Adhesives and Sealants

Dow Corning ® Adhesives are easily applied by a variety of methods. A general overview of these processes is given below. 

Unlike encapsulant and coating materials which usually cover or coat an entire circuit, adhesives are generally applied to distinct areas of an electronic part either by dispensing a bead of material, extruding a dot or glob near a particular component or even screen printing onto a portion of the substrate. 

Dow Corning ® Adhesives are supplied as either one or two part formulations, depending on the product being used.  Two-part products can be either a 1:1 or 10:1 mix ratio. Typically the two components are readily mixed with static or dynamic mixers. In addition many one-part adhesives are available which eliminate the need for mixing. They can be applied with simple needle dispensing systems. Automated systems are normally used for high volume processes with metering and mixing capability added for the two-part products.  In low volume, manual dispensing may be appropriate. These dispense systems will be discussed in more detail below. 

For assistance in designing a process tailored to the needs of your application, contact Dow Corning’s technical experts in the Application Center.

Automated Dispensing

Automated dispense systems are typically used for higher volume production processes. For both one-part and two-part products the adhesive is typically dispensed using either using a pump or other means of pressurization to move the material. If gas pressure is used, a barrier should be used to separate to the gas from the material to reduce bubbles in the cured adhesive. The adhesive is moved from a reservoir, which can be equipped with filler agitation or vacuum capability, through lines for dispensing. The material is then metered and extruded through a needle or similar dispense head. . For high volume processes, the pattern of application is generally robotically controlled. The dispense head moves to allow the material to be applied in the desired configuration on the part. Snuff-back valves are often used on the units to reduce stringing and dripping of the adhesive between dispense operations.

For two-part products the dispense systems consist of a separate feed reservoir for each component. The systems are set to achieve the desired mix ratio of the two components. The materials are then fed through a dispense head attached to a static or dynamic mixer which is used to achieve a uniform mix. The dispense head or valve is used to control the amount of material that is dispensed onto the part. Static mixer tubes may need to replaced or cleaned when the process is shut down for long periods to avoid having the material cure in the mix tube.

In some products volatiles may form in the adhesive during normal storage. These volatiles can create voids or bubbles in the adhesives when they are heat cured. If bubbles in the cured material are a problem, it may be necessary to de-air the adhesive in its original container or in the reservoir before it is applied. Also, if gas pressure in a pressure pot is being used to dispense the adhesive, the gas pressure should never be applied directly to the top surface of the material because the gas will dissolve into the adhesive and will create bubbles in the cure material. Bubbles caused by volatiles generally are not a problem for any of the room temperature products. In addition, some of the newly developed heat-curing adhesives were formulated for very low bubble formation. 

Also, during normal storage, filler separation may occur in some of the adhesives. The filler particles, used in some formulations, are dense enough that gravity can cause them to settle over time, leaving a clear polymer layer on top. The longer the adhesive is stored, the more likely some filler separation will occur. In some cases, it may be necessary to re-mix the material before being used in your application and it may be helpful to have a gentle agitation system in the feed reservoirs to keep the fillers uniform.  Care should be taken to avoid introducing air into the product when re-mixing the filler.

Dow Corning can help you select the proper dispensing system for your needs through our Equipment Alliance .

Manual Dispensing 

Many adhesives are available in cartridges or in tubes that can be used in manual dispensing equipment for low volume applications. Many different types of manual dispense systems are available. Some products are supplied in aluminum toothpaste tubes or caulking cartridges used with a manual gun. Both of these types are generally supplied with a nozzle tip that can be trimmed to regulate flow. There are also plastic syringes with plungers or cartridges.  Some are simply squeezed by hand while others use gas pressure to dispense the material. Syringes and cartridges with separate chambers for two-parts are available. Two-part packages typically have static mixer attachments to achieve mixing.