Solder Paste Metal Loading And Print Quality

Solder paste is a suspension of metal powder in a flux vehicle, and the metal loading is the fraction of its mass that is metal. A typical paste is between eighty five and ninety percent metal by weight, which corresponds to roughly half its volume. That figure sounds like a small detail and it decides most of the printing behaviour of the paste.

This article covers what the loading means, how it affects printing and slump, and how it interacts with the stencil and the joint.

What Metal Loading Means

A high metal loading means less flux and less solvent in a given volume of paste, so the deposit contains more solder and shrinks less during reflow. A low loading means more vehicle, so the deposit contains less metal and leaves more residue. The volume of the deposit is set by the stencil aperture, so the loading determines how much metal that volume actually delivers.

The relationship between mass and volume matters because the two are quoted differently. A paste at ninety percent by weight may be only about fifty percent by volume, because the metal is far denser than the flux. When a specification gives a figure it should say which basis is used, and when a joint is short of solder the volume fraction is the number to examine. The way the deposit is formed on the pad is described under placement order and pad positioning.

Effect On Viscosity And Printing

Adding metal raises the paste viscosity, because the particles have to move past each other for the material to flow. A higher loading gives a paste that holds its shape after printing and resists slumping, and a lower loading gives a paste that flows more readily and releases more easily from the aperture. Both behaviours matter, and they pull in opposite directions.

The printing window is the region in which the paste releases cleanly from the stencil and then holds the shape of the aperture. A paste whose viscosity is too low smears on release and slumps afterwards; one whose viscosity is too high does not fill the aperture evenly and leaves a deposit with voids or a rounded top. The viscosity is also shear dependent, which is the behaviour described under solder mask ink thixotropy in the context of mask ink, and it is what allows the paste to flow under the squeegee and then stop flowing once printed.

Slump And Deposit Shape

Slump is the movement of the deposit between printing and reflow. A deposit that slumps spreads beyond the pad boundary, and the paste that lands on the mask reflows into separate balls rather than joining the joint. Slump also reduces the height of the deposit, which reduces the volume of metal available and can leave a joint that is thin.

The resistance to slump comes from the loading and from the flux chemistry. A paste with a high loading and a well controlled yield stress holds its shape, and one with a low loading or an excess of solvent does not. The measurement is a slump test, in which a printed pattern is left for a defined period at a defined temperature and then compared with the pattern immediately after printing. A paste that passes the test on a test pattern may still slump in a hot room, so the test conditions should match the area where the printer actually stands.

Interaction With The Stencil

The stencil sets the volume, and the paste determines whether that volume is filled and released. The area ratio of the aperture, which is its opening area divided by its wall area, decides how much of the paste adheres to the wall rather than transferring to the pad. A low area ratio transfers poorly, and one way to compensate is a paste that releases more easily, which usually means a slightly lower loading.

The deposit also has to release without leaving a residue of paste on the stencil, because that residue accumulates and dries and is then deposited on a later board. The cleaning interval and the paste chemistry both contribute, and both are part of the printing process rather than of the paste alone. The design measures that make the printing window wider are described under design guidelines for manufacturability.

<img src="https://www.gopcba.com/wp-content/uploads/2026/05/smart-healthcare-PCBA.jpg" alt="Paste deposit volume measured after printing” />

Reflow And Residue

A high loading leaves less flux per unit volume, so the flux has less capacity to remove oxide from the powder and from the joint surfaces. That is the reason a paste with a very high loading needs a more active flux to compensate, and the reason such a paste can be less forgiving on an oxidised board. The same reduction in flux means less residue after reflow, which is an advantage for a no clean process.

A low loading leaves more flux and more residue, which gives more oxide removal capacity and more tolerance of a contaminated surface, at the cost of a residue that may need cleaning and of a deposit that shrinks more during reflow. The shrinkage matters for a joint whose volume is critical, because the paste volume that produced the joint was larger than the metal it contained.

Choosing And Controlling

The choice of loading is made with the printing requirement rather than in isolation. A fine pitch footprint printed with a thin stencil has a low area ratio and benefits from a paste that releases well, which usually means a slightly lower loading. A footprint that needs a large joint volume benefits from a higher loading, because the aperture can then be smaller for the same metal. Where both requirements exist on the same board, the paste is chosen for the more difficult one.

Control of the loading in production is indirect, because the loading is a property of the paste as supplied. What the process controls is the paste’s condition: the storage, the warming before the container is opened, the time on the stencil and the mixing. Paste that has lost solvent has effectively a higher loading and prints differently from the same paste when fresh, and paste that has absorbed moisture behaves as though it were lower. Both effects are controlled by the interval between printing and reflow, and that interval belongs in the process specification with a number rather than a note.

Two complaints account for most paste problems reported from the line. The first is a deposit that looks correct but produces a joint short of solder after reflow, which usually means the paste has lost solvent, so its effective loading has risen while the printed volume has fallen. The second is a deposit that spreads before reflow, which means the paste has absorbed moisture or its yield stress has fallen through working. Both are better answered by measuring the deposit volume than by changing the paste.

FAQ

Is a higher metal loading always better? No. It gives more metal per unit volume, less residue and less slump, and it also makes the paste harder to print and less tolerant of an oxidised surface. The right figure depends on the aperture and the board.

Why do two pastes with the same loading print differently? Loading is only one variable. The particle size distribution, the flux chemistry and the rheology modifiers all change how the paste behaves, and two pastes with an identical metal content can differ substantially in slump and release.

Does the loading change during a print run? The metal content does not change, but the solvent content does, as the paste is worked and exposed. The result is a paste that behaves as though its loading had risen, which is why the working life on the stencil is specified.