SMT Adhesive Dispensing And Dot Profile Control
Adhesive dispensing is the operation that places a bead of surface mount adhesive to hold components to the underside of a board so that they survive wave soldering, and sometimes so that they survive a second reflow on the other side. It is applied as a dot by a dispensing head, and the whole process rests on one small object: a bead of paste with a defined height, diameter and position that must behave the same way for every one of the thousands of dots on a panel. Small changes in that bead become large changes downstream.
This article covers what the dot has to do, how it is produced, what makes it drift, and how a dispensing process is set up and kept in control.
What The Adhesive Has To Do
The adhesive has three jobs. It must hold the component in place from placement until cure, so that the part does not slide when the board is handled or when it passes over a conveyor. It must survive the thermal shock of wave soldering without softening enough to release the part or to let it shift. And once cured, it must not impose so much stress on the joint that a ceramic chip capacitor cracks when the board flexes during depanelisation.
Those requirements pull in opposite directions. The tack needed for placement is a surface property of a soft material, while the strength needed for wave soldering comes from a cured thermoset. The dot geometry is the variable that lets both be satisfied: a tall, narrow dot gives a compliant column that reaches a component body standing on its own standoff, while a wide, low dot gives a large bonded area for a part with little clearance.
The Dot, Its Geometry And Its Profile
A useful dot is described by its diameter, its height and the shape of its top. The height is set so that the adhesive contacts both the board and the component, with the component pressed into the dot by the placement force so that the adhesive wets up the side of the body. If the dot is too low, the part is not held; if it is too high, the part is lifted off its pads and the solder joint suffers.
The dot profile matters because the material is not a simple liquid. A dot that has been dispensed leaves a tail, and a tail that falls over can bridge to a neighbouring pad or trap a component lead. A dot with a concave top indicates that the needle has pulled material back up as it withdrew, which is a sign that the valve is opening slowly or that the material is too viscous for the settings. A smooth, domed dot is the target.
How The Dot Is Produced
Three methods dominate. A time-pressure dispensing valve applies air pressure to a syringe for a fixed time, and the volume delivered depends on the viscosity, the needle diameter and the amount of material left in the barrel. An auger dispensing valve uses a rotating screw to meter material and is much less sensitive to viscosity, which makes it the usual choice for small dots and fine pitch. A jetting dispensing valve fires a droplet without touching the board, which allows dispensing over uneven surfaces and at high speed.
Each method has a different failure mode. Time-pressure drifts as the barrel empties, because the column of material above the needle changes the pressure that reaches the tip. An auger dispensing valve wears at the screw and the seat, so the volume creeps up over a long production run. A jetting dispensing valve is sensitive to the gap between the nozzle and the board and to the temperature of the adhesive, since both affect the droplet that forms.
What Changes The Dot On The Line
Viscosity is the largest single influence, and it changes with temperature and with time. Adhesive that has warmed in the syringe dispenses a larger dot at the same settings, and material that has been open to the air thickens as solvent evaporates and as it begins to cure. Bath temperature control and a defined working life for the syringe remove most of this variation, which is why a machine is usually allowed to reach thermal equilibrium before the first panel is run.
The hardware contributes the rest. A worn needle delivers a larger and less repeatable dot, the gap between the needle and the board changes the shape, and the speed of the axis affects the tail. Board topography matters as well, because dispensing over a via or a tall trace gives a different result from dispensing onto bare laminate. All of these are reasons to check the first boards of a lot rather than to trust the recipe.
Cure And Final Strength
The cure schedule is chosen to match the thermal budget of the assembly. A typical epoxy adhesive is cured in a tunnel oven at around 120 to 150 degrees Celsius, and the cure is judged by the strength that results rather than by the time in the oven. Under cure leaves the material soft and produces components that shift in the wave; over cure makes it brittle and transfers stress into the ceramic.
The adhesive strength is measured by pushing a component sideways with a defined force and recording the value at which it moves, which is the push off test. The figure is compared with a limit set from the product requirement, and the test is repeated after the wave solder operation where the risk of shift is highest. Related materials and protection methods are described under potting, dispensing and adhesives.
Inspection And Common Defects
Visual inspection after dispensing looks for the dot that is missing, the dot that is off the pad, and the dot that has slumped. Missing dots are usually a clogged needle or an air bubble in the barrel, and off-pad dots come from a fiducial that was not taught correctly or from a board that moved in its fixture. Slumped dots point to material that has thinned or to a board that was left too long before placement.
After placement, the check is whether the component sits flat and whether the adhesive has spread around the body instead of staying under it. After wave soldering the check is for shifted or lost parts, and after cure the check is for cracked terminations. Each of these inspections tests a different part of the same chain, and the assembly process as a whole is set out under PCBA development process and placement order and pad positioning.
Interaction With The Wave Solder Process
The adhesive is only one half of the operation; the wave it has to survive is the other half. Contact time, wave height, preheat and the direction of travel all affect whether a small part stays where it was placed. A short contact with a low wave is more forgiving, and a board that is run across the wave in the long direction presents fewer parts to the full force of the solder.
Where a part is known to be difficult, the fix is usually geometric rather than chemical. A taller dot, a larger bonded area or a small mechanical feature such as a hole or a routed slot that traps the adhesive will do more than a stronger material, because the load path is what decides whether the part moves. The same reasoning applies to the order in which parts are placed, which is described under placement order and pad positioning.
FAQ
Should the adhesive touch the solder pads? It should be kept away from them. Adhesive on a pad prevents the solder from wetting and can hold a lead away from the joint, so the dot is placed on the laminate between the pads, not on them.
Why do the dots get larger towards the end of a syringe? In a time-pressure system the shorter column of material lets more of the applied pressure reach the tip, so the volume rises unless the pressure is trimmed as the barrel empties.
Can an under cured dot be cured again? It can be passed through the oven a second time, and this is common practice, but the bond should be retested afterwards, because a second pass also exposes the components and the solder joints to another thermal cycle.
