Paste Jetting: Design Rules and Process Limits
Solder paste jetting and stencil printing both put a measured volume of paste onto a pad, and beyond that they have almost nothing in common. Printing transfers paste through a foil that is aligned to the board, so the deposit is defined by an aperture that was made for that product. Jetting ejects a droplet from a nozzle that moves over the board under program control, so the deposit is defined by the valve, the paste and the motion. The two routes fail differently, scale differently, and suit different production patterns.
What Each Process Can and Cannot Do
Printing produces every deposit in one stroke, so its throughput is essentially independent of the number of pads and depends instead on the number of boards. That makes it the fastest route for a high-volume product with a stable layout. Its limits are the stencil: apertures below an area ratio that the paste can release from will not fill reliably, and a board that is not flat will not seal against the foil.
Jetting places deposits one at a time, so its throughput falls as the pad count rises. Against that, it can deposit on a board with height variation, on a surface that no stencil can reach, and on a single pad in the middle of a panel without tooling. Where a product has few deposits, or where the board carries a step or a cavity, jetting is the only practical route, and it can also add a second deposit next to an existing one, which printing cannot do.
Deposit Volume and Its Control
In printing, the volume is set by the aperture area and the foil thickness, so it is a design parameter. Variation comes from the release, the paste and the board flatness, and the control loop is the printer’s own inspection system. A change of volume therefore requires a new stencil or a new foil thickness, which is a tooling change with a lead time.
In jetting, the volume is set by the valve’s stroke and by the rheology of the paste, so it is a program parameter. Variation comes from the paste’s viscosity drift, from the nozzle condition and from the firing frequency. A volume change is a program edit, which makes jetting attractive for a product that is still being tuned, and risky in a shop that does not control paste temperature and working time closely.
Paste Rheology and Jetting Requirements
A jet will only fire a paste whose viscosity, thixotropy and particle size suit it. The paste must break cleanly from the nozzle rather than forming a thread, and it must be able to pass through a nozzle orifice typically between 0.1 mm and 0.4 mm without the largest particles bridging it. Metal loadings above about 88 % and particle sizes above a quarter of the orifice diameter are the usual limits, which excludes some high-reliability pastes from the jetting route.
Temperature control is therefore part of the process. Paste that is 5 °C warmer dispenses a different volume, and the effect accumulates through a shift as the syringe sits near the machine. Jetting installations usually include a syringe holder with cooling or a defined warm-up routine, and the first deposits of a shift should be weighed rather than assumed. Where a paste is changed, the valve parameters must be re-established, because a setting that produced the correct volume with one material will not transfer to another.
Aperture Limits and Area Ratio
Printing is governed by the area ratio of the aperture, which is the area of the opening divided by the area of its walls. Below about 0.66 the paste tends to stay in the aperture rather than transferring to the pad, and below 0.5 the transfer becomes unreliable regardless of the paste. This is what forces a reduction in foil thickness for fine-pitch work, and it is the reason a 0.4 mm pitch product uses a 0.10 mm or 0.12 mm foil rather than a thicker one.
Jetting has no equivalent geometric limit. The droplet is the aperture, and a 0.2 mm deposit is produced by a valve setting rather than by a hole in a foil. The practical limit is the accuracy of the placement and the spread of the paste after it lands, which for a small deposit is a significant fraction of its diameter. A jet can therefore print pads that no stencil can, and it is the standard route for fine-pitch deposits on a board that also carries a step or an irregular surface. Where a stencil is used, the aperture design rules govern what is achievable.
Changeover, Stencil Logistics and NPI
A printed product needs a stencil, and a stencil needs to be ordered, received, inspected and stored. For a stable product that cost is amortised over many boards, and the stencil pays for itself in throughput. For a product built once or twice, or for a prototype that changes daily, the stencil becomes a schedule item: each revision needs a new foil, and the delay between the design change and the first printed board is measured in days.
Jetting removes the stencil from the critical path. A new product can be built from a program, and a design change is a software edit. That is a large advantage in new product introduction, where the layout is expected to move. The trade is the time to establish the valve parameters for each new paste and pad combination, which is measured in hours of engineering rather than in days of waiting, and the ongoing discipline of holding the paste within its window.
Throughput and Line Integration
For a board with several hundred deposits, printing is an order of magnitude faster. The printer completes the whole board in a single cycle measured in tens of seconds, while a jet needs a fraction of a second per deposit plus the travel between them, which for a dense board becomes minutes. Where the line is balanced around the placement machines, the jet may become the constraint, and the printing route is the only one that keeps up.
Jetting also changes the inspection arrangement. Because the deposits are placed individually, the inspection system must verify each one, and the data volume is larger. Many jetting installations integrate inspection into the head, which gives a deposit-by-deposit record rather than a board-level pass or fail. The paste inspection method has to be chosen with the deposition method, because a system tuned for a printed pattern may not measure a jetted dot correctly.
Defects Specific to Each Route
Printing produces bridging, insufficient paste, slump and stencil-related defects such as a clogged aperture or a damaged foil edge. The defects are usually correlated across the board, because a single stencil condition affects every deposit it makes. Jetting produces satellites from a poorly formed droplet, inconsistent volumes from a drifting valve, and clogging of the nozzle by dried paste, which produces a missing deposit rather than a small one.
The diagnostic path differs accordingly. A printed board with a repeating pattern of the same defect points to the aperture or the wipe; a jetted board with a random scatter of volume errors points to the valve and the paste. Knowing which route produced the board, and which failure pattern belongs to it, shortens the investigation considerably. The same reasoning applies to the printing control data, which should be recorded per board so a trend can be seen.
Choosing Between Them
The decision usually comes down to volume, stability and geometry. A stable product above a few thousand units a year with no unprintable features belongs on a stencil. A product with few deposits, a stepped or irregular surface, or a layout that is still changing belongs on a jet. Many shops run both, using jetting for prototypes and for the odd board that the stencil cannot serve, and printing for everything else.
Where both are available, the choice for a given board should be made on measured throughput and measured first-pass yield rather than on a preference. Running a production board through the jet because the stencil is not ready is occasionally right, but it should be a decision with a number behind it, because the jet will produce a different deposit volume and the profile may need to be adjusted to suit it. The paste volume figures for the two routes are not interchangeable, and the acceptance limits should reflect the route that produced the board.
FAQ
Can a jetted board use the same reflow profile as a printed one? Usually yes, if the deposit volumes match, but the profile should be verified rather than assumed. A jetted deposit has a different shape and often a slightly different flux-to-metal distribution, which changes the volatiles released during the ramp.
Does jetting require a different paste? Often it does. The paste must be able to break cleanly from the nozzle and pass through the orifice, and a paste developed for printing may string or clog. The paste supplier’s jetting grade should be used where one exists, and the parameters re-established for it.
Is jetting economical for a medium-volume product? It can be, where the board is difficult to print or where the changeover cost dominates. The comparison should include the stencil cost, the changeover time, the scrap during setup and the yield of both routes over a realistic batch, not just the deposit rate.
