Solder Paste Jetting: Practical Checks for Production

Solder paste jetting deposits paste by firing a droplet from a valve rather than by squeezing it through an aperture in a stencil. The process removes the stencil, the squeegee and the board support from the equation, and it gives a non contact method of placing a defined volume of paste exactly where it is needed. It is used for prototypes, for rework, for boards where a stencil cannot be used and for the small volumes that appear beside a printed area.

How the Valve Works

A jetting valve holds a small chamber of paste under pressure and fires it through a nozzle with a piezo electric or a pneumatic actuator. The actuator strikes the paste and ejects a droplet whose volume is set by the chamber geometry, the stroke and the paste properties rather than by a screens aperture.

The droplet leaves the nozzle with a velocity that has to be matched to the gap between the nozzle and the board. Too slow and the droplet does not detach cleanly, too fast and it splashes and forms satellites beside the pad, so the stand off distance is a controlled parameter.

Because there is no contact, the board does not have to be flat and it does not have to be supported against the force of a squeegee. This is what makes jetting attractive on a populated board, where a stencil would have to avoid the tall parts, and in machines where the board support is impractical.

Paste Properties for Jetting

Jetting paste is formulated differently from printing paste. The viscosity is lower and the rheology is more shear thinning, the particle size is smaller and the metal load is usually slightly lower, because the paste has to pass through a narrow nozzle repeatedly without separating.

The droplet forms because the paste breaks cleanly at the nozzle. A paste that is too thick forms a tail that stretches and breaks into satellites, while a paste that is too thin wets the nozzle and drips. The window is narrower than in printing, so the paste specification and the storage conditions matter more.

Particle size is critical because the nozzle is small. The largest particle must be well below the nozzle diameter, and the ratio between them should be checked against the supplier data rather than assumed from the mesh size on the label, since the useful figure is the 90th percentile.

Jetting Against Stencil Printing

Stencil printing is faster and cheaper per board once the volume is high, and it produces the most consistent deposit because the aperture fixes the volume. For a production board with thousands of pads, printing remains the right answer.

Jetting wins where the volume is low, where the pad layout changes often, where the board already carries tall components on the print side, or where a small number of pads in an awkward position have to be filled. The absence of a stencil removes the lead time, the cost and the storage of tooling, and it makes a design change a matter of editing a program rather than ordering a new screen.

The two methods are often combined. A stencil prints the main array while a jetting head adds paste to the pads that the stencil cannot reach, such as a cavity, a step or a region beside a connector that would be damaged by the squeegee.

Process Parameters

The volume of each droplet is set by the valve settings and the number of droplets per pad. The machine usually deposits a pattern of several small droplets rather than one large one, because small droplets wet and slump predictably while a large drop does not.

The stand off distance, the nozzle diameter and the firing frequency are the parameters that are adjusted most often. A shorter stand off improves the placement accuracy but risks contact with the board, and a longer stand off gives more clearance but a wider deposit.

Firing frequency affects the volume because the chamber refills between shots. A valve run at the top of its range may not refill completely, so the droplet volume falls and the defect appears on the pads that are filled last. The machine program should be written with the refill rate in mind.

Accuracy, Volume and Verification

Placement accuracy is limited by the vision system and by the mechanical stability of the head. Because the paste is deposited from above, the droplet lands where the nozzle pointed, and any vibration in the gantry appears directly as a placement error.

Volume verification is done by weighing a defined number of deposits or by measuring them optically. A balance gives the mass, which converts to a volume from the paste density, and an optical system gives the diameter and the height. Measuring the first board of a run and at intervals through it is enough to catch a drift in the valve before it turns into a defect on the line.

The deposited shape matters as well as the volume. A deposit that is taller than it is wide tends to slump unevenly during reflow, while a deposit that is spread too flat has a surface to volume ratio that promotes oxide formation and poor coalescence.

Defects and Maintenance

The characteristic defect of jetting is the satellite, a small sphere of paste thrown beside the pad. It comes from a stand off that is too large, a firing pressure that is too high or a nozzle that has partially dried paste on its tip.

Nozzle clogging develops gradually, and the first sign is a reduction in volume rather than a missing deposit. A missing deposit points to a complete blockage, and an intermittent one points to a paste that has begun to cure in the chamber and is breaking away in pieces.

Maintenance is mostly about keeping the paste moving, because a valve that sits idle is a valve that will clog. The valve should be purged at the start of a run and after any pause, the nozzle should be wiped on a schedule rather than when a defect appears, and the paste should be brought to room temperature before it is loaded.

Practical Rules

Choose the paste for the valve rather than for the printer, keep the stand off and the firing settings in a program that is recorded, and verify the volume on the first board and at intervals. Purge after every pause.

Record the valve settings, the paste batch and the volume measurements with the build records and the printing defect history, and use the paste inspection data to compare a jetted deposit with a printed one.

FAQ

When is jetting better than stencil printing? For low volume, for changing layouts, for boards that already carry tall parts and for a few pads that a stencil cannot reach. Printing remains better for high volume.

Why does jetting paste have a different formulation? It has to break cleanly at a small nozzle and pass through it repeatedly. The viscosity is lower, the particles are smaller and the metal load is usually slightly lower.

What is the most common jetting defect? Satellites, small spheres of paste beside the pad, caused by too large a stand off, too high a firing pressure or a nozzle with dried paste on its tip.