-
- News
- Books
Featured Books
- smt007 Magazine
Latest Issues
Current IssueBox Build
One trend is to add box build and final assembly to your product offering. In this issue, we explore the opportunities and risks of adding system assembly to your service portfolio.
IPC APEX EXPO 2024 Pre-show
This month’s issue devotes its pages to a comprehensive preview of the IPC APEX EXPO 2024 event. Whether your role is technical or business, if you're new-to-the-industry or seasoned veteran, you'll find value throughout this program.
Boost Your Sales
Every part of your business can be evaluated as a process, including your sales funnel. Optimizing your selling process requires a coordinated effort between marketing and sales. In this issue, industry experts in marketing and sales offer their best advice on how to boost your sales efforts.
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - smt007 Magazine
Step 4: Printing
December 31, 1969 |Estimated reading time: 12 minutes
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.
By Joe Belmonte, Bob Boyes and Alden Johnson
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.
Process Considerations
Developing an electronics manufacturing process using trial-and-error will result in a process that is “out of control” statistically. A process that is developed using trial-and-error has good days and bad days. We cannot use a process and hope it will produce acceptable products. There is one method to develop an efficient, high yield, stable electronics manufacturing process; and that is using formal experimentation and statistical studies to identify and optimize critical operating parameters. Many aspects of designing and optimizing the process require efficient, accurate experimentation. Statistically designed experimentation is used to obtain maximum information at a minimum cost of time and resources. Conclusions drawn from experiments determine the best course of action in establishing a process. Therefore, controllable variables of the process can be set at optimum levels in an objective manner - supported by data to produce the desired outcome. Once the process is stable, it should be monitored using statistical process control (SPC). To achieve best-in-class performance, it is vital to monitor the process to prevent defect occurrences.
Material Considerations
Circuit Board Design - PCBs should be as rigid as possible with a minimum of cut-outs and routings. If a PCB is extremely rectangular and rigid, the substrate will be more stable for printing. The PCB also should contain at least three fiducial marks that are on the copper-etch pattern, and not on the excess panel material, if possible.
PCB design and fabrication materials and methods used in circuit-board construction should be examined first. These consist of three critical elements: pad size, pad plating (finish) and solder mask. Identifying these parameters dictates the materials and equipment selected to complete the process.
Stencil Design - Four elements define typical stencil design: material, thickness, image pattern and aperture size. However, no single combination of these elements can be recommended as the most suitable choice. Instead, all available options must be considered in the context of the overall assembly process. One key element in stencil design is maximizing the amount of solder paste transferred through the stencil aperture onto the PCB pad. This is called “transfer efficiency”. Proper stencil design ensures that the force that adheres solder paste to the PCB pad will overcome the force that retains the solder paste in the stencil aperture. Two calculations that must be considered to maximize solder paste transfer efficiency are aspect ratio and area ratio.
Aspect ratio considers the ratio between aperture width and stencil thickness. Area ratio considers the ratio between the opening of the aperture (the area of the PCB pad that will be covered with solder paste) and the total surface area of the aperture walls. For small components where the stencil opening nearly is equal to the area of the aperture walls, area ratio is a vital calculation to design a stencil that prints well with minimum aperture clogging and maximum solder-paste transfer efficiency. An aspect ratio of 1.5 or greater and an area ratio of 0.66 or greater is required to ensure maximum solder-paste transfer efficiency and minimum aperture clogging (Table 1).
Paste Selection - Factors affecting paste are rheology and particle size and shape. For example, Type-4 solder paste 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- to 0.4-mm pitch, stencil openings should be between 0.005" and 0.008" wide. Because Type-4 paste has solder particles <0.0014", this criterion is met statistically. Table 2 shows recommended paste types, based on lead pitch.
Adhesives - In terms of equipment, any stencil printer capable of printing solder paste for a fine-pitch SMT assembly can be used for adhesive printing. In fact, adhesive printing does not require any special features from the stencil printer. Often, semi-automatic equipment is adequate. The most important aspect of the printer is programmable snap-off (or printing gap) capability between the board and stencil. The printer also should have a programmable, controlled separation of stencil and substrate.
Adhesive printing and solder-paste printing differ in materials (paste and stencil) used. Adhesives must possess several characteristics to produce reliable depositions. More importantly, printable adhesives must be suitable for long-term exposure to ambient humidity. To suit printing, an adhesive should be capable of being left on a stencil for three to five days without adverse effects. Printable adhesives also must be thixotropic so materials flow freely into apertures when sheared. Lastly, whether dispensed or printed, adhesives must have sufficient green or wet strength to hold components during the placement process, as well as a high-yield point to avoid slump and offer appropriate dot profiles.
