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Written by Chris Munroe   
Monday, 31 March 2008 19:00

Designers unfamiliar with Lean can actually add process steps – and cost.

Getting Lean An EMS provider’s facility can completely embrace Lean manufacturing principles, but if the OEM customers’ products aren’t designed for them, the result is inefficiency and missed cost-reduction opportunities.

Optimizing designs for Lean manufacturing requires designers to balance their goals for functionality, material cost and long-term product enhancement flexibility against throughput cost and logistics efficiency. Here, we focus on key elements in optimizing design to minimize throughput and logistics costs.

A basic premise of Lean is to eliminate non-value added activity. Unfortunately, many designers actually add non-value activity in the form of additional process steps in their quest to reduce component cost or fully use PCB real estate. Much of what is discussed here involves the basis of their contribution to better production quality. However, the (added) cost of inefficient throughput is equally important. In some cases, design constraints may drive tradeoffs that impact throughput; however, many of the examples below illustrate issues that can be avoided simply by following industry design standards, such as those published by IPC. Adherence to EMS provider design guidelines is also important, as these guidelines are more specifically aligned to the manufacturing processes used.

Minimizing the number of process steps and standardizing processes to minimize changeover time are key drivers of efficient throughput. Single-sided assemblies and assemblies that are either 100% SMT or 100% through-hole are most efficiently processed. 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. Standard designs or panelizations also reduce costs in terms of driving better PCB pricing and minimizing setup and changeover time.

PCBs that are 100% SMT are typically more efficiently processed than their mixed-technology counterparts. A through-hole part that appears cheaper in terms of unit price may actually be on par with with a slightly higher priced SMT component when throughput costs are considered. Not all product can be designed as 100% SMT, of course. However, when only one or two through-hole parts are involved, the benefits of selecting SMT options should be considered.

Any design element that drives manual processing adds significant throughput cost and may reduce quality. For instance, hand insertion vs. automatic placement triples the labor cost. Hand soldering vs. wave or reflow soldering results in triple the cost of automated soldering multiplied by the number of leads being hand soldered.

Improperly sized through-hole pads and holes will result in unacceptable solder joints, which means added inspection and touchup. Lack of fiducials, improperly placed fiducials, or fiducials in the wrong shape or size affect SMT component placement accuracy.

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SMT parts on the assembly bottom-side that cannot be wave soldered drive the need for a double-sided reflow process. While the process does not impact overall product quality, it adds the cost of selective solder pallets and additional solder paste. Bottom-side SMT pads that will be wave soldered require modified (larger/exposed) lands.

Incorrect orientation of the bottom-side SMT components can increase the frequency of opens and shorts, resulting in lower quality and added inspection and touchup. Likewise for incorrect SMT land patterns, which can cause opens, shorts, tombstoning, etc.

Simple steps, such as specifying that component fastening hardware exposed to the bottom side (solder side) should always be stainless steel, can minimize cost. This is because zinc-plated hardware attracts solder and requires masking.

Specifying preformed component packages, such as reels, instead of individual components, simplifies processing. Wires are not a preferred wave solder option and generally require a special offline assembly process. Wave soldering may be possible if Amp-in terminals on solder ends of wires rated at 105°C or higher are used.

While sockets on programmed parts potentially increase component-use flexibility, they can create reliability issues and typically have a higher cost than a soldered component. Programmed parts soldered to the board that use flash technology offer future programming flexibility and a more efficient processing option. It is important to ensure a means to reflash or reprogram the device in-circuit.

Test strategies based on Lean principles typically use standardized test platforms. Efficient ICT requires a PCB designed to IPC guidelines with good access points. According to an Agilent study of manufacturing defect root causes, 10 to 15% of defects are actually attributable to nonfunctioning parts or defective materials, rather than being process-related. We have seen similar statistics in internal defects analysis. A robust test process helps catch these “embedded” component defects prior to assembled product shipment.

Test point access can be a significant cost driver. If a design doesn’t have test point access sufficient to permit automated ICT, 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. Custom functional test systems alone may not provide as robust a test process and typically increase test time and overall test cost.

Logistics Cost Drivers

Materials selection also drives significant cost and can impact throughput. The greater the number of line items managed, the greater the cost. Similarly, the less flexibility an EMS provider has in supplier choices, the greater the opportunity for issues such as component availability constraints or increased cost. A designer’s ability to design a Lean bill of materials (BoM) can significantly reduce cost.

Consolidation of resistor and capacitor values on the BoM is one area for cost reduction. Many BoMs have a large number of values for resistors and capacitors, and each specific value drives management of additional component line item, as well as the opportunity for misplaced components on the board. When a design engineer tries to minimize the number of different values, it has the net effect of streamlining material management processes and minimizing the potential for manufacturing errors.

Noncancelable, nonreturnable (NCNR) custom parts are also a cost driver. While it may be impossible to completely eliminate custom components, minimize their use. Besides the excess inventory status of NCNR components when a product reaches end of life, these single-sourced components are vulnerable to availability constraints.

Allowing an EMS provider to source small components such as standard ICs and discretes off its internal approved vendor list offers the best options for cost reduction and logistics efficiency. Providing multiple sources for these components on the supplied AVL is the next most efficient option, as multiple sourcing minimizes availability issues and opens the door to robust pricing negotiations.

Dfx for Lean isn’t difficult. Many of the optimum practices discussed here are widely used. The key is combining these steps as part of focus to increase throughput in every stage of product realization from components procurement through manufacturing and ultimately shipment to end market. Product design can either be the element that adds cost along that journey or the focus that reduces cost every step of the way.

Chris Munroe is director of engineering at EPIC Technologies (epictech.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it . His column appears bimonthly.

Last Updated on Friday, 28 March 2008 06:04
 

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