The First Quality Gate — Before a Single Component Is Placed
A PCB that fails during assembly costs far more than a PCB rejected at incoming inspection. Components are wasted. Machine time is wasted. And if the defect is subtle — a near-separation in an inner layer, a thin spot in a via wall — the board may pass ICT and FCT, ship to the customer, and fail in the field.
IPC-A-600 is the standard that prevents this. It defines what is visually acceptable on a bare (unpopulated) printed circuit board. While IPC-6012 defines the performance requirements that a PCB fabricator must meet, IPC-A-600 provides the photographic reference images and written criteria that an inspector uses to judge whether a board is acceptable.
At Superb Automation, every incoming PCB lot is sampled and inspected against IPC-A-600 before release to production. If a board doesn't meet the acceptance criteria, it doesn't enter the SMT line.
What IPC-A-600 Covers
The standard is organized by defect type and board feature. Key categories:
Surface Defects
| Defect | What It Looks Like | Why It Matters |
|---|---|---|
| Measling | White spots or crosses at weave intersections in the laminate | Indicates internal delamination from thermal or mechanical stress. Can grow into a conductive path between layers. |
| Crazing | Connected white spots forming a network pattern | More advanced than measling; indicates laminate fracture. Usually cause for rejection in Class 3. |
| Blistering | Raised areas where laminate layers have separated | Trapped moisture or volatiles expanding during thermal stress. Always a defect. |
| Delamination | Separation between laminate layers or between laminate and copper | Loss of structural integrity. Reject — the board will fail under thermal or mechanical load. |
| Weave exposure | Glass fibers visible on the surface | Can trap contaminants and cause CAF (conductive anodic filament) growth in humid environments. |
| Haloing | Light area around a drilled hole in the laminate | Indicates mechanical damage during drilling. May affect hole wall quality. |
Plating Defects
| Defect | What It Looks Like | Why It Matters |
|---|---|---|
| Voids in hole wall | Missing plating in the barrel of a plated through-hole | Creates an open circuit or a high-resistance connection. Must meet minimum plating thickness per IPC-6012. |
| Nodules | Rough bumps on the plated surface | Can cause solder bridging or prevent proper component seating. |
| Burrs | Raised metal at hole edges | Can puncture solder mask, cause shorts, or prevent flat seating of components. |
Solder Mask Defects
| Defect | What It Looks Like | Why It Matters |
|---|---|---|
| Solder mask on pads | Green/colored residue on pads that should be bare copper | Prevents solder wetting — the joint will fail. |
| Solder mask skip | Missing mask where it should cover a trace | Exposed copper can corrode or short to adjacent conductors. |
| Solder mask blistering | Bubbles or lifted mask | Can break during soldering, exposing traces. |
| Solder mask registration | Misalignment between mask and pad pattern | Can encroach on pads or leave traces exposed. |
Dimensional and Physical Defects
Bow and twist — board is not flat. IPC-A-600 specifies maximum allowable deviation: 0.75% for SMT boards (surface mount), 1.5% for through-hole only.
Hole breakout — the drilled hole breaks through the edge of the annular ring. Reduces the solderable area and weakens the connection.
Scratches and cuts — mechanical damage to conductors. A scratch that reduces the conductor cross-section by more than 20% is a defect.
How We Use IPC-A-600 at Superb
Our incoming QC process for bare PCBs:
Lot sampling: Per ANSI/ASQ Z1.4 (or customer-specified AQL), a sample of boards from each incoming lot is selected.
Visual inspection: Under 10×-40× stereo microscope with ring light illumination. The inspector checks a representative set of features on each sample board — surface condition, hole quality, solder mask registration, conductor integrity.
Dimensional check: Critical dimensions (board outline, hole positions, slot dimensions) are verified against the fabrication drawing.
Cross-section (destructive): For new suppliers or high-reliability orders, one sample board is cross-sectioned to inspect inner layer registration, plating thickness, and laminate integrity — the visual checks IPC-A-600 doesn't cover.
Pass/fail decision: If the sample passes, the lot is released to production. If it fails, the lot is quarantined and the PCB fabricator is notified with photographic evidence of the defect.
Class 2 vs. Class 3 — What Changes
IPC-A-600 has the same Class 1/2/3 structure as IPC-A-610. The difference between Class 2 and Class 3 is particularly important for bare PCBs:
| Feature | Class 2 (Most Industrial) | Class 3 (High-Reliability) |
|---|---|---|
| Measling/crazing | Acceptable if limited in area, not bridging conductors | Typically reject |
| Hole void size | Up to 5% of hole wall area | Up to 2.5% of hole wall area |
| Nodules | Acceptable if not reducing spacing below minimum | Stricter — smaller nodules, especially in fine-pitch areas |
| Solder mask registration | Mask may encroach on pad by up to 50µm | Stricter — less encroachment allowed |
| Scratch depth | May not reduce conductor by >20% | May not reduce conductor by >10% |
What IPC-A-600 Means for Your Boards
A board that passes IPC-A-600 incoming inspection has been verified to meet the physical quality standards required for reliable assembly. It won't delaminate during reflow. Its holes won't produce defective solder joints. Its mask won't peel or cause shorts.
If a PCB fabricator delivers you boards that consistently pass IPC-A-600 Class 2 or Class 3, they have their process under control. If a contract manufacturer doesn't perform IPC-A-600 incoming inspection, they're gambling that the PCB fabricator got it right — and when they don't, the cost lands on you.
At Superb, we don't gamble.
