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Translating Lean Principles to DfX Improvements

Fri, 01/06/2012 - 11:08am
Chris Munroe

Cost pressures abound for just about every industry, but few experience the dual pressure to reduce cost without sacrificing quality at the level found in the medical industry. A true benefit of a strong Lean manufacturing philosophy is that it reduces cost by eliminating inefficiency, and, as a byproduct, contributes to improved quality.

Nowhere is that better illustrated than in the design for manufacturability/testability/assembly/logistics (Dfx) equation. The core philosophy of Lean manufacturing revolves around minimizing non-value added activity. In terms of a design philosophy, that translates to minimized variation. Consequently, designs should contribute to minimizing the number of required process steps or process variations.

As an example, single-sided assemblies and assemblies that are either 100% SMT or 100% through-hole are more efficiently processed than those with mixed technology or a double-sided design. Assemblies that place mixed technology on both sides of the board typically drive additional automated and manual processing time, additional tooling costs and a greater potential for handling or thermal shock related defects. Standardization of designs or panelizations also reduces costs both in terms of driving better PCB pricing and in terms of minimizing set-up and changeover time. While it can be difficult to completely eliminate the use of mixed technology, if there are only a couple of through-hole parts on the board, the benefits of redesigning to 100% SMT should be considered. Fewer processes and higher levels of automation ultimately cut overall costs and reduce the potential for defects.

Board layout should be another area of focus. As an example, improperly sized through-hole pads and holes create unacceptable solder joints. This in turn drives added inspection and touch-up. A two-sided design requiring double-sided reflow increases solder paste use and tooling costs. Incorrect SMT land patterns can cause opens, shorts, tombstoning, etc., resulting in a reduction in quality and added inspection.

From a logistics standpoint, consolidation of resistor and capacitor values on the bill of materials (BOM) is one area for cost reduction focus. Many BOMs have a large number of values for resistors and capacitors, and each specific value drives management of additional component line items, as well as the opportunity for misplaced components on the board. When the number of different values is minimized, material management processes are streamlined and the potential for errors in manufacturing are reduced. Developing an approved vendor list with multiple sources for all components provides the widest range of sourcing options over the life of the product.

The Lean focus on standardization also has benefits in test. Test point access is critical in controlling costs. Adhering to IPC design guidelines and any additional guidelines provided by the manufacturing site help ensure efficient test processes. Conversely, if a design doesn’t have enough test point access to allow automated ICT, automated optical inspection (AOI), x-ray inspection or flying probe test are higher cost alternatives. Those tools have longer test times, are less effective in fully testing the product and require greater operator interface time. Use of custom functional test systems alone may not provide as robust a test process and typically increase test time and overall test cost.

Making Tradeoffs
As discussed, it isn’t always possible to design entirely within Lean best practices. EPIC Technologies uses a scorecard system to rank recommendations made to its customers.

In the EPIC system, each customer data package is analyzed during the project launch phase using advanced product quality planning (APQP) techniques. While APQP was developed by the automotive industry, its basic philosophy makes sense for any Lean product launch process. Key points include:

  • Understand customer requirements
  • Use a robust process for product design and development verification
  • Use a robust process for production process design and development verification
  • Validate both product and process
  • Use a focused product launch process with feedback, assessment and corrective action mechanisms to ensure that product meets customer requirements.

The first step in this process is the design review summary. This report lists the recommendations of the Design for Manufacturing Evaluation and scores each based on relative importance.

Each importance level is defined as follows:

  • 5 – Will not build the assembly with this issue unresolved
  • 4 – Major design issues. Can build product as designed but at increased cost and quality risk
  • 3 - Issue should be corrected. Can build product as designed at increased cost
  • 2 – Minor design issue
  • 1 – Nice to Have Item

With this scale, OEMs have an easy-to-use formula for evaluating the recommended changes and the EMS provider can easily point to the benefit each recommendation will achieve. The analysis is done using internal Design for Manufacturability and Testability guidelines optimized for the specific production environment. Those standards are based on industry-accepted guidelines including those published by IPC.

A robust DfX approach based on Lean principles combined with an understandable ranking system streamlines cost-benefit analysis in product development or product redesign. While it is inevitable that some tradeoffs will be made, implemented improvements will contribute to reduced cost and increased quality.

Chris Munroe is director of engineering at EPIC Technologies. He can be reached at chris.munroe@epictech.com.

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