Viscosity automated dispensing

The number of dots that can be dispensed per unit of time also depends on viscosity. The use of low-viscosity adhesives increases the number of dots that can be dispensed. However, there is a tradeoff between the speed at which the adhesive can be dispensed and the speed of device placement. Most automated component-placement equipment operate faster than adhesive-dispensing equipment. A comparison of the effect of viscosity on dispensing and placement times is shown in Table 4.19 for an 86-lead plastic-encapsulated leadless chip carrier (PLCC). ... 

Besides premature aging, an increase in temperature during dispensing is also critical for example, a 5 °C rise above room temperature alters the quantity of material dispensed as well as the shape of dots and patterns. Increased temperatures during automated dispensing lower viscosity and increase the mass of dispensed adhesive. The increased amount of material may interfere with adjacent circuitry or with... 

A property related to viscosity is the thixotropic index, ameasure of the dependence ofviscosify on shear-rate. The thixotropic index (TI) is the ratio of viscosities measured at speeds that have aratio of 1 10 rpm, for example, at 2 rpm and 20 rpm. For non-Newtonian materials used in automated dispensing, shear thinning is apparent from TI values of 2-6. In general, capillary-flow underfills have thixotropic indices of 1, or close to 1. 

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