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Design For Manufacturability (DFM) for Printed Circuit Boards

This guide is meant to provide the basics on Design for Manufacturability (DFM) of printed circuit boards. Design for manufacturability is the process of designing products that are manufacturable with ease. The beginning stages are critical in modeling products that meet specific criteria since a large portion of the costs associated with a product from idea to completion occur during these beginning stages.

Manufacturers seek to create designs that meet the changing needs of consumers, that is - designs that can be brought to market without much delay while successfully meeting the needs of customers.

When a model is created it is sent to manufacturing for feedback. This review is typically referred to as design review. This process is important since, if not carried out properly, designs that cannot be produced may make it to the manufacturing stage. This is often the result of poor reviews and inadequate feedback during the review stage. This must be avoided. If each DFM guideline is not followed, it may result in a loss of time and money.

A printed circuit board, also referred to as PCB, and sometimes referred to as a printed wiring board, is used in most electronic devices. Its function is to support and connect electronic elements through conductive tracks. The conductive tracks are etched from copper sheets that have been laminated onto a supporting material called a substrate.

Before advanced technologies, PCBs were assembled with the method referred to as 'through hole' construction. Following that practice was a method that required component leads to be inserted into a copper foil pattern. With the new technologies of board lamination and etching, the process evolved into the standard fabrication process currently utilized.

Best practices for Printed Circuit Boards: Remember: keep it simple!

•Verify that parts are available in quantity
•Use as many common parts as possible
•Use standard tools
•Minimize the number of components
•Minimize number of processes
•Minimize manual soldering
•Minimize manual insertion and assembly
•Minimize the assembly planes
•Minimize assembly directions
•Eliminate hidden features
•Use standard processes that are understood
•Model parts for efficiency in testing
•Model parts using symmetry
•Maximize compliance for assembly
•Evaluate if environment friendly
•Considered temperature ranges of all components
•Identify and assess tolerance issues

Checklist for Printed Circuit Boards:

• Signal Checks: conductor width; spacing; annular ring; drill to copper; hole registration; text features; missing copper; features connection, missing holes; unconnected lines; rout to copper.

• Plane Checks: drill to copper; annular ring; spacing; conductor width; thermal air gap / spoke width; missing copper
rout to copper; drill registration

• Solder Mask Checks; solder mask clearance; coverage; rout to mask; spacing; missing solder mask clearance; exposed lines; partial clearances.

• Silk Screen Checks: silk screen to mask spacing; silk screen to copper spacing; silk screen to hole spacing;
silk screen to rout spacing; line width; text height; silk screen over copper text

Tips and Hints:

• Decide on width of circuit board traces based on acceptable temperature ranges.

• Decide on distance between circuits according to applicable standard requirements.

• Position filter capacitors so leads travel via printed circuit board traces that carry mainstream for filtering current.

• Decrease voltage occurence along ground path by using the ground plane for control circuit.

Know the acceptable manufacturing tolerances.

• Familairize yourself with the effects of non-zero trace impedance and signal communication from circuit to circuit.

• Eliminate tracking between conductors by ensuring proper clearance between traces.

• Use ground plane to act as a shield between power and control circuits when possible (multilayered boards).

Cost Efficient Best Practices in Printed circuit Board Manufactoring:

• Flow Restrictors: if conductivity meters are not used, flow restrictors are an inexpensive way to reducing the use of water. Flow restrictors are available in a range of sizes. They are most useful when used with timer controls to ensure that rinse water only flows during operations.

• Timer and Conductivity Controls: timer controls have similar cost savings as conductivity controls, but do not result in quality improvement. Conductivity controls are used to operate rinse tanks at a conductivity set point. Conductivity meters are worthwhile investments to evaluate the condition of rinse water.

• Spray Rinsing: spray rinsing is performed over plating, drag-out and rinse tanks. It reduces how much rinse water is used and can reduce the use of chemicals if rinse is recovered into the plating tanks.

• Reduce Peak Demand/Tank Heating: equipment that uses large amounts of power should be started in stages to reduce the peak demand rating on electric meters. Heating tanks can account for more than 40% of energy costs.

• Specific PCB Processing Equipment: equipment used can use large quantities of power. Be sure appropriate operating of equipment to maximize utilization during operational periods and eliminate operating equipment when it's not needed.

• Conserve Compressed Air: compressed air is the most expensive industrial utility. Check for air leaks and repair quickly. The cost can be hundreds to thousands per year.

• Heated Tanks: the loss of heat through the walls of tanks can be reduced by constructing them with double walls and packing the space between with high quality lagging material.

• Extraction Systems: ensure the extraction fan is sized correctly and is controlled via an AC inverter, so its speed can be varied and set easily.

• Monitoring: a key element for conserving energy is knowing how much energy is being used and where it is being used. Monitoring energy consumption can yield a substantial energy savings yearly.

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