Dispensing Nozzle Choices for Paste Jetting

Solder paste jetting applies paste without a stencil, using a valve to fire small deposits onto the pads as the board moves underneath. It exists because some products cannot be printed: boards with severe height variation, assemblies where a stencil cannot be supported, small batches where the stencil cost is disproportionate, and applications where a few deposits must be added to a printed board. The technology does not remove the physics of solder paste, but it changes which variables the process engineer has to control.

Where Jetting Fits

Jetting is at its best where the stencil is difficult. A board with components already on the top side cannot be printed on that side without support, and a product with a large height difference between its lowest and highest points can defeat a squeegee. Small production runs benefit when the cost of a stencil and the time to order one exceed the value of the boards. Rework and prototyping applications benefit most of all, because a deposit can be placed exactly where it is needed without any tooling at all.

It is less attractive for a large board covered with thousands of deposits, because the deposition rate is lower than printing and the cycle time is set by the number of dots. For high volume production the printed process remains faster and better characterised, and the comparison is essentially one of tooling cost and cycle time against flexibility and speed of change. The decision is economic before it is technical.

Deposit Volume and Shape

A jetted deposit is built from one or more dots, so its volume is the sum of what the valve fires and its shape is the result of how those dots merge and slump. Both depend on the paste rheology, the nozzle diameter, the firing pressure and the distance between the nozzle and the board. A paste that is suitable for printing is not automatically suitable for jetting, because the shear history inside the valve is different and the material must recover quickly after each shot.

The deposit shape matters for the joint as well as for the placement. A dot that sits high on the pad can interfere with the placement nozzle, while one that spreads too far can bridge at fine pitch. Operators learn quickly that the visual difference between acceptable and unacceptable is subtle, which is why the process needs a measured criterion, such as the deposit height and area compared against a target range, rather than a visual standard. Paste volume verification applies to jetting exactly as it does to printing.

The Valve and Its Nozzle

The valve is a precision device with a moving element, a paste chamber and a nozzle, and its condition changes with use. Wear on the sealing surfaces changes the shot volume, dried paste in the chamber changes the flow, and a partially blocked nozzle produces a deposit that is smaller and misdirected. Because the valve fires thousands of times an hour, small changes accumulate quickly, and the process drifts unless the shot volume is checked regularly.

Nozzle diameter is selected for the smallest deposit the product requires, and the same nozzle is used for larger deposits by firing more dots. Using the smallest nozzle for everything reduces throughput, while using a large nozzle for a fine-pitch part produces deposits that are too wide. A practical setup keeps two or three nozzle sizes available and records which is used for which product, tooling documentation being the model for that record.

Paste Handling for Jetting

Paste for jetting is often supplied in a cartridge or a syringe rather than a jar, and the handling rules are stricter because the material spends longer in a closed system at room temperature. The pot life of the paste in the valve is not the same as its pot life on a stencil, and the supplier’s guidance for the specific package should be followed rather than assumed. Paste storage practice covers the thawing and the labelling that apply to both formats.

Air bubbles are a particular concern. A bubble in the chamber produces a missing or a partial shot, and the resulting defect is random and hard to attribute. Settling the cartridge, purging before use and avoiding agitation are the usual countermeasures, and the first deposits after a pause should be discarded or checked. Where the defect pattern shows occasional missing deposits in an otherwise stable process, a bubble is one of the first explanations to test.

Programming and Verification

A jetting program records the position of every deposit, the number of dots, the firing parameters and the order in which they are placed. The program should be verified against the pad layout, since a single wrong coordinate produces a defect that may not appear until the joint fails. Where the board is printed and jetted in the same process, the two sets of deposits have to be reconciled so that no pad receives paste twice.

Verification of the process rather than the program means measuring deposits. A camera integrated into the machine or a separate optical measurement system gives the volume, area and height for each deposit, and the data can be trended over time. Setting warning limits on the shot volume, so that the machine flags a drift before the deposits leave the tolerance band, is the equivalent of the control chart used in printing, and it turns a periodic manual check into continuous supervision.

Defects and Responses

The characteristic defects of jetting are missing or partial deposits, satellite dots caused by a poor break-off, and deposits that slump enough to bridge. The responses follow the physics: a missing dot suggests a bubble or a blockage, satellites suggest the nozzle height or the paste rheology, and slumping suggests the deposit volume or the board temperature. Working through them in that order avoids changing parameters that are not responsible.

The process also has a thermal dimension. A board that is warm makes the paste slump, and a board that is cold resists adhesion to the pad; both produce defects that vary with the time of day and the state of the room. Recording the board temperature, or simply ensuring that boards coming from an oven have cooled before they reach the valve, removes a variable that is otherwise invisible during the investigation. Step region quality in printing deals with the same transfer physics from the opposite direction.

FAQ

Can jetting replace printing entirely? For low volume and for boards that cannot be printed, yes. For high volume the printed process is faster and better understood.

What causes occasional missing deposits? Usually a bubble in the chamber, a partially blocked nozzle or a firing parameter at the edge of its range. Check in that order.

Is special paste needed? Usually yes. Jetting pastes are formulated for the shear conditions inside the valve, and a printing paste may not perform consistently.

How is deposit volume verified? Optically, by measuring volume, area and height against a target range, with limits set from the joint requirement rather than from the current output.