Solder Paste Volume Control In SMT Printing

Solder paste volume is the single number that links the printing process to the quality of the joint. The volume of paste printed into an aperture, less what is lost during reflow, becomes the solder fillet, and a fillet that is too small or too large produces a defect.

Volume is controlled by the aperture area and by the stencil thickness, and it is measured by a solder paste inspection system. Understanding the relationship between those three things is what allows a process to be set up and then kept stable.

Why Volume Matters

The solder in a joint comes almost entirely from the paste, since the pad and the component lead contribute the surface but not the bulk. A deposit that is 20 percent short produces a fillet with insufficient solder, which is a reliability concern on any joint that sees thermal cycling.

A deposit that is too large is equally problematic. Excess solder can bridge to a neighbour, can lift a small chip component during reflow, or can form a ball that detaches and rolls across the board. The pad geometry that receives the deposit is described under PCB pad design standards.

Aperture Area And Stencil Thickness

The volume of a printed deposit is the aperture area multiplied by the stencil thickness, assuming the aperture fills completely and releases cleanly. Thickness is therefore the simplest lever for changing volume across the whole board, while aperture area is used to tune individual pads.

Stencil thickness is usually 0.10 to 0.15 mm for fine pitch work and up to 0.20 mm for a board with large pads and coarse features. A stepped stencil allows two thicknesses on one sheet, so that a fine pitch area and a connector can both receive the correct volume.

The Area Ratio And Release

The aperture area ratio is the area of the opening divided by the area of its walls. A low ratio means the paste has more contact with the wall than with the pad, so it prefers to stay inside the aperture instead of transferring to the board.

The practical rule is to keep the area ratio above 0.66 and ideally above 0.75. When the ratio falls below that, the deposit is short and inconsistent, and no amount of squeegee pressure will recover it. The stencil design rules behind this are described under design guidelines for manufacturability.

Deposit Height And Shape

Deposit height should be close to the stencil thickness, with a flat top and vertical walls. A deposit that is rounded rather than flat indicates that the paste is not releasing cleanly, which reduces the volume that transfers.

Slump is the tendency of the deposit to spread after printing. A paste with too much slump closes the gap to its neighbour and increases the risk of bridging, while a paste that holds its shape too strongly releases poorly from the aperture.

Measurement By Inspection

Solder paste inspection measures the volume, area and height of each deposit and compares them with the design. The measurement is optical, using structured light or laser triangulation, and it produces a map of the board showing which pads are outside tolerance.

The tolerance is usually expressed as a percentage of nominal volume, with typical limits of plus or minus 20 to 30 percent, and tighter limits for a fine pitch area. The results should be trended rather than judged one board at a time.

Correcting A Volume Problem

The first thing to check is the stencil itself: its cleanliness, its wear and the condition of the aperture walls. A stencil that has been used for a long run can develop plated aperture walls, which improve release considerably.

If the stencil is sound, the printer parameters are examined in order of effect: separation speed, squeegee pressure and print speed. Paste condition and room humidity also matter, and both are easy to overlook. How the requirement reaches the production recipe is described under PCB design and fabrication.

Paste Properties That Affect Volume

The metal content of the paste by weight is typically 88 to 90 percent, with the remainder made up of flux and solvent. A paste with a lower metal content deposits the same volume but leaves less solder after reflow, so a volume specification implicitly assumes a particular paste rather than any paste at all.

Rheology matters just as much. A paste that thins under shear flows into the aperture easily, and one that recovers its viscosity quickly releases cleanly when the board separates from the stencil. Both behaviours come from the flux chemistry and from the particle size distribution of the powder.

Printer Setup And Its Effect

Squeegee pressure should be the minimum that produces a clean wipe of the stencil surface, because excess pressure forces paste under the stencil and increases the cleaning requirement. Squeegee speed and separation speed then control how the paste releases from the aperture.

Separation speed is often the parameter that turns a marginal print into a good one. A slow separation lets the paste detach cleanly from the aperture wall, while a fast one pulls the deposit apart and leaves a short print behind on the pad.

Stepped And Rework Stencils

A stepped stencil changes the thickness locally by milling or electroforming a step into the foil. It allows a connector with large pads and a fine pitch area to receive the correct volume from the same printing stroke, which would otherwise be impossible on a single sheet.

A rework stencil is a separate, smaller stencil used to print paste onto a single site during repair. It has different requirements from a production stencil, because it is aligned by hand and used once, and the aperture is usually oversized to compensate for the poorer alignment.

Volume Differences Across The Board

Volume is rarely uniform across a whole panel. The first pads printed after a fresh paste addition receive more volume than the last, because the paste on the stencil becomes drier as the print cycle continues. A steady rhythm of small paste additions reduces that drift considerably.

Board position matters too. A panel printed with a squeegee that travels in one direction can show slightly more volume on one side than on the other, and the effect grows as the squeegee wears. Inspecting the same reference pads on every board makes the trend visible before it turns into a defect.

A large aperture also behaves differently from a small one on the same stencil. Paste in a large opening releases later and can leave a taller deposit at the trailing edge, which is one reason a large pad is often split into a grid of smaller openings.

FAQ

Can volume be increased by using a thicker stencil? Yes, but only if the area ratio allows clean release. On a fine pitch pattern a thicker stencil lowers the ratio and can reduce the volume rather than increase it.

How often should paste volume be measured? Continuously with an inspection system, or on a sample at intervals where the process is stable. Any change of paste, stencil or printer setup should trigger a fresh check.

What causes a deposit to be short on only a few pads? Usually a partially blocked aperture or a local stencil defect. The pattern of the short deposits across the board normally points to the cause.