Contract Manufacturing: Efficient Line Changeover

Estimated reading time: 7 minutes

Optimizing line-changeover efficiency is a necessity to implement lean manufacturing in high-mix, low-volume (HMLV) electronics assembly environments.

By Douglas Farlow

How has our industry defined efficient line changeover? Various definitions in the electronics manufacturing industry have been applied to rapid line changeover. Terms such as single-minute exchange of die (SMED), single-digit exchange of die (SDED), Six Sigma and lean manufacturing are some of the metrics used to measure changeover efficiency. Line changeover encompasses key areas of production equipment, of which screen printers and placement equipment are the two most time-consuming. Screen printers require stencil change, re-programming, paste loading, conveyor change and setup of board-support tooling. Pick-and-place machines require conveyor change, re-programming, feeder exchange and board-support setup.

Moreover, changes in domestic PCB assembly have evolved into a dominant shift toward high-mix, low-volume (HMLV) manufacturing requiring a higher frequency of line changeovers. Reducing changeover duration and optimizing changeover processes are critical factors to implementing lean manufacturing in an HMLV operation. For extreme HMLV applications, such as new product introduction (NPI), prototyping or short-run manufacturing, rapid line changeover is a necessity.

Quality Considerations

As board complexity has evolved, so has the creation of substrate-support methods. Total support of the circuit board during screen printing is essential to ensure high-quality printing. Proper support might be the most important process parameter to a successful print. Without it, solder bridging, insufficient solder or too much solder can result. In the past, single-sided boards have required simple support pins or bars. However, as fine-pitch devices, lead-free solders, BGAs, 0402 and 0201 discrete components, double-sided boards and thinner substrates have emerged, the demand for improved support has forced the industry to examine alternative methods.

Because electronic designers create boards that have components occupying the majority of both sides, available and usable support pin real estate has all but disappeared. This has forced board assemblers to find creative and expensive support solutions. Custom-milled support plates provide one solution. These plates accomplish support requirements, but are not suitable for flexible manufacturing because of cost and turnaround time. Another alternative, such as programmable-pin placement, has limited HMLV application due to an inherent lack of flexibility. Solid support is required during the placement process to address density, component size and the use of lead-free solders. Increased density of underside components is causing operators to devote significant time looking for open areas to place support, which in some cases is becoming nearly impossible. Even when the bottom-side space is available, it may be difficult to locate support pins accurately. With the introduction of lead-free solder pastes lacking “self-correcting” characteristics of eutectic solder during the reflow process, accurate placement becomes more important. Without proper support during the placement process, the natural “bouncing” caused by the placement nozzle contacting the board can affect placement accuracy adversely.

As the industry faces these issues, it has become necessary to upgrade substrate-support tooling in the majority of screen printers and placement equipment. What is needed is a highly flexible, rapid setup and highly reliable support-tooling method. Recent developments in substrate-support tooling have enabled board assemblers to reduce the time required for line changeover without compromising quality. A new family of substrate-support systems is available to combine setup speed, flexibility and reliability into a single unit (Figure 1).

Figure 1. Rapid changeover of a substrate support system.

To overcome density problems associated with current SMT assemblies, next-generation substrate-support systems use a concept similar to a “bed of nails.” The system is made of a number of pins, each independently movable and lockable. Some of the models are pneumatically activated, while others are spring-loaded. In either case, units are positioned in the printer or placement machine, the PCB is transferred in, and tooling pins are raised and locked into place. This system contains support pins that contact both the bare board and components, irrespective of differences in component height and position. The pins, made of a soft, compliant material, eliminate concern for physical or ESD damage to the board or components.

Setup and Changeover Challenges

Advances have been made to improve screen-printer use. Features such as automatic stencil loading, self-contained squeegee heads, vision assist, programmable conveyors and offline programming have become mature features on many machines. These have improved print quality; however, support tooling remains the largest component of screen-printer changeover (Figure 2).

Figure 2. Typical screen printer changeover operations.

