Increasing Your Competitiveness in PCB Assembly

Transcription

1 Increasing Your Competitiveness in PCB Assembly This article offers a seven step model for analyzing and integrating production systems. This phased approach can reduce costs, improve on-time delivery, and increase profitability by eliminating excess inventory and avoiding unnecessary shortages that can hold up production. These steps will get you moving in the right direction toward increasing your competitiveness in the world of PCB assembly. Because life cycles for many electronic products can be as short as a year or less, it often seems that consumers are already looking to the future for new features even as they buy the latest mobile telephone, television, or computer. Reflecting these volatile fluctuations in demand for electronic products, manufacturing strategies that address market conditions today may need to be reversed or redirected to adapt to different circumstances tomorrow. These market dynamics are forcing producers to consider new approaches in manufacturing products, managing materials, and facilitating operational planning. This means increasing the flexibility of their manufacturing operations by developing an infrastructure that allows them to modify their production systems on relatively short notice. In many cases, this means adding or enhancing in-line, integrated systems. An in-line, integrated system, in this sense, means one that is designed for your specific production requirements. It emphasizes flexibility by linking individual assembly cells to work together ( in line ), taking into consideration production flow, equipment configuration, and other logistical concerns so that the individual pieces function as one production unit ( integrated ). This integration not only optimizes the manufacturing process, but also enables the transition from forecast based planning to a system based on real time manufacturing flows. With this infrastructure to gather real time data on production trends, purchasing contracts can be driven by actual daily production requirements, as opposed to monthly production forecasts. Integrated systems therefore not only maximize the efficiency of production, but also offer 1

2 Figure 1 - Material flow in a traditional batch manufacturing process has a tendency to accumulate work in p r o c e s s. greater flexibility and effectiveness in responding to constantly changing market conditions. STEP 1: Analyze Current Production Operations Conducting a detailed analysis of current production operations can root out unnecessary costs you may not have recognized, and provide the basis for determining the best integration options for your present and anticipated production needs. Analyzing current operations involves taking a close look at several aspects of your production, from material flow and process obstacles to throughput and inventory management. There is, however, a common denominator: materials at all stages of the manufacturing process. Material Flow If the product manufactured is a high-volume industrial product using traditional batch material requirement planning (MRP) and stand-alone cell manufacturing techniques, then material flow is suspect. For example, an automated insertion process for building printed circuit boards (PCBs) using stand alone through hole equipment has a tendency, by design, to accumulate work in process (WIP). This standing inventory means wasted dollars. F i g u r e 1 illustrates this process: three component types are used, and operators load and unload the machines from typical 50 PCB tote bins. Assuming that two tote bins are used for input and two for output, there could be 100 PCBs of input and 100 PCBs of output beside each operator. This generates 200 PCBs of WIP per machine, which multiplied by three machines yields 600 PCBs of WIP. It does not take long for WIP and the cost of inventory to get out of control with this batch manufacturing method. Process, Throughput, and Inventory Just like material flow problems, process obstacles such as damaged boards or components being dislodged from their placement holes are indicated when partially finished goods accumulate between processes. Throughput problems are marked by the slow, manual movement of materials through your plant. Stacks of printed circuit boards have the potential of standing in front of manufacturing steps and test steps in the process. Inventory management problems are indicated when there is a lot of material, you don't know what you have or where it is, buyers don't know what to buy, stock is needed to support new production, and there is a lack of space. With all of these problems, there are severe cost factors. First is the cost of the materials alone. Second is the cost of labor to stack and move components and semi finished goods. Third is the cost of lost space. Fourth is the cost of money. All of this adds up to a substantial financial drain. STEP 2: Design System to Production Requirements There are many options in system design. The key is to design the right system for your production requirements, based on the market life span of your electronic product which can be as short as one year or less. Product dynamics like this require in-line manufacturing techniques to minimize time to market and cost of production. In-line manufacturing systems as shown in F i g u r e 2 reduce the time to market from days to hours for the production of either through hole or surface mount PCBs. Conveyors force things to speed up. An in line process helps reduce inventory, and it s also easier to know what has 2

