Evaluating SMD Adhesives

It�s uncommon to test green strength. [Whatever �green strength� is. Some chip bonder adhesive suppliers have taken to taking about improvements in �green strength� their literature. We have asked sales types to quantify this and have never heard back from them. Un-named technical types imply that this is marketing type.] Maybe you supplier can offer a suitable test scheme.

Viscosity is a more common measure of the properties of uncured material. Two systems for directly measuring the stress in a liquid are Haake [popular in Euroland] and Carrimed [popular in the US]. Each of these systems can produce a "flow curve" [shear stress vs. rate] and a "viscosity curve" [viscosity vs. shear rate].

However because of the thixotropic characteristic of chip bonders, different viscosity values can be obtained for any given shear rate, depending on the shearing history of the sample before measurement. This means that viscosity values cannot be directly compared unless measured according to the exact same methodology. DIN 54453 defines one standarded method.

Brookfield is out of fashion because it is a comparator measurement system.

Listen, we do not have problems with components falling off the board before cure. We break components from the board during handling between cure and wave. [Read this as stupidity.]

The cured adhesive joint needs only enough strength to hold the component to the underside of the PCB while passing through the soldering station. The forces on the joint are quite low, typically less than 2N for a flat chip [according to Loctite]. The biggest challenge is surviving the thermal shock as the joint moves from the preheat section into the solder wave. This requires a combination of good adhesion, strength, and flexibility.

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