The greatest changeover/setup demand for placement equipment is time required to change feeders, programming and board support (Figure 3). Rapid feeder and program changeover have been a focus with many equipment manufacturers offering feeder-trolley systems that allow the operator to “kit” feeders offline. Other features, such as tape splicing, component and reel verification, offline programming and machine optimization are commonplace.

Figure 3. Typical placement-equipment changeover operations.

The emphasis on board support for pick-and-place machines is a newer concern. In the past, the majority of placement machines were supplied with rudimentary support. Today, this is inadequate and can cause placement machines to share some of the same challenges as screen printers. In the case of screen printers, print-deposition precision is needed, whereas pick-and-place machines require placement accuracy. Improved board support has taken on an increased importance to achieve and assure consistent placement accuracy with the lower self-aligning characteristics of lead-free solder paste.

Cost of Equipment Utilization

Inadequate equipment usage has obvious and hidden costs for electronics manufacturers. The root cause of costly rework, non-revenue generating operations, expensive downtime, excessive work-in-process and poor quality can be traced to inadequate equipment usage.

In most industrialized countries, it has become cost-prohibitive to perform post-assembly rework. Building the product right the first time is essential. Unfortunately, many production operators receive a contradictory message. They are told to changeover lines quickly without sacrificing quality. As line changeover becomes more frequent, the opportunity for human error increases dramatically. Equipment downtime and non-revenue-generating line stoppages increase as operators struggle with antiquated tooling and processes. The solution is to implement systems and procedures that enable production operators to changeover lines with minimal difficulty, without compromising quality.

The following example reflects the challenge of improving line changeover efficiency. One Tier-1 EMS provider with multiple assembly lines focused attention on increasing equipment usage by reducing line changeover and setup time. Because changeover of existing board-support tooling required more than 15 minutes per machine, their primary objective was to reduce changeover time of support tooling. Their objectives were to reduce changeover and setup time of board-support tooling by 50% to less than 7.5 minutes per machine, improve operating efficiency by eliminating equipment idle time and maintain process reliability in terms of solder-paste deposition and placement accuracy. To achieve this, they converted existing board-support tooling to substrate-support systems for all screen printers and placement machines. Because their operation involved an average of two changeovers per line a day, with more than 5,000 changeovers per year effecting ten assembly lines - each with one automated screen printer and two placement machines - the operational impact would be significant.

Setup and Changeover Efficiency

Prior to converting to substrate-support systems, setup and changeover times were measured using existing board-support tooling, accounting for operator-to-operator and shift-to-shift variations. Manual setup of existing support tooling required an average time of 15:22 for each screen printer and placement machine. After converting to substrate-support systems, setup and changeover time was reduced to 5:44 per machine within two weeks of installation. Setup and changeover time continued to improve to 2:08 per machine.

Cost savings provided by setup and changeover reduction were considerable in terms of direct-labor savings using a full-absorption hourly rate. Line changeover using the old support-tooling method required more than 3,840 hours and cost more than $115,200 annually. Conversion to substrate support systems reduced changeover time to 533 hours, or $15,990 per year, yielding an annual cost savings of $99,210, an 87% improvement.

To ensure that screen-printing operation process reliability was not compromised with the substrate-support system, solder-paste deposition was monitored using visual inspection, concentrating on print quality and bridging.

Maximum improvement in operating efficiency can be obtained by eliminating defects during a manufacturing operation. Furthermore, a corresponding reduction in non-productive equipment changeover time results in an improvement in operating efficiency and faster cycle time. A savings also was realized in improved operational efficiency. Reducing board-support setup and changeover time improved overall equipment utilization by 9.6%, an 18% improvement in operating efficiency. Based on these improvements, converting all 10 production lines to the substrate-support system reduced setup and changeover time, resulting in a return on investment (ROI) of less than nine months.

Conclusion

With continued productivity challenges in mind, board-level assemblers have programs in place to improve line changeover efficiency and equipment utilization. An area such as board-support changeover can yield substantial operational savings. Equipping an assembly line with this new generation of substrate-support systems is essential to implement lean manufacturing practices.

Douglas Farlow, president and CEO, Production Solutions, Inc., may be contacted at (858) 486-1088; e-mail: dfarlow@production-solutions.com.