By Alden Johnson and Bob Boyes, Speedline Technologies Inc.
Optimizing fine-pitch electronics assembly requires a careful review of the complete process. Each variation of device, lead spacing, substrate, material and build-rate schedule requires specific parameters to ensure high yields. Additionally, selecting the best combination of materials, equipment and data analysis systems can greatly reduce development time and costs. In stencil printing, defects are typically caused by one or more of the following: poor alignment between the substrate and stencil, incorrect paste chemistry or variations in the amount of paste deposited. To eliminate defects, the capabilities of the printing equipment and the materials selected (substrate, paste type and stencil design) should be examined closely. Process Considerations It is no longer reasonable to define the high-yield PWB production process by trial and error. Many aspects of designing and optimizing the process require efficient, accurate experimentation. Statistically designed experimentation can be employed to obtain maximum information at a minimum cost in time and resources. Conclusions drawn from experiments will help determine the best course of action in establishing the process. For fine-pitch assembly, printing parameters must be individually tailored to each application. It is possible, however, to offer some general guidance as a starting point for experimentation and process development. Material Selection Circuit Board Design-The printed circuit board design (and the many materials of fabrication and methods used in circuit board construction) should be examined first. These may be separated into three critical elements that directly affect the printing process: pad size, pad plating (or finish) and solder mask type. Identifying these parameters will dictate the materials and equipment needed to complete the process. Stencil Design-Material, thickness, image pattern and aperture size are the key elements of the stencil design. No single combination of these elements can be recommended as the best choice for fine-pitch printing. Instead, the various options available must be considered in the context of the overall assembly process (Table 1). Paste Selection-The factors that affect paste are its rheology, particle size and shape. Type 4 solder paste, for example, is required for pitches under 0.4 mm, based on experimentation showing that four or more solder particles are needed to span the stencil aperture to achieve consistently good solder paste deposition. Similarly, with 0.3-0.4 mm pitch, the stencil openings should be between 0.005" and 0.008" wide. Since Type 4 paste contains <0.0014" solder particles, this criterion is statistically met. Recommended paste particle sizes, based on lead pitch, are provided in Table 2.
Squeegee Blades-Metal squeegee blades (Figure 1) enable a more controlled and consistent print height across the entire board area. Good results, however, may be obtained with urethane squeegee blades if there are few large pads on the board. A harder urethane (or high-density polyethylene) blade minimizes the potential for "scooping" on all but very large pads. Durometer hardnesses range from 60 to 120; however, 90 to 110 durometer blades provide excellent results. Appropriate Equipment After the design of the PWB has been determined and the materials have been selected, the next step is to review and select the appropriate equipment. There are several key criteria that a stencil printer should meet: Adequate Positioning Capability-The printer must align the stencil to the substrate with precision and repeatability. Because of the dimensions inherent in fine-pitch processing, positioning capability is the most important factor in printer selection and is essential for high throughput. For a 0.020" pitch device, the typical designed pad size is 0.012" to 0.013" wide, while for a 0.012" pitch device, pad size is 0.007" to 0.008" wide. The stencil opening should be designed 0.001" to 0.002" less than the pad width to ensure good gasketing during the paste deposition process. These dimensions allow only 0.001" to 0.002" between either side of the opening and the edge of the pad. If the printer alignment system is incapable of maintaining this relationship and paste is deposited beyond the edge of the pad, there is a possibility of bridging to an adjacent pad, which may cause a solder short. In volume production, the printer must reproduce this precise alignment at production speeds. The most efficient and reliable alignment systems incorporate discrete motion control for each axis and should be controlled by a closed-loop vision system with the capability of viewing multiple fiducial points on the stencil or substrate. Mechanical Stability-The ability of the printer to provide the necessary placement accuracy and repeatability requires a robust design. Structurally, the machine must be rigid enough to prevent relative motion between any axis of motion and the rest of the machine. Lack of rigidity may cause variations in the system's response, resulting in assembly defects. Rigidity is also required to ensure that all movement of the stencil in relation to the board is accomplished in planes that are parallel within narrow limits. Squeegee pressure is based on length of blade and material. For urethane or polyethylene, the starting pressure should be around 1.6 to 3.0 lb/in. of squeegee. When using metal blades, the starting pressure should be 1 lb/in. of squeegee. Based on the quality of the print, the pressure should then be adjusted to give complete fills of the apertures and clean wipes of the stencil surface.
