SMD Adhesive Dispensing Before Wave Soldering
On a mixed technology assembly, the chip components on the underside are held in place by dots of adhesive before the board crosses the wave during wave soldering, where the contact geometry is covered in the notes on wave height and contact. The adhesive is not an electrical material and it does not appear in the schematic, but the process that applies it decides whether the board survives the solder wave with every component still in position. A dot that is too small lets a part float off, and one that is too large lifts the part off its pads and produces an open joint.
The window is set by three quantities that have to be correct at the same time: the volume of the dot, its position relative to the pads and the component, and the strength it reaches after cure. Each is measured, each drifts, and each has a different corrective action. The article follows them in that order.
Why Adhesive Is Used
Adhesive holds the component against the board while the solder is liquid. Without it, a chip capacitor on the underside would be supported only by surface tension, and the turbulence of the wave is easily enough to move or lose it. The adhesive also keeps the part in contact with its pads during the thermal expansion that precedes the wave.
The alternative is to avoid the situation: to place the small parts on the top side, or to use a selective soldering process that fills only the through-hole joints. Where the design requires parts on both sides at a wave, adhesive is the conventional answer, and the process controls that follow are what make it reliable.
Where the Dots Go
The dot is placed under the centre of the component body, not on a pad. Material on a pad contaminates the joint and prevents wetting, and material between the pads is squeezed onto them when the part is placed. For a two terminal chip part the dot sits between the terminations, and for a small outline transistor it is placed under the body away from the leads.
The position tolerance is tight in one direction and loose in the other. Movement along the axis between the pads changes the adhesive area that contacts the part, while movement towards a pad risks contaminating it. The placement program uses the same fiducials as the component placement, and the dot pattern is checked against the pad geometry at the first article, using the alignment discipline described in the notes on placement offset control.
Dot Geometry and Volume
The dot is described by its diameter, its height and its volume. Typical values for a 0805 or 1206 chip component are a diameter of 0.8 to 1.5 mm and a height of 0.3 to 0.6 mm before placement, which gives enough material to form a fillet around the body without lifting it. The height after placement is what matters mechanically, and it is the height that the placement force sets.
Volume is controlled by the dispensing method and monitored by weight or by an optical measurement on a sample. Where the volume drifts high, the part is lifted and the joints open; where it drifts low, the bond is weak and the part may move in the wave. A daily check of the dot height on a sample board is a more useful control than a periodic check of the syringe pressure.
Dispensing Methods: Pin Transfer, Screen and Jet
Pin transfer uses a tool with pins that dip into a film of adhesive and transfer a dot to the board. It is fast, it applies many dots in one stroke, and it depends on the film thickness being held constant, because that thickness determines the volume. The tool wears and the film changes with time and temperature, so the volume is checked on a sample at a defined interval.
Stencil printing applies a pattern of adhesive through a screen or a stencil, which gives excellent volume repeatability at the cost of a tool per product. Jet dispensing places a measured droplet without touching the board, which suits uneven surfaces and high mix production, and it needs the nozzle and the valve to be maintained because a partially blocked nozzle changes both the volume and the direction of the dot. The choice among the three is a throughput and mix decision rather than a quality one, provided the volume is measured in each case.
Placement Force and Dot Deformation
The placement head presses the component into the dot, and the force determines how far the adhesive spreads under the part. A force that is too high flattens the dot until it reaches the pads, and a force that is too low leaves the part sitting on top of the adhesive with its terminations in the air. The useful range for a chip part on a standard nozzle is a few newtons, set by the machine and verified by measuring the bond line after placement.
The deformation also depends on the viscosity of the adhesive at the machine temperature and on the dwell time between dispensing and placement. An adhesive that has begun to cure on the board is harder to deform, and one that is too warm spreads further. The interval between the two steps should be specified and held, because it changes the result even when both machines are correct.
Cure and Its Effect on Strength
Cure converts the dot into a structural bond, and the schedule has to be matched to the adhesive and to the thermal limits of the assembly. A typical snap cure runs at 150 °C for 90 seconds, while a slower material may need 30 minutes at 120 °C. Under-cured adhesive retains solvent, develops less strength and can release gas into the wave.
Over-cure is also a risk. A material cured beyond its schedule becomes brittle, and a brittle dot fractures at the edge of the component during thermal expansion rather than flexing with it. The cure profile is verified with a thermal profile on a test board or with a differential scanning calorimeter check on a cured sample, and the method that suits the material is agreed with the supplier. The same cure discipline appears in the notes on underfill flow and cure.
Adhesive and Solder Interaction
Adhesive and flux do not mix well. Flux that reaches an adhesive dot can soften it, and adhesive that reaches a joint prevents wetting and leaves a void in the fillet. Wetting quality is measured with the method described in the notes on wetting balance testing. The design rule that keeps the two apart is a clear pad area on both sides of the dot, and the process rule is that the flux volume is set for the joint rather than for the whole board.
The wave also affects the adhesive thermally. The dot sees the solder temperature for the contact time, and a material selected for the assembly has to survive that without softening. Where a component moves in the wave, the cause is more often an under-cured dot than an oversized wave, and the cure record is the first thing to check.
Inspection and Verification
Verification has three levels. The dot is inspected by an automated optical system or by a person after dispensing, the component position is checked after placement, and the retention is confirmed by the fact that the parts are still in place after the wave. A sample board can also be inspected after cure to measure the bond line and to confirm that the adhesive has formed a fillet around the body.
A push-off test on a sample assembly gives a number for the component retention strength and is used to qualify a material or a cure change. It is destructive and is not a production test, but it is the evidence behind the process window. Where a customer requires a specific retention, the test method and the acceptance value belong in the specification, and the flow of an unsupported part in the wave is exactly the failure the test predicts.
Process Control and Records
The controls are the adhesive lot with its expiry and its storage temperature, the dispensing method and its parameters, the dot volume check, the placement force, the interval between dispensing and placement, and the cure profile. Each is recorded per shift, and the dot volume check is the one that catches a drifting process earliest.
The records should be linked to the machine maintenance that produced them: a new pin tool, a cleaned jet valve or a replenished film thickness all change the dot volume, and the defect that follows such a change is diagnosed in minutes when the maintenance log sits next to the process log. With that pairing, component loss in the wave stops being an occasional mystery.
FAQ
Can adhesive be omitted if the components are small? Small parts are the ones most likely to move in the wave. Adhesive is required whenever a surface mount part faces a solder wave on the underside.
Why do joints open after adhesive dispensing? Usually an oversized dot lifts the component off its pads. The dot volume check is the first thing to review.
Does a harder cure always give better retention? It gives more strength up to a point, and then the bond becomes brittle and fails during thermal expansion. The cure window in the material data sheet is the guide.
