LED Illumination for PCB and Electronics Inspection: Soldering, Components and Trace Verification

PCB and electronics inspection is one of the most demanding vision applications. Component sizes shrink with every product generation. Solder joints are often smaller than 0.5 mm. Moreover, gold pads, tin coatings and solder paste all reflect light differently. Therefore, an approach that works for one task may be useless for another on the same board.

The engineer must treat PCB inspection as a multi-task problem. Different tasks need different geometries, often on the same line. Therefore, understanding the optics of each target is the starting point. This page maps each task to the right technique and lists the matching RODER Vision illuminators.

The PCB Challenge: Miniaturisation and Reflective Surfaces

A modern PCB packs many surface types into a small area. Bare FR4, solder mask, paste, joints, copper traces, gold pads and markings may all sit within a few square centimetres. Each material has a different reflectance and texture. Consequently, each responds differently to direct, diffuse, coaxial and darkfield light.

Miniaturisation makes this harder. As packages shrink from QFP to BGA to CSP, the resolution demand rises. Higher magnification gives a smaller field of view. Therefore, the light must hold intensity and uniformity over a smaller area. At the same time, it must keep the correct geometry for the task.

Solder Joint Inspection: Coaxial and Darkfield

Solder joint inspection is the most critical SMT task. The goal is to detect insufficient or excess solder, bridges, cold joints and tombstoning. Each defect has a different 3D form. Therefore, each has a distinct optical signature.

Coaxial Illumination for Flat Solder Surfaces

A good joint on a flat pad has a mirror-like top surface. Coaxial light travels along the camera axis. Therefore, it returns a bright, uniform reflection from that flat surface. A defective joint is rough or tilted, so it reflects less light back. As a result, it appears darker. This brightness difference is the basis of coaxial solder inspection.

Coaxial light also suits gold pads before soldering. The gold surface is highly specular, so it appears uniformly bright. However, contamination, oxidation or damage disrupts the reflection. Therefore, the defect appears as a darker region. This allows pre-reflow pad inspection as well as joint verification.

Darkfield Illumination for Solder Defects

Low-angle darkfield light suits raised solder defects. Under grazing incidence, the flat mask and pads appear dark. However, raised features scatter light and appear bright. Bridges, solder balls and lifted leads are typical examples. Therefore, small defects become visible even at a fraction of a millimetre in height.

Component Placement Verification: Backlight and Ring

Placement verification checks presence, orientation and position tolerance. Different components need different light. Therefore, the choice follows the package type.

Backlight for Leaded Component Outlines

Leaded through-hole and large SMT packages show visible lead outlines. For these, backlight gives the highest contrast for lead position and bend. The leads appear as dark silhouettes against the bright field. Therefore, coplanarity, lead spacing and missing leads are measurable with sub-pixel accuracy.

Ring Light for Presence and Polarity

Ring light gives even front illumination from above the board. Therefore, it suits component presence and polarity checks. It also reads markings on flat package tops. A shallow angle adds contrast for body edges and polarity indicators.

Trace and Via Inspection on Bare Boards

Bare board inspection checks the copper before assembly. It verifies trace continuity, width, spacing and via presence. The target is the copper pattern on the solder mask. Therefore, coaxial light works well, giving even contrast on the reflective copper.

Via Inspection

Under coaxial light, via holes appear as dark circles against bright copper or mask. Therefore, the diameter, spacing and fill status are measurable. An unfilled via that should be plated looks different from a filled one. For thin boards, backlight can also transmit light for via inspection.

IC Marking and Serialisation Reading

Marking verification and code reading are common electronics tasks. The mark is laser-etched or ink-printed on a plastic or ceramic body. The contrast depends on the marking type, package colour and light. Therefore, a direct or low-angle light is chosen to maximise it. For laser marks, low-angle light raises the relief contrast.

Handling Gold and Tin Surfaces Without Glare

Gold connectors, tin leads and wetted solder are very specular. Under direct front light, they create intense hotspots. These saturate the sensor and hide the features. Therefore, three approaches are used to manage them.

The first is coaxial light. It uses the specular reflection constructively along the camera axis. Therefore, flat, perpendicular features appear bright, while tilted ones appear dark.

The second is dome or flat dome light. It fills the hemisphere of angles, so reflections spread across the dome interior. Consequently, it removes hotspots, at the cost of some feature contrast.

The third is polarisation. A polariser on the light and a cross-polarised analyser on the lens block specular glare. Therefore, only the diffusely scattered light passes through. As a result, glare from gold and tin disappears, revealing the hidden features.

RODER Vision Illuminator Families for PCB Inspection

The families below address the common PCB inspection needs. Each one offers high uniformity, stable output and strobe support. Therefore, the right choice follows the task and the surface.

The right choice depends on the task. For solder joints and pads, coaxial or DC6 ring light leads. Gold and tin glare instead calls for the FD2 dome or polarisation. Traces and vias suit the DL6 matrix, which gives strong contrast. Lead outlines favour the BL3 backlight, which is ideal. In every case, RODER Vision provides engineering support for geometry and wavelength. Therefore, define the target, the surface and the defect first, and then choose the matching light.

Frequently Asked Questions