For many years, numerous manufacturers world-wide have been printing glue with an 80 mesh screen or a stencil using conventional printing technology typical to solder paste printing.

Principle Explanation:

In the conventional printing process the adhesive is transferred completely from the stencil to the board. The stencil thickness and aperture diameter govern the volume deposited which can be calculated using the formula for a cylinder:

Stencil Thickness (determines the GDH)

Stencil thickness must be selected to be slightly greater (e.g. +2 mils) than the component stand-off (distance between the board surface under the component and the bottom of the component after placement). The stand-off after placement can vary for the same component, depending on the:

• Thickness of solder resist and of the copper tracks which may be passing between the pads
• Pad thickness - if the pads are higher than the solder resist, than it is necessary to increase the GDH/stencil thickness.
  
  

  Fig 1. Aperture versus Final Diameter.

It is easy to select the optimal stencil thickness when all components on the board have a similar stand-off.
However, if on the board we have both active (SOICs, QFPs etc.) and passive components (chips), then the passive components become the limiting value because their stand-off distance is typically no more than a 4 mils.
After the placement of the component, the excess glue is compressed and it flows in the space between the component and the PCB.

Fig 1. Aperture versus Final Glue Dot Diameter.

The information from the diagram assumes that all of the glue is transferred from the stencil to the PCB and that the shape of the dot after placement of the component is a cylinder.
Stand-off (component to board gap) indicates the distance between the component and the PCB after placement.

If chip components are placed in a glue dot which is too high, the following problems might occur:

• the glue spreads too much at placement and contaminates the soldering pads or
• the gap between the chip and the PCB after curing might be too large, which will generate "skips" during wave soldering.

A typical stencil thickness is 6 mils for designs with only passive (chip) components and 10 mils for designs with both passive and active (leaded) components .
Generally, the glue will be transferred completely from the stencil, if the stencil aperture diameter to stencil thickness is » 4:1 or more.

In order to specify an aperture diameter for a passive component, the distance between the inside edges of the SMT pads must be known or measured. This distance, minus the print alignment tolerance, then becomes the maximum diameter of the post-placement adhesive deposit.

Many companies are testing thicker plastic and metal, machined and laser-cut stencils, which can print glue dots, having not only a different diameter (GDD) but also a different height (GDH).

Principle Explanation:

When printing solder paste, it is desired that all paste from the stencil opening is being transferred to the PCB.
A new printing process takes advantage of the fact that some glue remains in the stencil (which is thicker than for solder paste) after the separation of the PCB and the glue (see Figure 3). The process is based on the surface tension between the glue and the stencil openings, (stencil thickness is constant at 0.3mm or 12 mils):

• If the stencil opening is small, e.g. 0.3 mm (12 mils), the surface tension between the glue and the stencil is so strong that the majority of the glue remains in the stencil. Glue dots on the PCB have the small height (GDH very low).
• With the bigger stencil opening, e.g. 0.8 mm (32 mils), the major part of the glue is transferred from the stencil. During the separation of the stencil and the PCB the stencil drags the glue and the dots will be very high (GDH very big).
• If the stencil opening is very large, e.g. 2 mm (80 mils), all glue is transferred from the stencil opening. The glue dots on the stencil will have a moderate thickness which is similar as the thickness of the stencil.

Fig 3. Aperture Width Vs Deposit Profile.

Diameter of stencil openings for different components:

The diameter (GDD) and the height (GDH) of the glue dots depend on:

• Diameter of the stencil openings
• Stencil thickness
• Viscosity/Rheology of the glue
• Surface roughness of the stencil apertures - surface tension forces between the glue and the stencil.
• Surface topography of the PCB

To find an optimal diameter of the glue dot /stencil apertures for different components, dots with different diameters were printed onto a glass substrate then populated with various components. These combinations of components and glue dots were then inspected for the maximum dot diameter that did not contaminate the component metallizations.

In the table below are approximate GDD (and openings in the stencil) for different components when printing with a metal, laser-cut stencil of 10 mil thickness using the printing parameters described:

Metal Stencils:
Some say "Acetone is the best solvent for cleanup from both a cost and solvency standpoint"
But if a non flammable solvent with equivalent solvency is desired then Acceton is not an option!!