The phenomenon of a single-thickness stencil producing various dot heights is key to successful adhesive printing. Adhesives do not release completely from apertures, leaving more in the apertures than is deposited on the substrate. As a rule, stencil thickness is determined by the highest dot height required. Generally, dot height should be about 1.5 to 2 times component standoff height. For high-yield point printable adhesives, a stencil thickness of 6 to 8 mils is recommended, while 10- to 12-mil thickness is suggested for SOICs.
Squeegee Blades - Metal squeegee blades enable a more controlled and consistent print height across the entire board area. However, good results can be obtained with urethane squeegee blades if there are few large pads on the board. Using a harder urethane or high-density polyethylene blade minimizes the potential for “scooping” on all pads except very large ones. Durometer hardness ranges from 60-120, however 90-110 durometer blades provide suitable results.
Figure 1. Rheometric pump head.
Another option is enclosed print heads. All enclosed print heads have the same goal - protect solder paste from the environment. The goal is to reduce the amount of solder paste being wasted due to its aging on the stencil and removal during product changeover. Enclosed print heads, whether for printing paste or adhesives, can offer superior results over blades (Figure 1). However, this depends on material formulation.
Equipment Considerations
After PCB design has been determined and materials have been selected, the next step is to review and select the appropriate equipment. It is important to consider future production demands when evaluating systems. There are several key criteria that a stencil printer should meet:
Adequate Positioning Capability. The printer must align the stencil to the substrate with accuracy and repeatability. In fine-pitch processing, positioning capability is the most important factor in printer selection, and is essential for high throughput and yield. For a 0.020"-pitch device, the typical designed pad size is 0.012" to 0.013" wide, and 0.007" to 0.008" wide for a 0.012"-pitch device. Stencil openings 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 cannot maintain this relationship, and paste is deposited beyond the edge of the pad, bridging to an adjacent pad could occur, producing a solder short.
In volume production, the printer must reproduce this precise alignment at production speeds. To ensure it can meet application requirements, the alignment process must be capable of greater precision than is required to meet physical alignment specifications. Ideally, a closed-loop vision system should control the alignment process to enable alignment to multiple fiducial points on the stencil and substrate.
Mechanical Stability. The printer’s ability to provide accuracy and repeatability requires a robust design. Structurally, the machine must be rigid enough to prevent relative motion between any axis and the rest of the machine. This can cause variations in the system’s response - resulting in assembly defects. Rigidity also is required to ensure that stencil movement in relation to the board is accomplished in planes that are parallel within narrow limits.
Squeegee Pressure, Speed and Downstop Level. Blade and material determine squeegee pressure. For urethane or polyethylene blades, starting pressure should be around 1.6 to 3.01 lb./in. of squeegee. For metal blades, starting pressure should be 1 lb./in. of squeegee. Based on the quality of the print, pressure should be adjusted to give complete aperture fills and clean stencil wipes.
Squeegee speed is driven primarily by paste formulation, which is recommended by paste manufacturers. Both pressure and speed can control print viscosity, with pressure contributing 80% and speed 20%, taking advantage of the shear-thin properties of typical paste. Maintaining consistent print pressure across the board during the entire print stroke, despite board topography, is an essential print-head task. Real-time control of downward print-head pressure with precise stencil and PCB relative positioning prevents process variations.
Sufficient Board Support. Proper board support is essential to ensure consistent print results and higher yields. Without proper board support, the force applied to the board across the entire PCB width will vary, and proper gasketing between stencil and board will not be achieved. Blade angle also can be affected, causing paste to be left on the stencil. Board supports should be distributed evenly across the width of the board, particularly with BGA and fine-pitch components, to prevent bridging and inconsistent paste deposition. Board supports also should be kept clean to ensure good contact with the substrate and reduce contamination.
Optimal Stencil/PCB Separation. Speed and distance are critical factors in stencil/board separation. If the board is separated from the stencil too quickly, bounce back (fast, repeated contact between the stencil and PCB) occurs and the board will need to be cleaned and reprinted. Paste will remain on the bottom of the stencil, causing problems during the next print cycle unless thoroughly cleaned. Lifting, or aperture paste retention, also can occur if separation is performed too quickly, causing insufficients and bridging; and the stencil will need to be cleaned prior to successive prints.
