Should getting more cost more? As pioneers in large scale production like Henry Ford have proven, it definitely should not. Building anything in mass quantities where the assembly team can specialize and be dedicated to a single task is an efficient means of production. This theory is evident in the business of mass producing items in the tens of thousands, where tooling costs can be absorbed over the large production run of the product. In the early 1960s, the Toyota Production System (TPS) built on Ford’s foundation and further streamlined large scale production.

Low volume, high mix manufacturing can benefit from the lessons learned from TPS. As production volumes are reduced, more effort needs to be devoted to making the correct decisions about each step in the process. The main considerations of small volume production are speed of assembly and the reduction of equipment downtime to
a minimum.

In both large scale and small scale production, the manufacturability of the product needs to be designed in from development. The concept of Design for Manufacturability, or DFM, is the balance of creating a product that performs the desired function and is efficient to manufacture. Considerations in electronics assembly include board size and shape, as there can be machine constraints related to both. One way around constraints of board size is to design small or odd shaped boards into easy to handle rectangular panels that can be separated after assembly. The IPC, an electronics assembly industry standards committee, has a reference document series (IPC-2220) that outlines considerations for DFM in printed circuits and wiring harnesses. Design for Manufacturability involves making each component as easy to assemble as possible. For electrical components, Design for Manufacturability should strive to avoid component placements that require extra steps in the manufacturing process.

An example of this is to avoid placing large heavy surface mount components on both sides of a printed circuit board, where components would need to be secured with adhesive prior to running the second side in the SMT line.

Using DFM for mechanical components, easier is better. Wiring harnesses built with Insulation Displacement Connectors (IDCs) are typically faster and more economical to fabricate. In places where a screw with a lock washer and flat washer would be used, a simple screw with a captive lock washer would be substituted to allow for a reduction in inventory items, less motion to install and maintenance of the integrity of the assembly.

Assembly speed can be increased by looking at work cell organization. The concept of labor loading in each work cell, so that all cells are turning out enough product to complete a final assembly in a given period of time, is discussed using Takt Time. Takt time can be thought of as the heartbeat of the production line. Work cells can be constructed to complete an assembly step as quickly as the cell after needs its next assembly. There is little time wasted for inventory build up in each cell, and workers get immediate feedback if there is a delay in their cell.

Another method of keeping speed of assembly up is to consider processes that can be automated. For the small volume - high mix manufacturer, there are many automated assembly equipment lines that allow flexibility without high tooling cost for each product line. For example, wave soldering technology is getting competition from selective soldering, which takes a solder fountain and moves it with CNC precision around a PCB panel for soldering through-hole components. These machines can be programmed quickly and have the flexibility to change the soldering parameters on each joint to achieve the optimum in reliability and speed. This type of machine is available from multiple manufactures with varying features and price points.

Reducing changeover time between products is also a consideration for the small volume manufacturer that is not as critical for the high volume player. As a high mix of products is run through the production lines, equipment needs to be changed over in as efficient a manner as possible. The quick changeover concept was introduced by Shigeo Shingo in the early 1960s. Shingo was consulting for Toyota and helped in the development of the Toyota Production System. As changeover time is reduced, the non-productive time of the factory is reduced and the Economic Lot Size can also be reduced without an increase in cost per part. Many semiconductor companies employ parallel changeover techniques. These practices and techniques allow much of the changeover to happen offline. Tasks like setting up the next feeder rack for a SMT pick and place machine can be performed while the equipment is running the current batch. The changeover then happens when the last printed circuit board of the current product clears the pick and place. The new feeder rack is wheeled into place and the pre-programmed product files are loaded into memory. A change that would take a few hours if started at the end of the batch now takes less than ten minutes of machine downtime.

It is possible to create an efficient manufacturing environment for lower volume, high mix product lines provided there is an understanding of the parameters that effect cost. These parameters will be slightly different for each manufacturer, but focus needs to be given to any activity that increases the time it takes to perform any task. Key places to look for time savings are in the changeover activities, maintenance activities and work cell labor loading.