3 Figure 2 - An In Line manufacturing process is likely to reduce the time to market and minimize cost of production. been shipped and what materials to purchase. Here are a few key areas to consider when designing your integrated system. Production Flow The flow of production in an in line process can be designed with board handling options to accommodate a wide range of production requirements. Conveyors can move PCBs between assembly machines, or can move boards automatically from one area of production to another. If floor layout or machine servicing needs call for personnel to safely cross from one side of the line to the other, some means of getting through will be necessary. Shuttle gate and lift gate modules can automatically open a piece of the production line to enable them to pass through. Using loading and unloading modules as buffers to manage the flow of materials through the line helps avoid bottlenecks. Other modules provide the ability to rotate, flip, shift, or change board direction to accommodate double-sided process requirements or space constraints. Production-Specific Configurations In line systems can be designed for both high volume and high mix environments. For high volume, low changeover manufacturing, systems can be designed to move PCBs through the entire assembly process without accumulating PCBs in off line tote bins like those used in the batch process. Boards can flow automatically through the manufacturing steps of screen printing, surface mount and/or through hole component placement, reflow, wave solder, manual assembly, post wave manual assembly, in circuit test, functional test, final assembly, and pack out. This helps eliminate standing inventory or WIP, and drives PCBs to final assembly or shipping at the rate of the slowest manufacturing step in the process. This approach reduces the cost of value added inventory. This is the value of WIP sitting on the production line, including the costs associated with adding components. For example, the in line system shown in F i g u r e 2 would have two boards on each conveyor and three boards in each machine for a total of 17 boards. Compare this to the batch system shown in F i g u r e 1 that had 600 PCBs of WIP. Long, totally integrated lines may not be suitable for every production environment. But there are many options to help make the system work. Board handling modules allow for a Figure 3 - A number of steps in an In Line process can be automated, but separated from each other to allow for quick changeover in a high mix environment. 3

4 Figure 4 - Applying Pull techniques to an In Line process helps avoid bottlenecks and achieve continuous material flow. great deal of flexibility in line design. Space constraints can be overcome by wrapping lines with diverters or board turn modules. A manufacturing line can take the shape of a U and head back on itself to best utilize floor space. Overhead conveyors can also be used to conserve floor space. In high mix environments, on the other hand, a number of steps in the process like top side vs. bottom side component placement can be automated within the line, but separated from each other to allow for quick changeover, as shown in F i g u r e 3. This facilitates changes to the manufacturing process line in a short period of time to respond to changes in product design. Product design or capacity requirements can be changed without affecting the total line configuration, which could take hours or days to rearrange. Pull Techniques Applying pull techniques to your system configuration will help you avoid bottlenecks and achieve continuous material flow. Using this design philosophy, the system will pull from assembly cell to assembly cell, keeping the PCBs moving. Clearing any temporary bottlenecks or interruptions will tend to automatically clear the buffers in between the manufacturing cells. F i g u r e 4 shows how this might apply to the in-line process shown in F i g u r e 2. Starting at the down-stream end of the process shipping you would design the line so that each preceding step in the manufacturing process is slightly slower as you move up the line. The pulling effect of this system design also has a tendency to pull along component inventory. After initial decreases in WIP by changing the manufacturing process and decreasing the buffering in between the machines, component inventories will decrease by default. Components may now be moved from the stock room to the manufacturing floor. Enough components for one day should be kept on the manufacturing floor, close to the appropriate process step. Only minimum component inventory should be kept in stock, keeping in mind transportation expenses and component costs based on volume purchasing. Purchasing is also adjusted to accommodate usage on a daily basis. STEP 3: Design PCBs for Integrated Automation Design guidelines should be a well-defined part of the computer aided design (CAD) criteria. Ensuring the highest productivity from each manufacturing process step requires the PCB to be designed for integrated automation. To ensure manufacturing discipline early in the design phase, designers should be involved in the manufacturing process from the beginning. Designers should maintain close communication with the manufacturing engineer, who should approve each design. Extensive research in PCB design has made it possible to begin standardizing design criteria. For example, the IPC has worked with manufacturers on criteria to help standardize PCB design for optimized production. These 4

