°C/85% RH, 50‑100V, 1000h. Monitor insulation resistance. Failure when resistance drops below 10⁸ Ω.
- Mitigation – Use high‑CAF laminates (e.g., Panasonic FR‑4 CAF, Isola 370HR), increase spacing, reduce voltage, improve lamination quality.
ECM – Electrochemical Migration (IPC‑TM‑650 2.6.3.7)
ECM occurs when ionic contaminants on the PCB surface dissolve in absorbed moisture, and under bias, metal ions migrate and form dendrites between conductors. It is a surface phenomenon.
- Critical factors – Ionic residues (chlorides, bromides, weak organic acids), high humidity, bias voltage, and exposed metal (solder, tin, silver).
- Most susceptible metals – Silver (especially immersion silver), tin (from HASL or solder), copper (if solder mask damaged).
- Test method – IPC‑TM‑650 2.6.3.7: surface insulation resistance (SIR) on comb patterns at 85°C/85% RH, 5‑50V bias, 168‑500 hours. Failure when resistance drops below 10⁷‑10⁸ Ω (depending on class).
- Mitigation – Maintain ionic cleanliness (<1.56 µg/cm², often stricter), use conformal coating, avoid hygroscopic residues, control process water quality.
Case example: A batch of immersion silver‑finished boards failed ECM testing due to silver migration under bias humidity. The supplier had not sealed the boards after finish application, allowing tarnish and ionic residues. After adding a post‑finish cleaning and vacuum sealing, ECM passed.
Test Method Comparison – When to Use Which
| Method | IPC‑TM‑650 | Test Structure | Bias Voltage | Duration | Failure Criterion |
|---|---|---|---|---|---|
| CAF | 2.6.25 | Parallel vias or comb pattern embedded in laminate | 50‑100V (or operating voltage) | 500‑1000h | Resistance <10⁸ Ω, or filament visible in microsection |
| ECM (SIR) | 2.6.3.7 | Surface comb pattern (IP‑B‑25 or similar) | 5‑50V (typical 10‑20V) | 168‑500h (168h for screening, 500h for qualification) | Resistance <10⁷‑10⁸ Ω, or dendrite visual |
| Combined CAF/ECM (modified) | 2.6.3.7 with embedded vias | Embedded comb patterns + surface comb on same coupon | Up to 100V | 1000h | Both criteria (bulk and surface) |
Which Test Should You Run? – Decision Matrix
Choose based on design and risk factors:
- Run CAF test (2.6.25) if: spacing ≤0.5 mm between biased inner‑layer conductors (especially vias), high voltage (>50V), high layer count, or low‑quality laminate (standard FR‑4).
- Run ECM (SIR) test (2.6.3.7) if: any surface conductors with bias, fine‑pitch BGAs, exposed finishes (ImAg, ImSn), or known cleanliness issues.
- Run both tests if: both conditions exist (e.g., high‑density automotive board with fine‑pitch connectors and tight via spacing).
Material and Process Requirements – Prevention Strategies
| Strategy | CAF Prevention | ECM Prevention |
|---|---|---|
| Material selection | High‑CAF resin (low moisture absorption, high glass‑resin adhesion) | Low‑outgassing, non‑hygroscopic, ionic cleanliness control |
| Spacing/design | Increase spacing (>0.5mm), stagger vias, avoid long parallel runs | Add conformal coating, avoid exposed metal, use non‑migratory finishes (ENIG) |
| Process control | Lamination quality (no delamination), drill cleanliness | Post‑assembly cleaning, ion chromatography monitoring |
| Testing/qualification | IPC‑TM‑650 2.6.25 for new materials and designs | SIR per 2.6.3.7 on first article and after process changes |
| Field mitigation | Use of conformal coating (slows moisture ingress) | Same – conformal coating is highly effective |
Supplier Audit Checklist – CAF and ECM Prevention
- Does the supplier have documented CAF testing capability (2.6.25)? Have they qualified their laminate for your spacing/voltage?
- Do they perform ion chromatography (2.3.28) to ensure surface cleanliness for ECM prevention?
- What is their standard procedure for surface finish handling (e.g., immersion silver tarnish prevention)?
- Do they offer conformal coating application with qualification data (humidity cycling + SIR)?
- Can they provide historical CAF and SIR test results for the specific laminate and finish combination?
Related Engineering Resources
Frequently Asked Questions
A> Not typically. CAF requires embedded via or internal structures; ECM uses surface comb patterns. However, a combined coupon with both embedded vias and surface comb can be used if designed appropriately.
A> Both are serious. CAF is often harder to detect because it occurs inside the laminate and may not show up in electrical test until a short develops. ECM can be arrested by cleaning or conformal coating. For high‑voltage applications, CAF is especially critical.
A> No, IPC‑6012 does not mandate CAF. However, many customers (automotive, telecom, medical) require it as an add‑on. Always check the procurement specification.
A> Conformal coating is very effective against ECM (surface dendrites). It can slow moisture ingress and reduce CAF risk, but cannot eliminate CAF if the laminate already has pathways (cracks, voids). CAF prevention requires robust laminate quality.
A> For new materials or designs, run both during qualification. For ongoing conformance, ECM (SIR) is often run quarterly or per lot for high‑reliability products. CAF is less frequent (annually or after material/process change) due to test duration (1000h).
Reference standards: IPC‑TM‑650 2.6.25 (CAF), 2.6.3.7 (SIR/ECM), IPC‑TR‑476A (CAF), IPC‑J‑STD‑001 (cleanliness).
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