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Article: Printed Circuit Board Design - Selecting the Right Technology

Printed Circuit Board Design-Selecting the Right Technology.

by Tim Mintzer EDC Inc.

Select the Right Technology:

Most PCB designs these days are required to pack "10 pounds of stuff into a 2 pound bag". This leads to more complex manufacturing and test techniques. By addressing the issues presented below, one can help to alleviate these problems before they hit the production floor.

Components-Surface Mount, Through Hole or Mixed:

Even though most common components are available in Surface Mount form, it is important to identify each component and package to be used in the design at the beginning. Top side components or Top and Bottom side components need to also be defined. If at all possible, all components should go on the Top Side.

Manufacturing Technologies-Reflow or Wave Soldering:

Although most OEMs and Contract Manufacturers have SMT capability, many boards require mixed technologies causing an additional wave-soldering step.

Higher density circuits sometimes require the use of Ball Grid Array (BGA) packages. This provides for higher levels of integration, but may require expensive X-ray inspection equipment. Knowing the capabilities of the Production Floor is imperative.

Testing-Automated or Manual:

Volume usually dictates this decision, however, a properly designed PCB will facilitate the migration from manual to automated. Even though they may not be used, it is a good idea if space permits, to add test points for In-Circuit Test (ICT). The addition of test/probe points will also help the tester look at critical signals in a manual test.

Design Methodology:

Many design these days use multi-layer PCBs. These do not add that much cost and allow a higher routing density for a given board area. In addition, properly designed power and ground planes will help keep noise and electromagnetic radiation from causing problems in the circuit.

High Speed and RF boards may require controlled impedances and a controlled dielectric constant of the material used. Identifying these signals up front will and designing accordingly will help ensure a successful design.

Most PCB fabricators can make PC boards down to 8 mil traces and 8 mil spaces or smaller. This is the industry norm. However, if not needed why use it? Using 10 mil traces and 10 mil spaces will increase the PCB houses' yield. Also, identify any high current traces (nets) and design those to handle the current. Over design those if possible, as the current goes up in a PCB trace, not only does the voltage drop due to losses, it will generate more heat.

Oh yes, on that subject, Heat. Consideration must be given to high heat dissipating sections of the circuit. Thermal mass must be given to components generating heat, whether with an add on heat sink or heat dispersing copper planes. This needs to be properly designed to provide maximum lifetime and low mean time between failure (MTBF).

Conclusion:

Although there are many different aspects to designing a Printed Circuit Board, I have tried to hit the main points in this article. Careful consideration given to the items mentioned above at the beginning of the design can eliminate problems early instead of during a production run which could cost lots of time and money. Don't design the PCB in a vacuum, soliciting input from manufacturing and test engineers will help achieve the desired results.

© 2005 - Tim Mintzer and EDC Inc. - Reproduction without permission prohibited. All Rights Reserved

 
 
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