The dispensing of substances is one of the core technologies of the micro assembly industry. Dispensing techniques ranges from dispensing of adhesive dots over dispensing of solder paste on eletrical contact pads to conformal coating for sealing complete surfaces. The applications are very different – but all have the challenge of being very precise.
Precision in this context means to dispense a defined amount of the substance. In the production of micro systems the required amounts of dispensed material can be in the nano liter range (1nl = 1×10-9l) and below. The used dispensing technology have to ensure that these parameters have to be reached – indepentently of varying material properties or environmental effects. Furthermore, another generic challenge of dispensing processes in the micro assembly industry are the required processing stability and processing capability. This is necessary to ensure a constant quality of the products. Often this is a functional requirement of the products function. Without a sufficient precision and reproducability the function of the product will be affected.
The requirements of precision and reproducability are relevant for the following parameters:
- volume V = f(x,y,z)
- position (x, y)
- contour accuracy (eccentricity of circles, straightness of lines, shape of points, etc.)
Further important values in terms of the processing of dispensing materials are:
- density ρ
- viscosity η
- surface tension σ
- thixotropy for filled materials (SMD adhesives, solder paste, etc.)
- filler materials
- curing process of adhesives (UV-curing, thermal curing, etc.)
These properties are often provided by the manufacturer of the dispensing materials and are published in technical data sheets. Due to diversity of the dispensing materials and technologies dispensing test are necessary to estimate the parameters for the required precision of the dispensing process. Furthermore, the parameters are also influenced by a multiplicity of environmental issues and the stochastic behavior of the fluid itself.
In the micro assembly production the dispensing is often combined with a pick & place process. Common dispensing materials are adhesives, lacquers or solder paste. For each different material there is a subsequent processes. For instance UV-curing of adhesives that has to be irradiated with high doses of UV light in order to activate the curing process. The subsequent process for solder pastes is often a pick & place process and following solder process. According to the requirements of the solder junction a reflow or laser solder process can be applied.
Fig. 1 Contact dispensing with single tilted, cone-shaped needle for dispensing application within the nano-litre range