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A simple recipe for a successful PCB Design

23 Mar

 

An electronics or embedded engineer should not limit themselves to writing firmware. Having thorough knowledge on hardware empowers the designer to understand what is happening and take the right path.

For example, the firmware written could be world-class, yet it may not stop an ADC count from oscillating whereas a simple decoupling capacitor may.  Before starting the building, one needs to properly lay down the foundations. Hardware is the base on which firmware lies, so this needs to be fundamentally right and stable.

Proper circuit design, topology and PCB routing is essential for any subsequent process to take us through further.

Points mentioned below may enhance the performance of the design while cutting the testing time shorter by making the process more convenient.

 

Plan from day one

1. Make a list of components you use often in your design. This could be your standard molex connectors, 0805

package resistors, crystals, etc. This list must contain all the standard components that you may use and those you might have used at least once in your career so far. It would be also helpful, if you could get the datasheets containing footprints of these components, wherever necessary.

2. Search DesignSpark for schematic symbols and PCB footprints which exactly match these components. In case matching schematic symbol and PCB footprint are in different libraries, you may want to create your own component which has the selected schematic symbol and footprint. You may also want to rename the component, symbol and footprint with similar names so that future retrieval is easier. DesignSpark has a nice wizard for creating such component libraries.

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3. It is also very important to see if each of the footprints have the right kind of pads by cross-verifying if the pad widths and holes are suitable to the physical component.

 

Component Wizard Tutorial Video

Let the Schematic have it all

1. Now, that, each of the components is ready, schematic can be drawn.  During schematic design, it is easy to relate to certain aspects of the design such as track width (depending on the current capacity needed),etc. Using “Settings” – “Design Technology” menu, we are able to select whether our net is a signal, power or ground. We may also create our own net classes with names like thick, thicker, thickest and so on. To specify the track width for the new net classes, we may have to select Settings – Design Technology, when we are inside the PCB file.

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2. De-coupling capacitors, as we know, are a must in any digital design. Most of them are connected to the DC supply and ground. It would help in identifying them easily in a PCB file, if we name them as CU1, CU2, etc, meaning CU1 is to be positioned near U1 and so on. This will ensure that we have not left any IC without a decoupling capacitor.

3. Depending upon the application, we may need to practically arrive at the value of certain components. For example, we may need to adjust the value of a resistor to change the frequency of an RC oscillator or setting a gain value. In a SMD design, this particular component may be selected as a through hole type. A production PCB can have all SMD components once the value is determined practically.

PCB Design

1. In multi-layer boards we use vias to switch between layers while laying tracks. Having vias of a larger size helps us in two ways. Firstly, if we need an additional connection from a net we could always have it by connecting a wire. Secondly, a larger via can act as test point. Design Technology Menu has settings where we can alter the default size of the via. I have found that a via of 70 mil diameter and 40 mil drill size to be very useful.

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2. In microcontroller designs, while it is important to position the crystal in close proximity to the microcontroller, it is equally important not to run any other track in between.

3. Copper pouring can be very useful in ensuring proper grounding. Copper pouring is basically filling areas where no nets are present while this filled copper would be connected to one of the selected nets. It would surround all areas around tracks with a standard spacing. We could do this by selecting “Add Area for Copper Pour” icon. Once the copper pour is drawn, connecting this copper pour area to a net is done by right clicking and selecting “Add to Net”.  Once we add the ground net, we get the ground filled.

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From Schematic design to design delivery, every step is important. If one can take care from the start,  valuable time can be saved.

DesignSpark – 28/101/11

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Schematic – Just drawing a circuit?

21 Mar

Is there more to it?

Having used a few CAD packages, including DesignSpark PCB, I was curious to look around to see how people work on drawing schematics. I was not interested in the way they draw or the design itself, but was only looking at the importance they attribute to drawing and configuring their schematics.  The general practice seemed to be – draw the circuit, translate to PCB and let’s work out things on the PCB side. I was wondering whether this approach was right.

The schematic is the closest in context to the design and layout provides the necessary connections physically. Whenever, we need to look into the design, our first action would be to refer to the schematic.

The most important aspect of a  PCB layout, perhaps, is the width of the track. Tracks are drawn based on the properties of the Nets specified in the schematic. For example, when we connect two component pins through a net, we can change the width of the track according to the current carrying capacity needed. We do this by clicking on the net and pressing ‘S’. We get to change the width in mils or the units we have chosen.

Image

For VDD and VSS, we may draw tracks with 60 to 80 mils whereas for a I2C signal such as SDA and SCL, we may go below 15 mil too, depending upon the size of the board.

As the schematic gives us a better idea about the current and voltages involved in a particular part in relation to the end design, specifying track widths would help us  to pre-fix the track widths to the appropriate values.

DesignSpark – 18/08/2011

 

 

PCB Design For Mixed Signal Circuits

12 Mar
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