Surface-mount adhesives curing

Single part epoxy presents another option. Some materials, such as surface mount adhesives, have the curing catalyst locked in a phase change material that is released with heat. Cure temperatures range from 100 to 150°C. The material supplier controls the mixing process, and these materials have excellent shelf life. 

Surface-mount adhesives 35-50% epoxy resin, 1-10% amine or polyamine curing agent, and 10-30% mineral filler and thixotrope Mondur MC/castor oil and castor oil polyols (>60%), filler 5%) Eccobond 125F (Henkel), Epo-Tek 70E-4 (Epoxy Technology), Epi-bond 7275 (Cookson Electronics) Flexobond 442 (Bacon Industries)... 

The spread and slump for surface-mount adhesives may be measured according to IPC-SM-817, Method 4.5.5. According to this method, sets of adhesive dots (minimum three per set), 0.65 cm in diameter and 0.25-mm thick, are deposited (for example, by screen or stencil printing) on each of two frosted glass microscope slides. One sample is stored 50-70 minutes at 25 5°C and 50 5% relative humidity and then measured for spreading of the uncured adhesive. The second sample is cured and measured for the increase in pattern from the original dimensions. The acceptance criterion is an increase of less than 10% of the original diameter of the dot or pattern. 

Coarse-pitch surface-mount components, especially passive devices, are affixed to the bottom side of the PWB using surface-mount adhesive. The adhesive is cured prior to wave soldering. The adhesive is in contact with the body of the component, which is aUgned with its respective pads on the circuit board in preparation for wave soldering. 

An example of a polyurethane adhesive used for surface mounting components consists of Mondur M cured with castor oil or with other polyols. The molecular weight and properties of polyurethanes are optimized when one equivalent weight of the isocyanate reacts with one equivalent weight of the polyol. The equivalent weights are based on the percent of hydroxide of the polyol and the percent of isocyanate of... 

Surface Mount Technology (SMT) Adhesives, Dymax developmental MULTI-CURE 997, Dymax Corp. (undated). 

Moisture absorption in uncured and cured adhesives is important for reliable adhesive bonding. Absorbed moisture in adhesives, particularly those used in surface-mounted parts, may result in voids in the bond hne due to moisture desorption or entrapment. Absorption of moisture after cure also affects reliability. Popcoming, the cracking of plastic parts during solder reflow, is due to the rapid vaporization and expulsion of moisture retained by the die-attach adhesive or the plastic encapsulant. 

Many aerobic formulations can be heat cured, but only between two surfaces. Table 1 shows the heat curing properties in an adhesive used to fix surface mount devices onto printed circuit boards compared to a typical epoxy and a typical anaerobic formulation used for the same purpose. 

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