Solder Joint Inspection Under Low-Angle Light

Solder is a mirror. What an inspector sees on a joint is mostly the reflection of whatever light is aimed at it, which is why the same joint can look perfect under one lamp and defective under another. Low-angle light, directed across the board rather than down onto it, converts the shape of a fillet into a visible gradient: a concave fillet catches light along its length, while a convex or flat surface throws it away. Learning to read that gradient is the core of the inspection technique.

Why the Light Angle Decides What You See

Under diffuse overhead light a joint is illuminated uniformly from all directions, and the reflection is dominated by the surface condition rather than by the geometry. That makes it easy to see contamination and discolouration, and almost impossible to judge the shape of the fillet. Under a low-angle beam, the surface that faces the light brightens and the surface that faces away darkens, so the curvature of the fillet becomes visible as a band of light with a defined edge.

The useful range is roughly 10 to 25 degrees above the board surface for a fillet of the size found on a 0.5 mm pitch part, and somewhat lower for a large through-hole fillet. Beyond about 30 degrees the effect weakens and the joint looks flat again. A two-lamp arrangement, one on each side at a low angle, is more useful than a single lamp because the shadow on one side of the fillet is filled by the other, and the band of light then describes the whole fillet rather than half of it.

Reading the Fillet and the Wetting Angle

A properly wetted fillet is concave and rises from the pad to the termination at an angle that is shallow at the pad and steep at the lead. Under low-angle light this appears as a smooth, bright band that narrows towards the top of the lead. The absence of that band, or a band that is interrupted or irregular, is the visual signature of a joint that has not wetted properly.

The wetting angle itself is judged at the point where the alloy meets the pad. A low contact angle, meaning the alloy feathers out onto the pad, indicates good wetting; a high contact angle, where the alloy meets the pad steeply, indicates a contaminated or oxidised surface. The check is best made with the light source behind the joint from the viewer’s perspective, so that the interface is outlined by the shadow. Where the joint is a no-clean joint, a small fillet and a visible wetting line are acceptable, but a joint that has balled up on the pad is not.

Detecting Lift, Cracks and Disturbed Joints

A lifted lead shows as a gap between the alloy and the lead, and the gap is easiest to see when the light passes through it. Aiming the beam along the lead rather than across it makes the gap a bright line. Where the lead is short or the joint is behind a component, a small mirror or a fibre-optic probe allows the light to be placed behind the joint without moving the board.

Cracks are visible as a fine dark line across the fillet, usually following the boundary between the alloy and the intermetallic layer. They appear most often after thermal cycling and are hard to see under diffuse light because the crack edge reflects in the same way as the surrounding surface. Under low-angle light, the crack interrupts the bright band and appears as a break in it. A disturbed joint, made while the alloy was partially solid, has a rough, dull surface that scatters light in all directions, and it is one of the few defects that is easier to see under diffuse light than under a low-angle beam; using both is therefore worth the extra lamp.

Magnification and Working Distance

Magnification is a trade against field of view and depth of field. At 3x to 5x an inspector can see a whole fine-pitch device and judge the fillets together, which is the right level for a general inspection. At 10x the individual fillet can be judged and the wetting line seen clearly, but the field of view is small and the board has to be moved. Above 20x, the depth of field becomes so short that the surface texture is visible but the overall shape of the fillet is not.

The choice should be driven by the defect being sought rather than by a preference for more magnification. Wetting and fillet shape are judged at 5x to 10x, cracks at 10x to 20x, and contamination at any magnification where the surface can be seen. A stereo microscope with a long working distance is more useful than a higher-magnification instrument with a short one, because it allows the light to be introduced at a low angle, which a short working distance does not.

Tools: Lamps, Rings, Fibre Optic and Digital

A ring light mounted around the objective produces diffuse light from all directions and is the worst choice for judging fillet shape, because it removes the shadow that carries the information. A ring light with a diffuser and variable segments is better, because individual segments can be switched on to produce a directional beam. A fibre-optic gooseneck or an LED bar with a flexible arm allows the angle to be set precisely and is the most useful arrangement for bench inspection.

Digital systems add the ability to record and to compare, at the cost of the view being two-dimensional. A system with a movable light or with multiple lights at different angles can reproduce the low-angle effect, and the images can be retained as evidence. Where a digital system is used for acceptance, the light angles and the exposure should be fixed and recorded, because a change in either alters what the operators see and therefore what they accept.

Documenting What Was Seen

An inspection result that cannot be reproduced by another person is not a result. Where a joint is judged marginal, the record should identify the board, the reference designator, the light angle used and the magnification, so that the same joint can be re-examined by a reviewer. A photograph taken under the same conditions is the best evidence, and a photograph taken under different conditions can be worse than no photograph at all.

Boundary samples are the other tool. Keeping physical samples of an acceptable joint and of a marginal one, made from the same product and photographed under the inspection lighting, gives the inspector a reference that resolves most disagreements. The samples should be replaced when the product or the joint geometry changes, and they should be stored so that the surfaces do not oxidise. The acceptance criteria themselves should name the light angle and the magnification, because a change in either changes the judgement, and the visual inspection checklist is the natural place to record which angle and magnification apply to each joint family.

Common Misjudgements

The most common error is to judge a joint under whatever light happens to be on the bench and then treat the judgement as objective. Closely related is the habit of judging a fillet by its brightness rather than by its shape: a shiny joint that is convex reflects strongly and can appear healthy, while a properly concave but slightly dull joint can appear defective. The remedy is to set the light deliberately and to judge the shape first, then the surface.

Another frequent error is inspecting a board before it has cooled completely. A joint that is still warm has a different surface appearance and a fillet that is still relaxing, and an inspector who looks at a board straight from the oven will see a different joint from the one that will be shipped. Where the board is inspected immediately after reflow, the inspection should be treated as a process check rather than as a final acceptance. The inspection standard for the product should state the point in the process at which the acceptance inspection is made.

Training an Inspector

Inspection is a learned skill and it decays without practice. A training programme should include the physical setting up of the light, the recognition of each defect type under the correct angle, and the use of boundary samples. Written criteria are necessary but not sufficient: an inspector who has never seen a lifted lead under a low-angle beam will not recognise one from a description.

Periodic requalification keeps the skill alive and gives the shop evidence that the inspection is consistent between inspectors, which is what a customer audit will look for. A simple requalification uses a set of boards holding known defects and known-good joints, presented without identification, and requires the inspector to classify them. The workmanship training standards and the certification arrangements describe the structure of such a programme, and both should be tied to the specific products the inspector will see.

FAQ

Is a bright joint always a good joint? No. Brightness describes the surface, not the shape. A shiny, convex joint that has not wetted the pad can look better than a properly formed joint with a matte surface. Judge the fillet shape and the wetting line first, and treat brightness as a secondary indicator.

Can inspection be done from a photograph? For a documented record, yes, provided the photograph was taken under controlled lighting. For a decision on a marginal joint, no, because the photographer chose the angle and the reviewer cannot change it. Marginal calls should be made on the physical part.

Does a low-angle lamp need to be a special type? No. Any light that can be positioned at a low angle and that is bright enough to see the fillet clearly will do. A flexible LED bar or a fibre-optic gooseneck is convenient, but a bench lamp placed close to the board and angled across it produces the same effect.