5 Sidebar 1 Basic PCB Manufacturing Recommendations Review and understand the different manufacturing steps. Select components based on product and manufacturing requirements, regardless of the latest packaging technology. Determine that leads meet component lead to hole size ratios. Maintain adequate separation for clinch clearances between through hole leaded devices and bottom side surface mount parts. Monitor the effect that lead material and plating has on the life of clinch cutters, and replace as necessary. Review surface mount pad design that is critical for PCB design, solder joint integrity, and component placement reliability. Maintain standard 3mm PCB edge clearances for reliable transport through the manufacturing system. Avoid cutouts along any of the PCB edges. Standardize to one size and style of tooling holes for in line through hole equipment and testers. Analyze component to component clearance to avoid solder shadowing during the wave soldering process. Keep the PCB flat during the wave solder process for increased solder joint integrity. Review component footprints and maintain clearances needed for head and clinch assemblies. Check surface mount component clearances required for placement capability and to prevent bridging problems. Design adequate clearances for test points, solder mask, and solder paste stencil, and separate test point vias from component pads so solder does not bridge to the test vias. Determine the correct process and cleanliness specification for solder paste, and compare solder paste stencil aperture size versus pad size. Compare stencil thickness to the size and type of component being placed, and determine if a step-down or lase cut stencil is being used. design guidelines can be incorporated into the CAD system being used. The guidelines should also be tested against automated design rule checks, such as the checks of surface mount device (SMD) pad footprints against IPC-SM-782. Others to test against are the design checks for clearance rules, such as trace to via, SMD to copper, SMD to board, SMD to drill, and component to component. Testable vias that interconnect different layers of circuitry can be probed in the front, back, or both sides of PCBs, thus supporting single side or clam shell testers. Design guidelines can follow basic standardslike IPC-SM-782 for surface mount pad design. Design for manufacturing should also include basic recommendations, such as those listed in Sidebar 1. Designs are also available from placement equipment manufacturers.* STEP 4: Evaluate Placement Equipment Since above-average effort is put into the design of many printed circuits, it is imperative to not compromise a design by using inferior placement equipment, screen printers, conveyors, and testers. Look for and evaluate placement equipment that is manufactured under stringent quality controls. Check to make sure the equipment has an internal quality run, with data available for review. This data should be in a common statistical process control (SPC) format that can be compared from manufacturer to manufacturer. The data available should represent statistical field data for machine uptime, downtime, mean time to repair, mean time between interrupts, and mean time between repairs. Accuracy and repeatability should also be measured. This is done using an optical view machine to detect the placement of precision glass slugs onto a precision glass plate graph. An equipment manufacturer should be able to show continuous improvement in the number of components per hour (cph) placed over the period of time the machines have been in production. This demonstrates the manufacturer's commitment to continually improving the quality of their placement equipment, insertion equipment, conveyors, and test equipment. 5

