Adhesive Dispensing: Design Rules and Process Limits
SMT adhesive exists for one reason: to hold components on the underside of a board while it passes over a wave. Everything about the process, from the dot size to the cure schedule, follows from that duty. The adhesive has to survive the flux, the preheat and the wave without releasing the part, and it has to do so without contaminating the solder joints or leaving a residue that prevents them from forming. It is a small volume of material carrying a large responsibility, and the defects it causes are usually attributed to the wave.
Why Adhesive Is Used at All
In a mixed-technology assembly, the surface mount parts on the secondary side are attached before the through-hole parts are soldered. Those parts have no other means of staying in place: the paste has already reflowed, and gravity works against them. Adhesive applied between the pads, cured, and then carried through the wave gives the component retention that the reflowed joints cannot. Without it, parts drop into the solder pot, which is the failure the process exists to prevent.
The alternative is to solder the through-hole parts first and reflow afterwards, or to use a selective process that does not immerse the board. Both exist, and both have their own constraints, but where a full wave soldering pass is used the adhesive is mandatory. The engineering question is not whether to use it but how much, where, and cured to what specification.
Adhesive Dispensing Methods and Their Windows
Time-pressure dispensing uses a fixed air pulse to push adhesive through a needle. It is simple and cheap, but the dot size changes as the syringe empties, because the same pulse produces a different volume when the column of material above the needle is shorter. Auger dispensing uses a rotating screw to meter the material and is far more consistent over the life of the syringe, which is why it is used for tight dot tolerances. Jet dispensing ejects a droplet without touching the board, which removes the needle-to-board distance from the equation and allows very high speed.
The choice follows the dot size and the volume. A dot of 0.5 mm diameter and above can be dispensed by any of the three; below about 0.3 mm the needle becomes the limiting factor and jetting is the practical route. Where the board has height variation, jetting is also more tolerant because the nozzle does not have to approach the surface. The trade is the cost of the valve and the need to control the material’s rheology closely, because a jet will not fire a material whose viscosity has drifted.
Dot Geometry and Placement Rules
The dot is placed between the pads, on the board surface, and it must not touch either pad. Adhesive that reaches a pad contaminates the surface and blocks the joint, producing an open circuit that is difficult to see. The usual rule is to keep the dot at least 0.2 mm clear of the pad edge, and to keep its diameter small enough that it does not bridge the gap between pads even if it spreads during cure.
The dot also has to sit where the component body will press it, not on the termination. For a chip component, the dot goes under the centre of the body. For a small outline package, it goes under the body between the lead rows, and for a large package it may need two or more dots spaced to prevent the body rocking. Where a package has a large central area, a single line or a cross pattern gives better retention than a single dot, because the material spreads to form a pad. The pattern should be defined in the placement program and drawn on the assembly drawing so that it can be checked.
Adhesive Volume and Component Mass
The volume of adhesive has to scale with the mass of the component and with the force the wave will exert on it. A 0603 chip needs very little; a large inductor needs enough that the bond line covers a useful area and the cured fillet reaches partway up the body. The working figure is a bond line of 0.2 to 0.5 mm and a cured fillet that contacts the body over at least a third of its height, which is what resists the wave rather than the adhesive under the part alone.
Too much adhesive is a defect in its own right. A large dot spreads under the component and can reach the pads, and the excess that squeezes out lifts the component off the board so that its terminations no longer sit in the paste. The visible symptom is a component standing proud of the board with a gap under one end, and the cure for it is less material rather than more pressure during placement.
Cure Profile and Its Verification
Adhesives cure either by heat or by ultraviolet light, and many are dual-cure so that the UV fixes the dot in position and the heat completes the reaction. A partially cured dot holds the component until the wave, but a dot that is under-cured will soften in the preheat, release the part and leave a sticky residue on the board that collects dust and interferes with later processes. The cure schedule is therefore a process parameter with a tolerance, not an approximate setting.
Verification is done with the same profile measurement used for reflow: a thermocouple on a board, run through the oven, with the time above the cure temperature recorded. Many adhesives also change colour as they cure, and a visible change from the wet dot to a matte, slightly darker fillet is a useful first check. Where the adhesive is UV-cured, checking the intensity at the board surface rather than at the lamp, and checking it at the end of the lamp’s rated life rather than when it is new, prevents a gradual loss of cure that shows up as components falling off.
Interaction With the Wave and the Flux
The adhesive has to survive the flux. A flux that attacks the cured adhesive softens it, and the part releases during the wave. This is one of the reasons a change of flux is never a small change on a mixed-technology line: the new chemistry has to be checked against the adhesive as well as against the alloy and the mask. A simple test, curing a set of dots and immersing them in the flux for the dwell time, answers the question in an afternoon.
Preheat is the other interaction. The board reaches 100 °C to 130 °C on the topside during preheat, and the adhesive has to stay rigid at that temperature. Where the cure temperature and the preheat temperature are close, the adhesive will soften in the oven even though it was correctly cured, and the parts will shift. The spray fluxing setup and the wave parameters should be reviewed together with the adhesive specification, because the process window is the intersection of all three.
Typical Defects and Their Signatures
A starved dot, too small to form a fillet, leaves the component held only by the reflowed joints. It often survives the wave but falls off during handling or vibration test, which makes the defect appear later than its cause. A squeezed dot, where the component was pressed down during placement, produces a bond line that is very thin in the middle and has material at the edges; it usually holds, but the component may sit unevenly. Stringing, where the adhesive forms a thread as the needle lifts, leaves fine filaments across the board that collect dust and can bridge pads.
Each signature points to a specific setting. Starved dots come from a low dispense volume, a partially blocked needle or a syringe that is nearly empty. Squeezed dots come from excessive placement force or from a dot that is too large. Stringing comes from a needle that is too high on lift, a material that is too cold, or a dot that is too large for the needle. Recording the defect with the dot pattern and the placement pressure makes the corrective action immediate rather than experimental.
Storage, Pot Life and Handling
Adhesive has a shelf life in the sealed syringe and a working life once it is opened, and the working life is much shorter. Material left on the machine overnight can cure in the needle, and the first dots of the next shift will then be undersized. The syringe should be purged at start-up onto a waste card until the dots are consistent, and the purge should be recorded as part of the first-article check rather than treated as a habit.
Temperature matters more than most operators expect. Adhesive stored in a cold room and used immediately will dispense a smaller volume than the same material at room temperature, because its viscosity is higher. The syringe should be allowed to reach room temperature in its sealed bag before opening, which also prevents condensation on the material. The same discipline applies to moisture-sensitive components, and the baking schedule notes cover the component side of the same problem. Every printed assembly should be checked against the inspection standard for dot placement and cure condition before the wave, because a defect caught there costs far less than one found after the parts have been soldered.
FAQ
Can adhesive be dispensed onto the pads by mistake? It can, and the result is a joint that does not form. The dispense program should be checked against the pad geometry on the first board of each batch, and any dot that has spread onto a pad should be removed and re-dispensed before placement rather than soldered over.
Does the adhesive have to be removed after the wave? No. A cured dot inside the finished assembly is part of the product, and it is expected to remain. What does have to be removed is uncured material that has spread onto areas where it will collect contamination or interfere with a later operation.
How is dot placement verified in production? By automated optical inspection before placement, where the machine can measure dot presence, position and diameter, or by operator inspection at a defined frequency. The measurement is far more reliable than a visual judgement of whether a dot looks right, and it gives a number that can be trended.