It is recommended to verify the compatibility of the cleanup solvent with the stencil frame adhesive.

Plastic Stencils:
Plastic stencils can be cleaned with the same recommended solvents as for metal stencils with a few additional considerations.

• A static charge may build up on the stencil during cleaning that can effect printing results.
• Plastic stencils are more susceptible to scratching during manual cleaning.
• Typically plastic stencils are much thicker than metal stencils and small apertures may require more aggressive cleaning such as high pressure spray and/or ultrasonic methods

The shape and the consistency of the glue dot depends on the rheology of the adhesive (yield point and plastic viscosity).
Pay attention that the adhesive must have been developed with a rheology designed for stencil printing, not hygroscopic and excellent adhesion with standard and difficult to glue components (Low-Stress Plastic Encapsulated Components).

Printing parameters have a crucial influence on glue dot shape and consistency. In laboratory conditions, optimal results have been obtained with the following parameters:

Advantages:
(Valid for Conventional and for the Printing Process with Thick Stencils)

• Higher throughput.
• There is no need to install dispensing machines on the line:
  - shorter lines
  - lower investment.

• Larger glue packages can be used:
  - less package waste
  - lower costs for the glue per quantity unit.
• This process is well suitable for double sided boards populated with SMD-components only.
• Adhesive deposits that are shapes are possible.

Disadvantages:
(Valid for Conventional and for the Printing Process with Thick Stencils)

• Less flexible as each PCB-Layout requires another stencil or screen.
• Thick stencils are more difficult to be cleaned than needles.
• It is not possible to print adhesive on a populated PCB.
• Not applicable for double sided reflowed boards, as it is only possible to dispense the glue after printing of solder paste (process: first side of the PCB - print solder paste, dispense the glue, place the components, reflow the paste and cure the glue ; second side of the PCB - print solder paste, place the components, reflow the paste).
• Adhesive is exposed to the environment over longer period of time - bigger temperature differences and water absorption can cause performance changes in the adhesive - it is recommended to use non-hygroscopic adhesives which have low sensitivity to temperature.
• There is a danger of dust/lint entrapment in the glue.

(Valid for Conventional Printing Process only

• It is difficult to process components with a big difference in stand-off (QFP’s and PLCC’s) as all glue dots have the same height.
  This assumes that the component stand-off height is > 10 mils.

Concerns:
(Valid for Conventional and for the Printing Process with Thick Stencils)

• There is a danger of air entrapment in the glue during the squeegee process. The danger of air entrapment is bigger with high viscosity adhesive. On the other hand, an increase of the temperature and smaller vertical squeegee movement decreases the risk of air entrapment. The use of metal squeegees, can significantly reduce the entrapment of air.
• If a printing technique is used for processing of PCBs which are populated both with SMD and also with through-hole components, then the process must be organised as follows: print the glue, place the SMD’s, cure the glue, insert the through-hole components then wave solder. During the placement of the through-hole components a strong bending of the PCB is possible which might cause a loss of SMD’s. This danger is less present when the glue is applied by dispensing because through-hole component insertion typically precedes the glue dispense and SMT placement operations.

(Valid for Conventional Printing Process only)

• If there are components on the board with a very big difference in stand-off, it may be necessary to dispense the glue manually or automatically .

(Valid for Conventional Printing Process with Thick only)

• During the cleaning process with plastic stencil a static charge can occur.
• When printing with stencils considerably thicker than 10 mils the dot consistency of small dot (< 20 mils) is compromised

The conventional printing process works fine with the limitation that all glue dots have the same height.
High dots with excellent consistency for the component range of 0402 chips to SO 28 leaded components can be obtained with a thick (10 mil) metal stencil.
A print-print print mode with a metal squeegee blade and slow board to stencil separation speed are recommended.
By adding snap-off or by leaving a controlled film of material over the apertures after the last squeegee stroke, very high large dot heights can be obtained.