Solder Paste Jetting vs Stencil Printing: Deposit Method Selection

Stencil printing and solder paste jetting both put a controlled volume of alloy on a pad, but they do it by different physical mechanisms and they fail in different ways. The choice follows from the smallest aperture on the board, the number of deposits, the flatness of the surface and the paste that has to be shared with the rest of the assembly.

What Each Method Deposits

Stencil printing forces paste through apertures with a squeegee and relies on the paste’s own adhesion to release from the aperture walls. Jetting fires a measured droplet from a nozzle driven by a piezo or pneumatic actuator and places it without the stencil touching the board.

The two methods produce different deposit geometry. A printed deposit is a truncated brick whose sides follow the aperture walls, while a jetted deposit is a stack of droplets with a domed top and a footprint that spreads slightly on impact, which changes how the paste behaves when it is reflowed. Deposit height differs as well: a printed brick is flat topped at the stencil thickness, while a jetted stack is tallest in the middle, so the collapse during reflow follows the alloy and the pad rather than the aperture.

Deposit Volume and Transfer Efficiency

Transfer efficiency is the ratio of deposited volume to aperture volume, and a good print holds 80 to 100 percent of the aperture volume for apertures above 0.4 mm square. Below an aperture area ratio of about 0.66, which is aperture area divided by aperture wall area, the print loses volume quickly and becomes variable.

Jetting has no equivalent wall area limit, because the paste is never released from a wall. A 200 micrometre droplet can be placed reliably on a pad with no stencil support behind it, which is why the method is used on stepped carriers and on boards that already carry tall components.

Where Stencil Printing Still Wins

Printing wins on throughput wherever apertures are large enough and the board is flat. A single stroke deposits every pad on the panel at once, so the cycle time does not grow with the number of deposits, and a 0.4 mm pitch device can be printed alongside chip components in the same pass.

It also wins on paste consistency, because the whole assembly shares one paste lot and one open time. On a jetting line the paste in the reservoir ages differently from the paste in the printer, and two ages on one board means the profile has to cover both.

Where Jetting Is the Only Practical Option

Jetting is used where a stencil cannot sit flat: a board pressed into a carrier, a populated second side, a connector standing above the plane of the pads. It is also used where a single site has to be reworked without disturbing the rest of the assembly.

It wins where deposit volume has to vary across the board, because each droplet is programmed individually. A connector shell that needs more paste than the signal pads can be given more without a stepped stencil, and the volume is recorded per site rather than per aperture. The programming effort is real, since every site carries its own droplet count and pressure setting, so the method repays that work on products with a stable build where the data is reused run after run.

Aperture and Pad Design for Jetted Paste

Jetted paste spreads on landing, so the solder mask opening has to contain it. A 200 micrometre pad receiving a 250 micrometre droplet will see paste on the mask, and the mask surface energy then decides where the deposit finishes rather than the pad geometry.

The working rule is to keep droplet diameter at or below the pad width and to let reflow pull the paste back onto the copper. Where the pad is smaller than one droplet, the deposit is split into two smaller drops placed side by side, which keeps the total volume without losing the containment.

Paste Rheology for Each Method

Printing needs a high viscosity paste with strong shear thinning so that it rolls ahead of the squeegee and releases cleanly from the aperture. Jetting needs a paste that breaks cleanly at the nozzle and does not string, which favours slightly lower viscosity and a tighter particle size distribution.

Type 4 powder, at 20 to 38 micrometres, prints well down to 0.4 mm pitch, while Type 5 at 15 to 25 micrometres is used for 0.3 mm pitch and for fine jetting. Particle size is the first thing to change when a deposit becomes inconsistent, before pressure or speed is touched, and the open time limits are the same as for any paste warm up routine. Particle size also sets the smallest usable droplet, because a nozzle cannot eject a drop smaller than roughly three times the largest particle in the powder; forcing it produces a spray that lands off target instead of a dot.

Nozzle and Stencil Maintenance Compared

A print process is maintained by stencil cleaning and paste refresh, which are scheduled by print count. A jetting process is maintained by nozzle cleaning and droplet volume verification, because the nozzle seat wears and the volume falls by a few percent over a few hundred thousand cycles.

Droplet volume is verified by weighing a fixed number of drops on a balance and comparing the mass against the expected figure for the paste density. Twenty drops gives a usable average, and the result is recorded against nozzle hours so that wear is visible before it becomes a defect. Nozzle hours are logged against the pressure set point, since a worn nozzle running at raised pressure hides the wear until the deposit degenerates from a dot into a spatter pattern.

Verifying the Deposit Before Reflow

Both methods are verified by solder paste inspection, which measures volume, area and height against a reference. Printing shows systematic volume loss on the smallest apertures, while jetting shows individual missing or stray drops from a partially clogged nozzle, and the two patterns need different limits.

Inspection limits should come from the joint requirement rather than from the process average. A deposit at 50 percent of nominal volume is often acceptable for a chip resistor and not acceptable for a fine-pitch lead, so a single global limit either passes bad deposits or rejects good ones; our SPI guide sets out how the limits are derived.

Choosing the Method for a Product

The decision comes down to four figures: the smallest aperture area ratio, the number of deposits per board, the flatness of the surface and the mix of products on the line. Printing is chosen when the first is comfortable and the others are favourable, and jetting when any of them is not. A fifth figure is the powder type the rest of the line already uses, because adding a second particle size for one product costs a changeover that only pays where the deposit quality cannot be reached any other way.

Several lines run both, printing the main pass and jetting the exceptions. In that case the two processes should share one paste specification, or the reflow profile will have to cover two paste ages and two particle sizes at the same time, and placement accuracy will be blamed for a deposit problem.

FAQ

Can jetting replace printing on a high-volume line? Not usually, because the deposits are placed one at a time and the cycle time grows with the deposit count. Jetting is used for the sites a stencil cannot reach or for products where the deposit has to vary across the board.

Is transfer efficiency a meaningful figure for jetting? Not in the printed sense, because there is no aperture volume to compare against. Jetting is controlled by droplet mass and count, and the equivalent check is the weighed drop test rather than a ratio to an aperture.

Which method is more sensitive to paste condition? Jetting, because a clogged nozzle produces no deposit at all rather than a small one. Printing degrades gradually as paste dries on the stencil, which gives the inspector a trend to see, while jetting fails site by site and needs inspection limits tight enough to catch a single missing dot.