6 Sidebar 2: Summary of Step-by-Step Analysis This summary lists the seven steps in analyzing a production system, and the typical results of the analysis. Step 1 - Analyze Current Production Operations High WIP is wasted dollars. An integrated system may be best to maximize production efficiency. Step 2 - Design System to Production Requirements In line manufacturing systems reduce time to market and cost. Board handling options help meet flow requirements. In line systems reduce WIP. Integrated lines can use many production options. Pull techniques maximize flow and minimize inventory. Step 3 - Design PCBs for Integrated Automation PCBs work best when designed for automation. Standardized PCB designs help optimize production. Design guidelines should follow basic standards and recommendations. Step 4 - Evaluate Placement Equipment Quality equipment is essential. Comprehensive data output is needed. Step 5 - Establish Measurement Capability Gathering real time data on production status is necessary. Data can be used for machine pattern programs. Data must be secure and easily transported. Step 6 - Analyze Performance Trends Trend analysis helps to respond to market demand. Data collection is required to complete management reports. Data is needed to evaluate the status of product lines. Step 7 - Adjust Manufacturing Processes and Flow Adjust accordingly STEP 5: Establish Measurement Capability A measurement capability is essential in order to provide management reports regarding the manufacturing process. This capability can be used to verify that the steps taken in board design, machine quality, material flow, and process flow are producing the desired results. To do this, a computer host provides real-time data collection in a common SPC format from the various machines in the process line. This data collection can also be used to monitor feeders, component placement, and testers, and compare operation to set limits and triggers. When errors happen, the manufacturing process can be stopped and the errors reviewed and corrected. Today s CAD systems are integrating more and more computer-aided manufacturing (CAM) programs. The computer data-collection host provides a medium to download pattern programs to assembly machines and testers. This allows machine pattern programs to be developed after a design is complete and downloaded to manufacturing equipment. When changes are made to the design, that information is instantly available to manufacturing, and bills of material are available to purchasing. To use the data, it is important to be able to pass it from system to system using transfer control protocol/internet protocol (TCP/IP) Ethernet connections and Windows NT. Windows NT is rapidly becoming the platform of choice at the expense of UNIX. With this operating platform available, data can be passed from the business mainframes to distributed networks on the manufacturing floor. The integrity of data is important and password control at different levels is critical. STEP 6: Analyze Performance Trends Time-to-market is a critical concern for new designs. It is imperative to have a system in place to collect and verify SPC data and analyze performance trends. This provides real-time verification that the manufacturing process is under control. This information should be available as real time displays for review by the manufacturing staff on the production floor and in chart form for review by management using remote terminals. 6

7 Dynamic data exchange (DDE) capability in the data collection host computer allows using the extensive network of existing applications. For example, management reports can be transferred to spreadsheet so the user can arrange the report in a desired format. Data may also be written to structured query language (SQL) databases to complete extensive work being done in business software packages like SAP and Oracle to support manufacturing flow techniques. The data collection host provides a window collecting the number of PCBs shipped on a daily basis. By transferring this information to the purchasing module, purchasing contracts can be modified daily, based on usage. This information provides the backbone for E-commerce. Having a central host with HTML (hypertext mark-up language) and ASP (active server pages) provides a window for management, service technicians, planners, purchasers, and other qualified individuals to evaluate the status of the product line or lines from their desks. By designing a product for automation, providing process lines for the best utilization of WIP, and selecting the best quality equipment, you can produce a predictable product flow. With various departments having access to real time manufacturing information, processes like Lean Manufacturing and Demand Flow can provide real financial gains. trends and actual daily production requirements will help you reduce costs and increase profitability. You can make the transition from forecast based planning to an operation based on real time manufacturing flows. These steps show the way. The next step is yours. About the Author Dennis Lockhart, BSEE, is a Systems Applications Engineer with Corp., where he has been developing and managing manufacturing system projects worldwide for 18 years. His experience includes assembly applications for surface mount, through hole, manual assembly, robotics, printed circuit board development, test, final assembly, facilities, custom line software, host software, project management, and full turnkey manufacturing systems. He may be contacted by at lockhart@uic.com. * For example, ' Surface Mount Technology Laboratory (SMT Lab) has suggested pad designs for the most common component styles used for surface mount designs. Universal has also developed guidelines to improve the quality of printed circuit board design for through hole components (see a copy of Universal's publication, Through Hole Design Guidelines, from a link at Universal's Web site, STEP 7: Adjust Manufacturing Processes and Flow Finally, taking the initial six steps outlined in this article will identify points that are specific to your PCB manufacturing operation. As an example, see the results of a sample step by step analysis in Sidebar 2. The points you identify, and the results of your analysis, should lead to an adjustment of your manufacturing processes and flow. Your analysis will show how to put increased flexibility into your manufacturing strategy. This includes an infrastructure that allows short-term adaptation of production systems, which is especially important because of the short product life cycles of today's electronic products. Adjusting your manufacturing processes and flows so that you can work with production 7