PCB Design for Manufacturing Report Cards

One of my earlier BLOGs talked about PWB Cost Drivers. What is required for the printed wiring board (PWB) Value Delivery Chain is a method of predicting the cost of fabrication for the printed circuit board (PCB) substrate, as well as the cost of PCB assembly and PCB assembly testing. With these tools and the wiring density metrics, tradeoffs can be performed early in the product's life. Many call this Design for Manufacturing and Assembly, I call it the DFM Report Cards because that’s what its creator, Tucker Garrison called it. With the two DFM Report Cards that I will outline here, an accurate estimate can be made of a proposed design’s cost. Placed into spreadsheets, this forms a powerful tool to select PCB parameters, and hopefully, to optimize a design.

Design For Manufacturing (DFM) Report Cards

Fabrication Report Card

The fabrication report card is a matrix supplied by a PWB fabricator which relates the various design choices on a PWB to design points. These points are based on the actual prices a fabricator will charge for these features.

Typical factors that PWB fabricators use to price a printed wiring board are:

• Size of the board and number that fit on a panel

• Number of layers

• Material of construction

• Trace and space widths

• Total number of holes

• Smallest hole diameter

• Solder mask and component legends

• Final metalization or finish

• Gold plated edge connectors

• Design specific features, etc.

Once the PWB fabricator has collected the factors that influence his prices, he collects these costs and normalizes the figures with the smallest non-zero amount. The report card would look like Figure 1.

Figure 1. Example of One Company’s fabrication design report card. [1]

Assembly Report Card

The metrics of PCB assembly "tradeoffs" relates factors of process, component selection, and test-to-assembly prices. Yields and rework are factored into the points of the Assembly Report Card. The point total provides an estimation of the relative prices of assembly and test.

The PCB assembly report card is a matrix supplied by the PCB assembler. It relates various process assembly and test choices that the assembler provides, along with various component sizes, orientations, complexity, and known quality, to the costs of providing for these design choices. Typical factors that affect PCB assembly costs are:

• One pass or two pass IR reflow

• Wave solder process

• Manual or automatic parts placement

• Odd shaped parts

• Part quality level

• Connector placement

• Test coverage

• Test diagnosability

• Assembly stress testing

• Repair equipment compatibility

By collecting all the costs associated with assembly, test, and repair and then normalizing these costs with the smallest non-zero value, a matrix such as shown in Figure 2 can be produced.

SMT Assembly Report Card: The IBM Assembly Report Card’s [2] 10 assembly trade-off factors are shown in Figure 2. The report card was the work of many PCB assembly engineers with the help of the accounting department. This report card has 10 factors that range in points from zero to thirty-five. The total points can affect the prices from a 30% discount to a 30% penalty. The matrix in Figure 2 is from the former IBM-Austin’s electronic assembly facility, the graph illustrates IBM's price point tradeoffs.

The report card was introduced in the early 1990s; within two years of using the report card, assembly point scores were averaging around 75 instead of the earlier 50, with its Gaussian distribution. Remember, higher scores indicate more producibility. This had a major effect in reducing the costs of many of the assemblies processed by the facility. This effect can be seen in Figure 3. The scores are no longer normally distributed, but the Project team has focused the designs to take advantage of the discounts. The visibility of the cost implementations of different design choices, provided to a designer while in the layout phase, allows the designer to check PCB manufacturability or producibility while they can still make timely design changes. Most of the time, manufacturability feedback is delayed until volume production when design changes are no longer possible!

For more details and additional examples, read Chapter 18, “Planning for Design, Fabrication, and Assembly” in McGraw-Hill’s PRINTED CIRCUIT HANDBOOK-Sixth or Seventh Edition edited by Clyde F. Coombs, Jr. and Happy Holden (2016).

Figure 2. IBM’s Assembly Design Report Card [2]

Figure 3. Improvement in producibility scores as a result of the Assembly Report Card use. [2]

Would you like to find out more about how Altium can help you with your next PCB design or have questions about our PCB design software? Talk to an expert at Altium or continue reading about the best software for PCB design and manufacturability guidelines.

References:

1. Holden, H.T., “PREDICTING COST TRADEOFFS IN DESIGN FOR MANUFACTURING” Surface Mount International Conference, Sept 1995, pp 659-665

2. Hume, H.; Komm, R.; and Garrison, T., IBM, "Design Report Card: A Method for Measuring Design for Manufacturability", Surface Mount International Conference, Sept 1992, pp 986-991