Unlike wave soldering, reflow soldering does not inherently include the solder in the soldering process. The solder needs to be introduced to the board beforehand and then melted in the reflow oven. Solder paste can be applied manually with fine instrumentation or, more realistically, modern inkjet style printers can precisely apply solder paste to the pads. However, for even small runs, using a solder stencil currently is the most common process. A PCB stencil provides a template that goes over the PCB and has openings only where solder paste needs to be applied. With a glob of solder paste and a squeegee, an entire board can have the solder paste applied within seconds. This also can be automated and done by machine. However, there are different styles of stencils and knowing exactly how they work can help in getting the best stencil for your project.
A typical assembly solder paste stencil is a flat, smooth surface of specified thickness made of either stretched Kapton flexible material, Mylar film, or thin stainless steel. The internal cutout edges are smoothed in some manner to assist in the release of the solder paste. Some stencils are made of stainless steel or nickel, which gives them a longer life and are either cut by a laser or have the pattern chemically etched out. The stainless stencils can be thicker (up to 16 mils) in order to place more solder paste on the pad. Minimum cut width is .002 mils, with .00025 inches positional tolerance and feature smooth side walls for better release of paste. A stencil size of up to 29 by 29 inches is available. Mylar and Kapton stencils are lower cost than stainless steel stencils and used mostly for small runs and prototypes. They have a lower cycle life and are cut with UV laser. They are typically .003 to .005 thick and supported by a stiff cast aluminum frame.
Mylar stencils are either polyester film or the stencil can be made from Kapton, which offers higher tensile strength and reliability than Mylar as well as the ability to maintain better physical, electrical, and mechanical properties over a wider temperature range. The main difference between Mylar and Kapton is that Kapton can remain stable in a wide range of temperatures, from -273°C to +400°C, while Mylar’s range is about -70°C to +180°C. Both feature 100 plus cycles, and both are inferior to stainless steel stencils for registration.
Besides the materials, there are different styles of stencils. Small, flexible BGA stencils have been developed that are to remain as permanent fixtures of the PCB, helping with rework if there are any issues with the chip. Multi-level stencils are more exotic stencils that have different levels, which allows different thicknesses of solder paste to be applied on mixed technologies, such as through hole and fine pitch SMT. The mix requires different mounts of paste for different parts, eliminating co-planarity problems. It is possible to print older paste pillars with this technology for BGA chips, along with the SMT components. To achieve this, solder paste is printed on, in and around the through-hole and pad. The step up stencil can achieve sufficient solder paste for this application. On top of the base material of the stencils, there are sometimes an extra coating to cover either the metal or plastic basis. The addition of a permanent, nanocoating to the stencil foil minimizes the ability of the solder paste to stick to the stencil apertures and the bottom side of the foil. With the nano-coatings, up to an eight time increase in the number of prints is possible before cleaning the stencil, as well as successful printing at surface area ratios below 0.45.
The Stencil Printing Process
The stencil printing process starts with aligning the stencil so that the final location of the deposited paste is on the pad then filling of the aperture with solder paste. The accuracy is important in achieving the best result of an accurate volume of paste (a brick) deposited to the correct pad location on the PCB. Stencil dimensional accuracy and printing positional accuracy is dependent on the quality of the CAM data, the stencil quality and the printing quality. The printing positional accuracy will be determined by the alignment pin methods used and the quality of the PCB to tooling hole accuracy. The first step in printing solder paste is to fill the stencil openings with solder paste by using a metal squeegee blade, pulling a bead of solder paste in front of the blade and therefore filling the apertures.
Several factors can contribute to the accuracy of the aperture fill process. The direction of the aperture with respect to the squeegee blade bead has an effect on the fill process. An aperture with its long axis in the same direction as the blade bead does not fill as well as an aperture with its smallest opening to the blade bead. Squeegee speed also influences aperture fill.
The squeegee blade edge has a large influence on how the paste fills a stencil opening. Best results occur as you print at the minimum squeegee pressure while still maintaining a clean wipe of the solder paste on the stencil surface. If squeegee pressure is too high, both the squeegee blade and the stencil may be damaged. Excessive squeegee pressure will cause paste to smear under the stencil surface.
Knowing the materials involved and the way a stencil is used is the first step towards design. If design is not the end goal, this information will help you understand the pros and cons of each type versus the costs and requirements of your particular project. Assembly houses have a great deal of experience in this field so they are a great resource to discuss your stenciling needs.