PCB Failure Analysis Flowchart
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PCB Failure Analysis Flowchart – A Systematic Root Cause Investigation Process
No structured process – root cause never found
A medical device manufacturer experienced intermittent field failures. Three different failure analysis labs were hired, each producing different conclusions (solder joint fatigue, CAF, pad cratering). The failures continued. The problem? No structured analysis process. Each lab started with a different hypothesis and only tested that hypothesis. A systematic, step‑by‑step flowchart would have forced a logical progression from symptoms to root cause. This document provides that flowchart.
The Seven‑Step Failure Analysis Process
Each step must be completed before moving to the next. Skipping steps leads to inconclusive or incorrect root causes.
Data Collection
NDT Screening
Electrical Test
Cross‑Section
Chemical/Material
Root Cause
Corrective Action
Detailed Analysis Steps and IPC‑TM‑650 Methods
Step 1 – Data Collection and Initial Assessment
Objective: Gather all contextual information before any destructive testing.
- Field environment: temperature range, humidity, vibration spectrum, voltage bias
- Failure mode: intermittent open, hard short, degraded insulation resistance, signal integrity loss
- Failure rate: isolated (single board) vs systemic (entire lot)
- Timeline: time from assembly to failure (infant mortality, random, wear‑out)
- Reference board: obtain a known‑good board from same production lot for comparison
Step 2 – Non‑Destructive Testing (NDT) Screening
Objective: Identify failure location without destroying the board.
| Method | IPC‑TM‑650 | What It Detects |
|---|---|---|
| Visual (10‑50×) | N/A | Solder mask defects, pad lifting, discoloration, obvious cracks |
| X‑Ray | 2.4.52 | Voids, misregistration, broken vias, missing pads |
| C‑SAM | 2.6.21 | Delamination, cracks at laminate interfaces |
| Electrical test | 2.5.1/2.5.2 | Pinpoint failing nets (continuity, isolation) |
Step 3 – Electrical Stress and Failure Reproduction
Objective: Reproduce the failure under controlled conditions.
- Apply thermal cycling (2.6.7) at field‑relevant extremes to see if crack propagates
- Perform bias humidity testing (2.6.3.7) for CAF/ECM suspicion
- Run TDR (2.5.5.13) to locate impedance discontinuities
Step 4 – Destructive Analysis (Microsection and Dye & Pry)
Objective: Examine internal structure and failure interface.
| Method | IPC‑TM‑650 | What It Reveals |
|---|---|---|
| Microsection | 2.1.1 | IMC thickness, voiding, cracks, plating voids, inner layer breakout |
| Inner layer ring | 2.1.5 | Inner layer annular ring (hidden defects) |
| Dye & pry | 2.4.57.2 | Cracks in BGA solder joints under components |
Step 5 – Chemical and Material Analysis
Objective: Identify contamination, material degradation, or outgassing.
- Ion chromatography (2.3.28) – quantify ionic residues (Cl⁻, Br⁻, Na⁺, etc.)
- FTIR – identify organic contamination (flux residues, oils)
- SEM/EDS – elemental analysis of corrosion products or IMC composition
Step 6 – Root Cause Determination
Objective: Identify the primary cause based on all evidence.
| Observed Failure | Likely Root Cause(s) | Confirmatory Test |
|---|---|---|
| Intermittent open (BGA) | Solder fatigue, pad cratering, black pad | Dye & pry, microsection, SEM/EDS |
| Low insulation resistance | CAF, ECM, ionic contamination | CAF/SIR (2.6.25/2.6.3.7), IC (2.3.28) |
| Via open (PTH) | Plating void, barrel crack, wedge void | Microsection (2.1.1) |
| Microvia open | Corner crack, target pad miss, resin recession | Microsection (2.1.1) |
| Delamination / blister | Moisture, poor lamination, thermal stress | C‑SAM (2.6.21), microsection (2.1.1) |
Step 7 – Corrective Action and Verification
Objective: Implement and verify fix.
- Design change (increase pad size, change via structure)
- Material change (higher‑Tg laminate, high‑CAF resin)
- Process change (improved cleaning, tighter registration)
- Supplier corrective action (8D report)
- Verification testing (re‑run qualification or focused stress test)
Failure Analysis Checklist – 10 Questions to Answer
- Is the failure isolated to one board or systemic across a lot?
- Did the failure occur during assembly, test, or field operation?
- What is the field environment (temperature, humidity, vibration, voltage)?
- Has the same failure occurred before? (If yes, previous root cause?)
- Is the failure location consistent (e.g., same via, same BGA corner)?
- Have non‑destructive tests (X‑Ray, C‑SAM) revealed anything?
- Has microsection been performed on the failing net?
- Are there any anomalies in production records (material lot, process drift)?
- Does the reference board (good) show the same features?
- What specific corrective action will prevent recurrence?
Related Engineering Resources
Frequently Asked Questions
Q: What is the most common mistake in failure analysis?
A> Skipping non‑destructive testing (X‑Ray, C‑SAM) and going directly to microsection. Without NDT, you may miss the failure location and section the wrong area.
Q: How many microsections are typically needed?
A> Start with 2‑3 cross‑sections of the suspected failing net. If inconclusive, increase to 5‑10 across the board. Always section a known‑good board as reference.
Q: Can a failure be caused by multiple root causes?
A> Yes – often a design weakness (e.g., thin annular ring) combined with a process issue (e.g., drill offset) triggers failure. Identify all contributing factors.
Q: What is the difference between root cause and contributing cause?
A> Root cause is the primary reason the failure occurred (e.g., black pad). Contributing cause accelerates or enables the failure (e.g., high humidity). Fix root cause first.
Q: How long does a typical failure analysis take?
A> Simple failures (visual or X‑Ray visible): 1‑2 days. Complex failures requiring microsection and chemical analysis: 1‑2 weeks.
Reference standards: IPC‑TM‑650 (2.4.52 X‑Ray, 2.1.1 microsection, 2.4.57.2 dye & pry, 2.6.21 C‑SAM, 2.6.3.7 SIR, 2.6.25 CAF).
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