Solder Paste Inspection Guide

Solder paste inspection measures the deposit on every pad before a component is placed. It measures a process that has no other in line check, and it does so at the point where a correction is still cheap, which is why the data is more valuable than the pass and fail decision that the machine produces. Understanding what the numbers mean is what turns the inspection from a gate into a control.

What Is Measured

A three dimensional system projects a pattern onto the board and reconstructs the height of the deposit. From the height map it derives the volume, the area and the maximum height for each pad, and compares each of them with the program limits.

Volume is the measurement that correlates best with the joint that will be formed, because the solder that ends up in the joint comes from the whole deposit. Area and height describe the shape, and a deposit can have the right volume with an area that is too large and a height that is too low, which behaves differently during reflow.

The measurements are expressed as a percentage of the nominal value from the stencil design, so a deposit at ninety percent of the intended volume is reported as ninety percent regardless of the absolute size.

Why It Is Worth Measuring Paste

Printing is the step that generates most of the defects that appear after reflow. A deposit that is short produces an open or a weak joint, one that is excessive produces bridging and solder balls, and one that is misplaced produces a tombstone.

By the time these defects are visible, the board has been through the oven and the components have been consumed. Rejecting a printed board costs a cleaning step, while rejecting it after reflow costs the components and the labour.

The data also separates the causes. A deposit that is uniformly low points at the paste or the foil, one that is low on one side of the board points at the printer, and one that varies between pads of different sizes points at the aperture and the area ratio.

Setting the Limits

The limits should come from the process capability rather than from a generic table. Measuring a production board for a few days and calculating the distribution gives a centre and a spread, and the limits are then set at a distance from the centre that produces a useful signal without flooding the line with false rejections.

Volume limits are usually expressed as a percentage window around the nominal, while the area and height limits are wider because the deposit shape varies more than its volume. A tight volume limit with a loose height limit catches the deposits that matter.

Different pad types need different limits. A fine pitch aperture releases less consistently than a large one, and applying the same window to both will either reject most of the fine pitch pads or ignore the problems on the large ones.

Acting on the Data

A single out of limit pad is a local event, most often a stencil that needs cleaning or a pad with an obstruction. A group of pads that are all low is a printer setting, a paste condition or a foil issue.

The trend is more informative than the value. A process that sits at the centre of the window today and drifts steadily across the shift is telling you about a paste that is drying, a stencil that is becoming clogged or a squeegee that is wearing.

The inspection should therefore feed a chart rather than only a reject bin. Reporting the average and the spread per board, and reviewing them at the end of a shift, is what makes a slow drift visible before it becomes a defect.

Limits of the Method

The system cannot see under a stencil, so a deposit that is correct on the board but wrong in the aperture is undetectable. It also cannot measure inside a hole, which matters for a pin in paste process where the paste is printed into a barrel.

A measurement on a reflective or a dark surface is less reliable, and a paste that is very glossy can produce a reflection that the sensor interprets as a different height. The programmes should be validated against a known sample rather than assumed to be correct.

The inspection says nothing about the paste itself. A deposit of the correct volume from a paste that has dried out or that has separated will still fail at reflow, so the material controls remain necessary alongside the measurement.

Integration with the Line

The inspection can run after printing and before placement, which gives the earliest warning, or it can run on a sample basis after placement. In line inspection on every board gives the most data and the highest cost per board.

The feedback loop to the printer is the most valuable integration. Where the inspection can adjust the printer automatically, the drift is corrected without an operator, and where it can only alarm, the response depends on someone noticing the alarm.

Sampling is a reasonable compromise for a stable process. Measuring every tenth board and reviewing the trend gives most of the benefit at a fraction of the cycle time cost, provided that the sample is taken at a fixed interval rather than when a problem is suspected.

Correlation with Reflow

The value of the inspection depends on the correlation between the deposit and the joint. That correlation should be established once, by measuring a set of boards and then examining the joints, so that the limits are known to correspond to something.

The correlation is not the same for every package. A large pad tolerates a wide volume range because the fillet can absorb the difference, while a small pad with a fine pitch has a much narrower useful window.

Where the correlation has been established, the limits can be tightened without increasing the false rejection rate, and the inspection becomes a control rather than a screen.

Practical Rules

Measure the volume, the area and the height, set the limits from the capability of the process and different limits for different pad types, and review the trend rather than the individual reading.

Record the data with the build records and the defect history, and compare it with the SPI practice and the printing defects whenever a reflow defect appears without a paste alarm.

Additional Considerations for This Build

Practical attention to process control pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating process control explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Process Control and Verification

On a design of this kind, area is the item that decides how the rest of the board is arranged. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

FAQ

Which measurement correlates best with the joint? Volume. The solder in the joint comes from the whole deposit, so a short volume is the most reliable predictor of a weak or open joint.

How should the limits be set? From the capability of the process, measured over several days, with different windows for fine pitch and large pads rather than one generic table.

What is the main limitation? The system cannot see paste inside a hole or under a stencil, and it says nothing about the condition of the paste itself.