Solder Paste Determines Speed Squeegee speed is determined primarily by the solder paste being printed. A range of 0.500" to 6.500"/sec. is typical, with higher viscosity levels requiring slower speeds to allow the paste to flow into the stencil openings. When printing ultrafine-pitch, a range of 0.500" to 1.000"/sec. should yield good results. Both pressure and speed can be used to control print viscosity, with pressure contributing 80 percent and speed 20 percent, to take advantage of the shear-thin properties of the paste. Maintaining consistent print pressure across the board during the entire print stroke, despite board topography, is an absolutely essential task performed by the printhead. Real-time control of downward printhead pressure with precise stencil and PWB relative positioning prevents process variations. Sophisticated programmable printheads with balanced pressure points, located at each end of the blade, offer significant advantages. "Downstop" level is dependent on machine type, with the major consideration being the printhead design. Downstop controls the distance that the printhead travels beyond the substrate and prevents coining of the stencil. Downstop should be set to a level that enables the squeegee to wipe the stencil clean and not over-deflect the stencil as the blades go beyond the substrate. Proper board support is essential to ensure consistent print results and higher yields. Without this support, the force applied to the board (across the entire width of the PWB) will vary, preventing proper gasketing between stencil and board. Blade angle may also be affected, and may cause paste to remain on the stencil. Board supports should be distributed evenly across the width of the board, particularly with BGA and fine-pitch devices, to prevent bridging (sagging) and inconsistent paste deposition. Additionally, board supports should be kept clean. Cleanliness will ensure flatness and eliminate the potential for paste deposits on the underside of the board. Optimal Stencil/PWB Separation The critical factors in stencil/board separation are speed and distance. If the board is separated from the stencil too quickly, "bounce back" (fast, repeated contact between the stencil and PWB during separation) will occur, requiring the board to be cleaned and reprinted.
Additionally, paste will remain on the bottom of the stencil, causing problems during the next print cycle, unless thoroughly cleaned. Lifting, or paste retention in the apertures, may also occur if separation is performed too quickly, causing insufficients and bridging, and the stencil will have to be cleaned prior to successive prints.
In stenciling, a reasonable grasp on true production fluctuations requires the combination of good inspection tools with active statistical process control (SPC) features. A good inspection system must be able to feed data back to enable process adjustments in real time so that the process engineer can take action to correct the situation before defective products are made. 2-D inspection systems (Figure 2) measure the amount of paste that is covering the target pad (Figure 3) and compare that against the required coverage. For automated systems, the operator can elect to allow the printer to automatically initiate corrective action. Verifying the results of a freshly printed board is the optimal way to determine that the print process is in control and that acceptable boards are being produced. Operator Intervention Automated operations minimize the need for operator handling, making setup easier and ensuring a consistent and repeatable process. Some advanced stencil printers (Figure 4) offer features such as programmable printheads that permit the operator to program squeegee pressure and downstop and to automatically level the blades before printing. Automatic paste dispensing systems add preprogrammed amounts of paste at preselected intervals, keeping solder paste from remaining on the stencil for extended periods and constantly refreshing paste supply. Automatic stencil wiping provides unassisted cleaning of the stencil, while a vacuum system cleans paste from clogged apertures. Bump Printing Stencil printing solder bumps onto wafers brings the promise of high yield and throughput, low tooling costs and full automation.
Including wafer handling and printing, a production rate in excess of 40 wafers/ hour is easily achievable. Adaptable for a variety of solder paste compositions, there is no penalty for wafer size evolution and print speed is independent of pattern density and bump size. However, process integration for wafer printing calls for more than just scaling down the stencil dimensions. The typical aperture size of roughly 125 µm violates the aspect ratio rule for stencils thicker than 85 µm. High-density patterns, needed to make larger bumps, require large, closely spaced apertures in thin foils. This requirement (to meet the aspect ratio rule) places increased demands on the paste release and stencil wipe processes and frequency. Additionally, the stencil represents a critical limiting factor in the quality of the printed wafer. Aspect Ratio-With a required aperture aspect ratio in excess of 1.5 for good paste release, the thickness of the stencil must be minimized to achieve consistent paste height. Fabricators must inspect 100% of the apertures for location and size. Several vendors have implemented machine vision and precision micro-manipulation to accomplish total aperture inspection, but the equipment and procedures are expensive.
Paste type is also of concern. Because wafer-printing stencils are typically 75-125 µm thick, a Type 5 paste PSD centering on 22 µm will give a typical printed brick only about 3-5 beads in height. A variation in paste volume of ± one layer of beads for this paste results in a variation of ± 25% of bump volume. Although the actual printing, aligning and passing the squeegee across the wafer takes less than a minute, none of the material and equipment operating parameters are completely independent of the others. Conclusion To develop the optimal PWB or wafer bumping production process, it is essential to consider all aspects of the process. The board design, the components to be placed, the materials to be used and the equipment selected must all work in harmony.
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