In the Loop
Defect prevention capability is only achievable with robust closed-loop process controls.
Closed-loop process control is the dream of every SMT process engineer. What is closed-loop process control? It is a method to continually monitor and adjust a process to maintain a particular target value of an output or outputs. For a closed-loop process control system to function we must identify the output or output factors, as well as the input factors that influence the variation of those outputs. For example, paste deposit height, weight, volume, shape, etc. may be considered the outputs for solder paste printing, while the inputs would be print process parameters, paste type, tooling, etc. Not only must these inputs and outputs be identified, but these factors also must be quantified through use of formal statistical tools such as design of experiments (DoE).
Presently, closed-loop process control is primarily seen in the component placement and reflow processes. One area where process control can be implemented with significant impact to the entire assembly process is the solder paste printing process. (Controlling the entire SMT line would be the ultimate goal, but this appears to be unrealistic at present.) Most assemblers now are looking for ways to prevent defects before they impact yields. We are starting to see genuine interest in a defect prevention capability, versus just defect detection capability. This can be achieved only with proven and robust closed-loop process controls.
Stencil printing is a critical first step in surface mount assembly. Paste printing is recognized as the most difficult process to control, yet the most cost-effective stage for defect detection, repair and reduction. It is often said that the solder paste printing operation causes some 50 to 80% of defects found in PCB assembly. Printing is widely recognized as a complex process whose optimal performance depends on the adjustment of a substantial number of parameters. It is not uncommon to hear that stencil printing is more art than science. In fact, the process is so complex that suboptimal print parameters usually end up being used. In addition, stencil printing produces relatively noisy data, which make the print process extremely difficult to control. Ideally, minimizing the variance of the deposited location and volumes will improve process quality and produce more reliable solder joints.
In an effort to improve the performance of stencil printers, continuous process monitoring and statistical process control techniques traditionally have been used. However, these techniques require constant process tweaking and depend highly on process expertise. Presently, manufacturing engineers tune control parameters to a recommended nominal value suggested by the equipment or solder paste manufacturers. In general, line engineers optimize control parameters by printing a few initial boards and “hope” the process stays in control. However, when a process disturbance or drift occurs, process yield typically degrades rapidly to the point of becoming unacceptable.
In general, there are two critical aspects to a printing process: put down the right “volume” of paste on the right “spot.” In another words, not only must the amount of paste volume be monitored, so too should the X, Y and Θ registration of the board. This issue is compounded when dealing with miniature components such as 0201, 01005, 0.4 and 0.3mm CSPs and Pb-free paste. Pb-free paste is known to have less spread, or wettability, and adds to the challenge. The paste deposit volume is a direct function of stencil thickness, although it can be influenced by the process parameters, whereas the X, Y and Θ (better known as positional accuracy) control is a direct function of printer capability.
This is the most common closed-loop control system applied to a solder paste printer. It deals with accurately placing solder deposits on top of PCB pads. Although most printers have automated systems that align the stencil to the PCB, it is not uncommon to see solder paste deposits end up at locations that are not ideal. These print errors come about from board-to-board variations, stencil stretch, inaccuracies in the alignment system or other sources. A closed-loop controller that corrects for deposit positional inaccuracies typically will measure the offset of the printed deposit with respect to the board pads and correct the relative position of the stencil to the PCB. All corrections are made in the plane of the stencil or board. In general, the deposits’ position relative to the pads can be measured within the printer. But, this comes at the price of slower cycle time. Alternately, if minimizing cycle time is critical, board inspection can be exported outside the printer and performed in parallel to the print operation.
One such scheme is the use of an external SPI system to inspect the board and feed the positional information to the printer for processing. In this scheme, following printing, the printer conveys the board and stroke direction information to the SPI. The SPI inspects the board and feeds the X, Y and Θ offset numbers back to the printer. At this point, the printer processes the information and applies a certain percentage of the offset value to the next board with the same stroke direction. Early results show them to be quite capable of holding the registration at the specified target value. This type of closed-loop process control will provide the board assembler the ultimate control of its process and reduce rework, repair, scrap, etc. Not to mention the highest possible first-pass yield.
Rita Mohanty, Ph.D., is director advanced development at Speedline Technologies (speedlinetech.com); rmohanty@speedlinetech.com.
