Solder Paste Printing Defects and Control

Solder paste printing is the step that decides the outcome of the whole assembly process, because a deposit that is wrong cannot be corrected later. Too little paste produces an open joint, too much produces bridging, and a deposit that is the right volume but in the wrong place produces a defect that appears at reflow rather than at print. The good news is that printing is a process with a small number of variables, and the defects it produces have well understood causes.

How the Process Works

A squeegee pushes paste across a stencil, filling the apertures as it passes. The stencil then separates from the board, leaving the paste behind on the pads. The volume that remains depends on the aperture geometry, the paste rheology, the separation behaviour and the condition of the stencil and the board surface.

The aperture geometry sets the theoretical volume, which is the area multiplied by the stencil thickness. The transfer efficiency, which is the ratio of the actual volume to the theoretical volume, describes how much of that is achieved, and it falls as the aperture becomes smaller relative to the stencil thickness. This relationship is why fine pitch printing requires thin stencils and careful aperture design.

The separation step is where many defects originate. As the board drops away from the stencil, the paste has to release cleanly from the aperture walls and split from the paste that remains in the aperture. The speed and the manner of the separation determine whether the deposit is left intact or torn.

Common Defects and Their Causes

Insufficient paste is the most common defect, and it appears as a deposit that is smaller or thinner than the pad. The usual causes are an aperture that is too small for the stencil thickness, a clogged aperture, a squeegee pressure that is too high and scraping paste out of the hole, a separation that is too fast or a paste that has lost its tack and its flow.

Bridging is the second, and it appears as paste connecting adjacent pads. It comes from excess paste, from insufficient spacing, from a stencil underside that is contaminated so that paste smears across the surface, and from a paste that slumps after printing because it is too warm or too old. The stencil cleaning interval is often the deciding factor.

Slumping and paste spreading occur after the print and before reflow. A paste with too low a viscosity, a high ambient temperature or a long delay between printing and reflow all allow the deposit to lose its shape, and the defect is discovered only at inspection after reflow. Recording the time between print and reflow makes this cause visible.

Print Parameters

Squeegee pressure should be the minimum that gives a clean wipe, because excess pressure deforms the stencil, wears the squeegee and forces paste out of the apertures. The correct pressure is found by increasing it until the stencil is clean and then backing off slightly, and it should be checked as part of the setup.

The squeegee speed determines how much paste is pushed into the aperture and how much is left on the stencil surface. Slower speeds generally fill the apertures more completely and are used for fine features, while faster speeds improve throughput at the cost of transfer efficiency. The setting should be chosen for the finest feature on the board rather than for the average.

Separation speed is the parameter most often left at a default. A slow separation allows the paste to release from the walls and improves the deposit, while a fast separation increases throughput and causes torn deposits. For fine pitch work the separation should be slowed, and the improvement is usually visible immediately in the print inspection results.

Stencil and Squeegee Condition

The stencil is a consumable and it changes with use. The aperture walls lose their plating, the tension falls and the underside becomes contaminated. Each of these degrades the transfer efficiency, and the degradation is gradual enough that it is attributed to other causes. The stencil tension should be checked periodically, and the stencil should be replaced when it falls outside its range.

The squeegee itself wears and develops a rounded edge, which reduces the pressure it applies at the stencil surface and reduces the volume of paste pushed into the aperture. A worn squeegee produces a gradual decline in paste volume that is easy to mistake for a paste problem, and the blade should be inspected on a schedule rather than when a defect appears.

The underside of the stencil should be cleaned automatically at defined intervals during the print run. The interval depends on the aperture size and the paste, and it should be short enough that no bridging appears between cleans. Running without in process cleaning is a decision to accept a rising defect rate over the run.

Paste Handling

Paste is a perishable material with a shelf life, a working life after it has been opened and a behaviour that changes with temperature. It should be stored cold, allowed to reach room temperature in its sealed container before use, and used within the documented window. Paste that is used cold prints differently, and paste that has been open for a shift loses solvent and tack.

The paste on the stencil should be replenished in small amounts rather than in one large addition, because a large bead of paste sits on the stencil for longer and changes consistency. The old practice of adding a full cartridge at the start of a run produces a paste condition that varies from good at the beginning to poor at the end.

Recording the paste lot number with the batch is worth the few seconds it takes. When a printing problem appears, the first question is whether the paste changed, and the answer should be available rather than reconstructed from the delivery notes.

Verification and Control

Defect control on the print process begins with verification, which is done with a paste inspection system, either in line or as a separate step, and the measurements that matter are the volume per deposit and the position. Volume is the better indicator of the process than height or area alone, because it is the quantity that determines the joint.

The inspection data should be trended rather than only compared against limits. A gradual decline in volume over a run indicates a worn squeegee, a clogging stencil or a paste that is drying, and the trend is visible before the limits are reached. This is the point at which the data becomes useful for control rather than for sorting.

The print parameters should be recorded with the batch, together with the stencil identity and the paste lot. When the same board is run again later, the settings are available, and when a defect appears the variables can be compared rather than guessed at.

Practical Rules

Use the minimum squeegee pressure that gives a clean wipe, set the speed for the finest feature on the board and slow the separation for fine pitch work. Check the stencil tension and the squeegee edge on a schedule, and clean the stencil underside at an interval that prevents bridging.

Control the paste by storage, by warm up and by working life, and record the paste lot and the print parameters with the build records. Print quality is the foundation of joint quality, and the paste inspection data and the defect list together show whether the process is being controlled or merely monitored.

FAQ

What is the most common printing defect? Insufficient paste. It usually comes from an aperture that is too small for the stencil thickness, a clogged aperture or too much squeegee pressure.

Why slow the separation? Because the paste needs time to release from the aperture walls. A fast separation tears the deposit, particularly at fine pitch.

How often should the stencil be cleaned? At an interval short enough that no bridging appears between cleans. The interval depends on the aperture size and the paste.