Solder Paste Jetting: Preparation, Placement and Process Control

There are two ways to put solder paste onto a pad. The traditional one is a stencil that is placed over the board, filled by a squeegee, and lifted away, which deposits paste on every pad in a single pass. The alternative is a jet valve that ejects a measured droplet of paste onto each pad individually, driven by the same motion system that places components. The two methods produce the same kind of deposit by very different means, and the choice changes the process rather than the product.

This article explains how solder paste jetting and stencil printing each work, where the practical limits lie, and how the decision is made for a given product and volume.

How Stencil Printing Works

The stencil is a thin metal foil with apertures that match the pads, held in tension in a frame, and the printer aligns it to the board using the fiducials and presses it flat against the surface. A squeegee then travels across the stencil, filling the apertures with paste, and the board is separated from the stencil by a controlled motion that leaves the paste clinging to the pad rather than to the aperture wall.

Three properties decide whether the transfer works. The area ratio, which compares the area of the aperture to the area of its wall, has to be high enough for the paste to release; the paste rheology has to allow the aperture to fill and then hold its shape; and the board has to be flat and well supported so that the stencil seals against it. The advantage of the method is that it is parallel: every pad on the panel receives its deposit in the same stroke, so the cycle time is almost independent of the number of pads.

How Jetting Works

A jet valve contains a piezoelectric actuator that drives a small piston against a nozzle, and each pulse ejects a droplet of paste of a defined volume. The head is mounted on a gantry, and the deposit on each pad is built from a programmed number of droplets, which allows the volume to be adjusted pad by pad rather than being fixed by the stencil thickness. No tooling is required for the board, so a new product needs a program rather than a stencil.

The consequences of that flexibility are significant. A board with components already mounted on one side can have paste placed on the other side without a stencil, a board whose surface is not flat can be printed because the nozzle follows a programmed height, and a single pad can be given extra paste to correct a known deficiency. Where a product is built in small batches with frequent changes, the absence of stencil cost and lead time is a bigger advantage than the deposit quality.

Where Each Method Wins

For a flat board in volume, stencil printing is faster and cheaper per board. The tooling cost is amortised over the volume, the throughput is high, and the process is well understood. For a high mix of products in low volume, jetting avoids a stencil for every change and removes the waiting time between the design release and the first build.

Jetting also wins on geometry. A board with a stiffener, a connector, or a tall component that would obstruct a squeegee, a three dimensional molded substrate, and a flex circuit that cannot be held flat are all cases where a stencil cannot seal and a jet can reach. The same applies to a selective process in which paste is added to a single component, for example after a repair or to supplement a pad that was found to be short of paste.

Limits Of Jetting

The throughput is the main limit. A jet deposits droplets sequentially, so the time per board rises with the number of pads, while a stencil printer does the whole panel in one stroke. Multiple nozzles reduce the gap, but the method remains serial, and it is usually applied to a product whose total pad count is modest or to a selective area of a larger board.

The paste is the second limit. A jetting paste is formulated differently from a printing paste, with a rheology that allows it to be ejected cleanly and to release from the nozzle without stringing, and using a printing paste in a jet valve produces inconsistent droplets and clogged nozzles. The droplet size limits the finest deposit that can be produced, the fine powder grades are more prone to clogging, and the volume of a droplet depends on the valve temperature, so the head has to be temperature controlled. Consistency across thousands of droplets is the parameter that has to be verified, and it is checked by weighing a defined number of droplets rather than by inspecting a deposit.

Limits Of Stencil Printing

The stencil needs a flat board and a support that prevents it from deflecting under the squeegee. A board that is not supported flexes away from the stencil, which leaves a wedge shaped deposit, and a board with a height variation larger than the stencil can accommodate cannot be printed at all. The aperture area ratio sets the smallest pad that can be printed reliably, and the stencil thickness sets the deposit volume for every pad on the board, so a design with both coarse and fine pads has to compromise.

The tooling cost and lead time are the commercial limits. A stencil is inexpensive in absolute terms but it takes time to order, it has to be stored, and it becomes obsolete when the design changes. For a product with a long life and a stable design, none of that matters.

Hybrid Approaches And The Decision

The two methods are not mutually exclusive. A common arrangement is to print the main side with a stencil and to jet the paste for a component that is added later, and a repair station may use a jet to place paste on a single footprint after a component has been removed. A line that carries several products can print the high volume ones and jet the low volume ones with the same placement platform.

The decision comes down to volume, geometry, and change frequency. Where the board is flat and the volume is high, the stencil is the answer. Where the board is unusual, the volume is low, or the design changes often, jetting is the answer, and the trade is throughput against flexibility. The pad geometry that both methods depend on is described under pad design standards, the layout consequences under manufacturable design guidelines, and the reflow behaviour of the deposit under component shift during reflow.

FAQ

Can any solder paste be jetted? No. Jetting needs a paste formulated for the valve, with a rheology that allows clean ejection. A printing paste clogs the nozzle and produces inconsistent droplets.

Is jetting more accurate than printing? The deposit position can be more accurate because it is programmed per pad, but the deposit shape is built from droplets rather than formed by an aperture, and the volume consistency has to be verified separately.

Does jetting remove the need for a stencil altogether? Only for the areas it covers. Where a board is flat and in volume, a stencil is still faster, and many lines use both methods for different products or different sides of the same product.