Soldering paste printing process

Referring to the solder paste printing process with a stencil, Pb-free solder pastes behave very similar to the Sn-Pb pastes for leaded and area-array pitches of greater than 0.5 mm. At the smaller pitches that are characterized by smaller apertures, it has been observed that the Pb-free solders have a slightly reduced transfer coefficient. The likely... 

In the solder paste printing process, defects typically are caused by poor alignment between the substrate and stencil, incorrect material selection (substrate, paste type and stencil design), or variations in the amount of paste deposited. Defect elimination relies on the engineer and operator to address these variables and monitor the process. 

The solder paste printing process is simple—place the correct amount of solder paste in the correct location at an acceptable rate. While this goal sounds easy, executing this requires the identification, understanding, and optimization of several factors that influence how well the process performs. 

Assembly requirements also place constraints on the circuit board layout. A very high part density requires a large number of apertures in the solder paste stencil, which can cause the stencil to become locally too flimsy to control the solder paste deposit. A surface-mount circuit board with a very wide range of component sizes and package configurations may require multi-thickness stencils to properly control the paste deposit. Solder paste printing quality is a determining factor in solder-joint defects observed after the reflow process. 

Morris, J. R., and Bandyopadhyay, N., No-Qean Solder Paste Reflow Process, Printed Circuits Assembly, February 1990, pp. 26-31. 

This chapter covers why manufacturers inspect printed circuit assemblies, how they have implemented and enhanced visual inspection, what automated inspection systems they are using, and how they have implemented these antomated systems. The scope of this chapter includes only inspection of printed circuit assemblies during the assembly process, as typically shown in Fig. 53.1. Thus, it includes inspection of solder paste after the paste printing process step, components after the component placement process step, and solder joints after the solder reflow process step. Not included, however, is incoming inspection of components and the bare printed circuit board (PCB). The focus of this chapter is on prodnction nse of inspection, not the collection of measurements dnring process development in a research and development (R D) environment. 

Solder Paste Measurements. Typical solder paste measurements, shown in Fig. 53.8(a), are volume, area of pad covered, height, and misalignment with the pad. These quantitative measurements provide information about the paste viscosity, stencil registration, cleanhness, snap-off, and squeegee speed and pressure that can lead to improvement in the paste printing process. 

Post-Reflow Solder Joint Measurements. Solder joint measurements, such as fillet heights, average solder thickness across the pad, void volume, and pm-to-pad offsets, as shown in Fig. 53.8(c), provide information about the paste printing process, the component placement process, and the solder reflow process steps. Attribute measurements, such as solder bridges, opens, or insufficient solder, are most common. Quantitative measurements of solder... 

Analysis of these physical thickness measurements of solder joints provides the information required for process characterization and improvement. For instance, variations in average solder thickness or volume for the solder joints across a single assembly or from assembly to assembly provide insight into the quahty level of the paste printing process as well as sources of defects. 

Surface Finishes. The search for alternatives to hot air solder leveling (HASL) has been ongoing for several years, primarily because of the inherent inconsistency in the quality of the HASL finish. For example, the thickness (and therefore, solderability) of HASL is difficult to control. In areas with a very thin layer of HASL, consumption of Sn by the formation of tin-copper intermetallics will render the areas non-wet-table. The HASL finish is typically non-flat (with a dome shape), making it difficult to deposit a consistent amount of solder paste during solder paste printing and difficult to place fine pitch (<25 mil) devices. The HASL process itself is not as clean and easy to control as some plating processes. The current move towards lead-free solder has provided the additional impetus towards alternative surface finishes. 

The process parameters key to the stencil printing process include the squeegee type and hardness, print speed, print pressure, and print gap. An optimal stencil printer setup provides a clean sweep on the stencil surface and a repeatable solder paste deposition process. Shore A scale polyurethane squeegee hardness level between 85 to 95 is appropriate to achieve repeatable solder deposits for lead-free solder pastes. Print parameters such as the print speed, print pressure, and the print gap are adjusted to accommodate the variety of solder paste rheologies available. There are no specific equipment modifications necessary to stencil printer lead-free solder pastes. 

The main advantage of stencil printing over screen printing occurs in applications where very small areas of paste have to be deposited. For components with pitches equal to or smaller than 0.65 mm, the stencil printing process is the only viable way for printing solder paste. Therefore, stencil printing has replaced screen printing in most cases. 

Conventional printing processes are less useful for applying conductive adhesives or solder paste. Dispensing of single dots in a complex geometric application is therefore necessary. Three-dimensional circuit carriers also decrease the freedom of component placement, which leads to restrictions for placement systems. 

Stencil printing is a fast efficient process for long production runs and is widely used in surface mounting of components. In surface mounting, the solder paste is generally dispensed first by stenciling, followed by needle dispensing the adhesive however, these processes may be reversed or both the solder and adhesive can be selectively dispensed by needle. 

As with most processes, numerous variables affect the final product. For solder paste deposition using stencils, Chouta and Heck identified 39 variables relating to materials, 32 variables to processes and equipment, and 10 variables to personnel and environment. Among variables critical for the successful stencil dispensing of adhesives are stencil design, the adhesive material and its rheological properties, and the printing process. 

Although stencil printing is widely used for both adhesives and solder paste, and has many advantages over other dispensing processes, there are several limitations that must be addressed as follows ... 

In the general process flow for SMT, printed-circuit boards are cleaned and dried, solder paste is stenciled or screen printed onto pad locations to which components are... 

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