Dispensing any fluid material onto very small hybrids has two basic requirements: Applying the material to the exact location required and dispensing a dot of exact volume. Due to the small size of substrates and the typical density of required material pattern, these are not minor problems. Figure 1 illustrates a typical hybrid substrate wil material dots of 8 mils in diameter. In applying material to populated substrates these problems become magnified and precision dispensing becomes even more critical. When the material has the idiosyncrasies of solder paste, another dimension is added.
Solder paste is a frequent choice for SMT hybrid applications because it has excellent electrical conductivity and, more importantly, it is re-workable. Solder paste is a slurry of fluxes and binders, and metal that can separate if not handled properly. Dispensing solder paste with the wrong type of dispensing head can cause it to separate. In this condition, metal separates from the flux, causing a poor solder joint. Since dots of solder for a physical as well as an electrical bond for devices mounted to the substrate, alignment of the dispensing pattern is critical.
Typical hybrid substrates or assemblies are small with dense patterns. Aligning the dispense pattern is more important than lining up the component to the substrate for mounting. The dispensed material can spread. If it is applied to the borderline of the proper spot and then a component is put on top of it, there is a good chance of destroying the assembly by shorting out the next pad. If the dots of solder paste are accurately dispensed (location and volume) but the component is "off" in placement slightly, there is generally more forgiveness.
Because of the small size of hybrids, the density of patterns, and the size of the dots to be dispensed, hand-held dispensing can be immediately ruled out. The only other feasable alternative to automated dispensing is screening. This has been done for years and when the hybrid is relatively large, typically an uncut substrate, and the dots of solder paste are not extremely small, it has served the application satisfactorily. But aligning the screen to a very small substrate with a small dot/high density pattern, and doing it with any real accuracy, can be a problem — if not totally impossible.
What is Solder Paste?
In order to understand the process of dispensing solder paste, it is important to have an understanding of solder paste. Many papers have been written explaining the proper selection and the various properties of solder paste in detail. For the purpose of this article, a brief synopsis is sufficient. Solder paste is a slurry made up of a metal alloy, generally in the form of balls, classified by mesh size. Balls of metal are held in a suspension by a mixture of flux, solvents, and binders. Therefore the physical qualities of solder paste are its alloy, type of flux, particle size, viscosity (slurry thickness), metal content, thixotropy (ability to stay put), and homogeneity.