Inspection/SPC Data Collection. For a reasonable grasp on true production fluctuations, inspection tools must be combined with active SPC features. A good inspection system must provide analysis tools so process engineers can correct the situation before it becomes a serious problem. Two-dimensional inspection systems measure the amount of paste covering the target pad and compare that against 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 acceptable boards are being produced. Correcting problems at this stage requires cleaning and reprinting the board, which is considerably less costly than repairing defects downstream.
The best in-line 2-D/SPC inspection equipment will offer an array of tools to streamline the inspection and data-gathering process and allow engineers to customize inspection patterns and timing to best meet the needs of the production line. For an SPC program to be effective, it must be consistently applied on the critical attributes.
Minimal Operator Intervention. Automated operations minimize the need for operator intervention, easing setup and ensuring a consistent and repeatable process. Some advanced stencil printers have features such as a programmable print head that allows operators to program squeegee pressure and downstop; and automatically level the blades prior to printing. Automatic paste dispensing systems add pre-programmed amounts of paste at pre-selected intervals, reducing solder paste exposure time and continually refreshing paste supplies. Automatic stencil wiping provides unassisted stencil cleaning, while a vacuum system cleans paste from clogged apertures.
Lead-free Solder Paste
The lead-free transition will have some impact on the stencil printing process. Printability of solder paste depends on flux formulation, but not for all alloy types. Experience has proven that there are no differences when printing lead-free solder pastes than when printing tin/lead solder pastes. Formal testing has verified that the printed volume of several lead-free solder pastes and tin/lead solder pastes were the same statistically when using the same stencil, printing equipment and boards (Figure 2).
Figure 2. Print volumes of three alloy types on different surface finishes for different devices showed that volumes for all alloys were the same statistically.
Printability of lead-free solder paste will not change, but its spread during reflow will, which may require tightening of the stencil-printing process. One possible issue is print accuracy, or the alignment of the printed solder paste onto the PCB pad. Because lead-free alloys do not spread or wet as well as tin/lead, any solder paste that is not accurately printed onto the PCB will stay close to where it was printed after the reflow soldering process. Figures 3 and 4 depict the same deposits before and after reflow for QFPs and passives.
Figure 3a. and 3b. QFP printed with lead-free solder paste before and after reflow. There is a lack of spread during the reflow process.
The major concern in this situation is the accuracy of printing equipment to align stencil apertures to PCB pads. When addressing the variation in stencil to PWB alignment, several sources must be considered - variation of positional accuracy of the PWB, variation of the alignment capability of the printer and stencil variation. If stencil variation is contained and the variation of a calibrated printer is known to be ±1 mil at Six Sigma, then the remaining factor is the positional accuracy of the PWB. PWB variation may be the largest contributor to misalignment. PWBs are known to “shrink” from CAD data as a result of the fabrication process. They also experience some shrink in the first reflow process, exacerbating misalignment issues when printing the second side of the board.
Figure 4a. and 4b. 0603 and 0805 printed with lead-free solder paste before and after reflow. There is a lack of spread during the reflow process.
To address PWB variation, the board can be measured and mapped so that a stencil is created to custom-fit the PWBs. Generally, all PWBs in a manufacturing lot shrink by the same proportion if oriented in the same direction on the vendor panel. For large-volume production, it is economical to request that PWB manufacturers measure circuit boards and provide positional data so that a customized stencil can be cut to match them. Yield improvements offset stencil costs. Measuring and mapping the board adds the benefit of programming pick-and-place equipment with actual location data, thereby reducing defects associated with pick-and-place, including misplacements, tombstones and soldering defects like solder balls or bridges.
Conclusion
To develop an optimal PCB printing process, it is essential to consider all aspects of the process. The design of the board being produced, components being placed, materials used and selected equipment must work harmoniously. Suppliers realize that they must work together to develop recommended process parameters to satisfy the requirements of customers’ applications.
REFERENCES
For a complete list of references, please contact the authors.
Joe Belmonte, project manager, Advanced Process Group, Speedline Technologies, may be contacted at (508) 541-4772; e-mail: jbelmonte@speedlinetech.com. Bob Boyes, product marketing manager for high-performance printers, Speedline Technologies, may be contacted at (508) 541-6422; e-mail: bboyes@speedlinetech.com. Alden Johnson, senior applications engineer, Speedline Technologies, may be contacted at (508) 541-4841; e-mail: ajohnson@speedlinetech.com.