Solder Paste Dispensing on Micro-Hybrid Substrates for Surface Mount Applications
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 feasible 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.
Typically, the alloy is chosen for an application based on desired mechanical and electrical requirements. The flux type is chosen based on the degree of cleaning required to achieve a good solder joint and the cleaning method used to remove any residual flux. Dispensing small dots of solder paste requires a specific particle size and viscosity.
Generally, the smaller the dot size the finer size particle required to dispense the dot through the tip. While metal particle sizes in a range of -325 to +500 (passes through a 325 screen, but does not pass through a 500 screen, as shown in figure 2) are common, smaller particles ranging from +625 are often used. The smaller the particle size, the greater the surface area per volume as
shown in figure 3. This added surface area places more requirements on the flux to flow the solder. Also, while -325 to +500 paste generally contains 85 percent metal by weight, the smaller particle size paste is generally only 70 percent by weight. In order for a dot of solder paste to remain in place during reflow, a relatively high viscosity is desired. A low viscosity material will slump, spreading the particles of metal over a wider area during the reflow process. Drying out the solder paste before adding components and reflowing is a method of managing the spreading problem.