Solder Paste Transfer Efficiency: What the SPI Numbers Mean

Transfer efficiency is the ratio of the paste volume that actually lands on the pad to the volume the aperture could hold. It is the number that tells a process engineer how well the stencil is releasing, and it is the number most often quoted without its context.

A transfer of 100 percent is not the goal in every case, because a somewhat larger deposit is often used to compensate for shrinkage during reflow. What matters is that the value is known, stable and adequate for the smallest aperture on the board. A mean of 88 percent with a tight distribution is a healthier process than a mean of 95 percent with a long tail.

What Transfer Efficiency Measures

The ideal volume is the aperture area multiplied by the foil thickness, and the printed volume is what the paste inspection system measures on the pad. Dividing one by the other gives the efficiency, usually expressed as a percentage of the ideal.

Both terms carry uncertainty. The foil thickness varies across a tensioned stencil, the aperture is not a perfect prism after laser cutting, and the inspection system has its own resolution. A single deposit measured at 80 percent may be well within the noise, which is why the useful figure is a mean with a distribution rather than one number.

Area Ratio and the Working Limit

The area ratio is the area of the aperture opening divided by the area of the aperture wall. For a square opening of side D in a foil of thickness t it is D divided by four times t, and it is the geometric property that predicts how well the paste will release.

An area ratio of about 0.66 is the practical floor for a conventional print, and the transfer falls away quickly below it. Above 1.0 the deposit is close to the ideal, and between the two the result depends heavily on the paste and the aperture wall. The relationship is geometric, so it can be calculated before the stencil is ordered and used to choose the foil thickness, as the area ratio discussion sets out.

Print Volume as a Percentage of Ideal

Production limits are usually written as a window around the ideal volume, for example 80 to 120 percent for general work and 60 to 140 percent for very small apertures where variation is unavoidable. The window is chosen with the joint requirement in mind rather than from the printer alone. A window wider than the joint can tolerate is a window that hides defects.

The measurement method has to be stated with the limits. An inspection system that includes the area of a paste smear reports a larger volume than one that measures only the coherent deposit, and comparing the two without knowing which was used leads to arguments that no amount of data settles. The measurement itself is covered in the notes on paste volume measurement.

Where the Paste Is Lost

Paste is lost in four places: it adheres to the aperture wall, it is left as a film on the stencil underside, it is smeared outside the pad area, and it is pressed back onto the stencil by the separation. The first two dominate at small area ratios.

That is why transfer efficiency is a stencil property as much as a printer property. A change of aperture wall quality, a change of foil thickness or a change of separation speed all move the number, and none of them is visible in a review of the printer settings. It also means that a change of stencil supplier is a process change, even when the drawings are identical.

Stencil Release and Surface Coatings

Release is improved by making the aperture wall smoother, by reducing the wall area, and by lowering the surface energy of the wall so that paste prefers to stay on the pad. Smoothness is set by the cutting method, and its effect is largest exactly where the area ratio is smallest. A nano coating addresses the last of those, and the effect is a measurable gain on the smallest apertures.

The gain is typically in the region of five to fifteen percent on fine pitch openings, which is the difference between a marginal print and a comfortable one. Coatings also wear, so the improvement decays with cleaning cycles and print count, which is the subject of the notes on stencil coatings.

<img src="https://www.gopcba.com/wp-content/uploads/2025/08/19.png" alt="Stencil aperture wall after paste release under magnification” />

Vacuum Printing as a Process Change

Vacuum printing applies a pressure difference across the stencil so that the aperture is filled from the top while air is drawn from below. Filling improves markedly on small openings, and the transfer approaches the ideal even at area ratios that a conventional print cannot manage.

The equipment and the stencil have to be designed for it, since the vacuum has to reach the aperture without pulling the paste away from the pad. Where the product has a fine pitch area alongside coarse features, a vacuum assisted print can remove the need for a step stencil, at the cost of a more complex setup. The vacuum path also has to be cleaned, since paste drawn into the tooling changes the filling behaviour.

Reading SPI Data Without Overreacting

Paste inspection produces a large amount of data and very few decisions. The useful view is a control chart of the mean volume for the smallest aperture on each board, with the distribution of the individual deposits behind it.

Chasing individual outliers is usually wasted effort, because a single low deposit rarely becomes a defect. A shift in the mean, a widening of the spread or a change in the shape of the distribution are the signals that point at a process change worth investigating. The shape also separates a stencil fault from a paste fault, because a stencil fault moves a group of apertures together.

Limits That Match the Product

Volume limits should be derived from the joint that has to be formed. A power pad with a large thermal mass tolerates a wide volume range, while a fine pitch connector with a small pad has a narrow one, and both can appear on the same board.

That means the inspection program needs more than one limit, applied by aperture size or by component type. A single window applied to the whole board either wastes capacity on the easy features or lets the difficult ones pass. The limits can be reviewed once the correlation with joint quality has been established.

Correlating Prints With Joint Quality

Transfer efficiency earns its place when it predicts joints. Printing a board, measuring the deposits and then reflowing it to inspect the joints builds the correlation, and the exercise usually shows that a narrow volume band is only necessary on a few features.

The correlation should be repeated after any change of paste, stencil or printer. It is inexpensive, it takes a handful of boards, and it converts a number into a specification that the line can defend, and it gives the inspection programme a reason for the windows it applies.

FAQ

What is a good transfer efficiency? Around 80 to 100 percent of the ideal volume for most apertures, falling towards 60 percent on the smallest. The acceptable figure depends on the joint requirement rather than on an industry number.

Why does transfer fall on small apertures? Because the area ratio falls with aperture size. Below about 0.66 the paste adheres to the aperture wall more strongly than it adheres to the pad, and the deposit leaves material behind.

Does a coating remove the limit? It improves release, typically by five to fifteen percent on fine pitch apertures, but it does not remove the geometry. A poorly designed aperture still prints poorly with the best coating available.