Practical Selection of Adhesives for Surface Mount Assembly

Surface Mount Technology (SMT) first started with the introduction of mixed technology designs incorporating chip components mounted on the underside of the circuit board. This process is often called a "Type II" assembly incorporating conventional components, with SM components on the underside. "Type I" contains only SMT components, with a "Type III" containing conventional components and SM parts on both sides. These terms were originally coined by the IPC in the USA for many of their specifications.

The use of wave soldering technology, and with it components mounted on the underside, requires the use of adhesives. Adhesive is used to hold the parts in place during placement, through the handling operations and the final through the soldering process. After soldering the adhesives are not required to provide any further function. In fact, if the material could completely disappear after wave soldering, this would be, in most cases, an advantage. 

The adhesives which have been used over the years have come in for a lot of criticism; poor dispensing, poor electrical characteristics, short storage and operating life. The materials have also been accused of causing considerable problems during the rework and repair of products. In most cases the criticism levelled has been partly the fault of process engineers not properly evaluating the materials and the suppliers for putting materials on the market without properly evaluating the formulations in real production environments 

The choice of adhesives for effective manufacture is now a little more straight forward for the engineering to assess. The choice of materials for mounting components is still quite wide if we consider the total range of adhesives available, but in most cases many of the available formulations can be discarded due to production requirements. 

The following are some of the adhesive characteristics which need to be evaluated :

Electrical properties
Dispensing
Adhesive strength
Slump/bleeding
Material compatibility
Cure time/temperature
Adhesive life
Outgassing

Today the use of one part systems is the norm due to the simplicity of supply and use. The one part systems come in epoxies, acrylates and cyanoacrylates. Whichever material is selected process control on the assembly line is still very important factor. The two methods for in process inspection are shear strength and degree of spread, both can be evaluated using SPC (Statistical Process Control). It is of course necessary to first evaluate the material, to establish control limits for production, if this is not done correctly SPC will not be beneficial. Control limits can easily be achieved with a simple process capability program. With the advancement of materials the ability of the production equipment has been called in to question with many engineers moving away from the standard production line dispensers supplied to the more flexible higher accuracy systems supplied by the likes of Camelot Systems.

Many engineers evaluate adhesive strength without asking the first basic question, how strong does a glue have to be for surface mount ? The actual force exerted on an SOIC16 during wave contact has been found to be less than 20-30 grams. So why do we loose components in the wave ? 

Because they were never correctly bonded in the first place!!!

In the case process monitored the degree of shear force required to remove components after glue curing can be easily tested. Shear strength measurement of scrap components mounted on the board after cure is a useful test. It can be used to determine correct cure cycle, limited glue dispense and contaminated components or circuit boards. The shear test may be conducted using components placed on the edge of the board or on the scrape area of a multi panel. Adhesive is dispensed as part of a glue pattern with component place in position specifically for this test. The board is put through the recommended curing operation and measurements are taken for shear force after the board has cooled.

The test results obtained may be affected by the solder resist type, the copper preservative coatings on the PCB, the release compounds on the components, quantity of adhesive, component stand off height and the adhesive quality. As a basic guide the minimum shear strength that should be expected on an SOIC16 should be over 500 grams. Normally the force will seen will be in excess of 1kg.

For in process inspection purposes it is also effective to provide a glue test pattern, the inspection staff can visually examine for stringing, the size of dot or tailing. Measurements may be taken of the diameter of the test dots with reference to limits which have already been defined using statistical techniques. The test pattern may also be used prior to adhesive cure for bond strength testing with scrap components. 

An alternative method used by some manufacturers is an active test pattern which may be used as a visual test for accuracy or glue slump. The test pattern shown is used on glue stations with a two dot pattern. The pattern consists of three sets of dots each at right angles to the next, each with a different separation. In the test pattern one dot pattern is touching the next, in the second they are separated. In the case of variation in the process excessive glue would cause each dot pattern to merge, limited glue would show each dot pattern to be separated. The active test pattern has been successfully run on a number of pick and place systems and again provided simple, but effective quality control. 

If legend ink is being used on the design it is beneficial to have an adhesive pattern box printed on the edge of the board. A white box makes visual and vision recognition far more effective. Looking at red inks on a green back ground is difficult.

Inspection standards are always necessary in production if test patterns are not appropriate. These must be based on the capability of the process and used as a reference for inspection as well as initial training programs. A reference sheet can be extremely useful when quality engineering are collecting SPC data, this guarantees that everyone is working to a common standard and the resulting statistical data is of some value. Examples of the inspection criteria is available on the authors Internet Web Site http://www.bobwillis.co.uk

Surface contamination or bleed is difficult to detect with some adhesive materials without surface analysis in a laboratory. The adhesive may affect soldering but not be readily visible on the surface of pads or component terminations. A simple method of evaluation is to dispense and cure dots of material under evaluation on to a small copper laminate sheet. The sheet of material may then be passed through a wave soldering process. The solder should fully wet the surface of the copper right up to the adhesive dot. There should be no evidence of copper visible as this would indicate adhesive contamination or bleed during curing.

To investigate the degree of adhesive spread before and after component placement a thin glass microscope slide may be used. Dots of adhesive are dispensed onto the glass slide of a size appropriate for the components. The components are then placed and the glue cured as recommended by the supplier. After curing the underside of the component can be examined for the degree of adhesive spread. This is a simple method to determine glue dot size and shows if the glue is likely to contact the component terminations.

The test samples also allow the curing to be assessed. If the adhesive is cured too fast voids may be present in the glue. The same can be said if the water content of the glue is high as it will result in void formation. This can be easily examined looking through the glass slide. Voids can lower the adhesive strength and may result in solder being trapped during wave soldering. Some engineers may not believe that solder shorts can occur through an adhesive but they can and it is a factory stopper !!

Some people feel that the use of adhesive in mixed technology designs only cause problems. Contaminating pads surfaces providing defects on the wave soldering process. This only occurs if the process is not in control. Adhesive can help to eliminate solder shorts during wave soldering and may be a real solution to difficult designs. During normal automatic dispense programs a dot may be placed between two termination which always short. It makes a nice change to see glue solving rather than causing problems.

Bob Willis is an independent process consultant. Further process advice and information is available on Bob Willis Internet Home Page:http://www.bobwillis.co.uk