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NEW to PCB Layout?
- by Matt Stevenson

I am new to PCB Layout.  What are the important design considerations that I need to make sure to take into account?

PCB layout at its core is a simple process, connecting all of the points of the components to their necessary endpoints.  However, in practice, there is much more to it than just routing traces from point A to B.  If you take these 6 design considerations into mind during the process not only will your design include all of the necessary connections but will also be more manufacturable, more reliable, and dissipate heat away from the electronics more readily.
   
Separation:  In most design applications separation of signal types is a very important consideration.  A classic example of this is keeping analog and digital signals and their associated grounds separate.  Grouping the sections of the schematic (digital and analog that share a common ground) together in close proximity and placing the ground plane directly below this grouping is a good design technique.  Similarly running only analog lines over (or under) the analog ground layer is another trick to help reduce capacitive coupling of signals. 
   
Minimizing EMI: This is such an important design consideration that I previously wrote a separate discussion on this topic that you can read here.  In short, here are 4 tips for reducing EMI from your design:

1. Choose SMD components over through-hole parts.  Generally, the leads on the through-hole parts create higher levels of inductance and thus an opportunity for EMI. 
2. Design with multilayers, adding a ground plane on the layer directly below the external signals (2 and n-1).  The presence of these planes in close proximity to the signal effectively will reduce the return path, keep signals clean and reduce EMI emission.  Connecting decoupling or bypass capacitors to these planes offers another effective technique for reducing EMI due to the short and logical return paths. 
3. Limit the operating current and or the rise times of the signals to help reduce larger fluctuations in current, offering lower EMI emission rates.  
4. Match the impedance on signals. This is a fairly critical practice of design, especially at higher signal speeds that will reduce the opportunity for signal reflection, harmonics, ringing, and overshooting digital signals, all of which increase the EMI radiation. 

Manage your Heat:  Heat is another potential pitfall to an electronic design if not managed and mitigated properly.  Most electronic components generate heat during use and likewise, most have some threshold to withstand heat.  This is another important topic that has had a dedicated piece written that you can read here.  In general, identify which components generate the most heat in your design, distance them from other heat generators and any heat-sensitive components.  Design in a way to remove the excess heat from the board (heat sinks, cooling fan, or thermal vias).  Much of the information on the heat generation and sensitivity to heat can be found on the component datasheets.

Component Placement:  When you have thought through function and separation, EMI, and heat and you are placing components on your layout, the next logical consideration is to optimize the placements from a design integrity standpoint but also to increase the assembly yields.  You want to shorten the distance of the trace runs as much as possible (less signal loss, faster speeds, and a cleaner overall signal), orient parts in a similar fashion and perpendicular to at least one edge of the PCB (if possible), consider the height and width of components and when possible use a stair-step approach to placement (don’t trap a short component between 2 taller components), place all SMD components on one side of the PCB (unless you really need both) and place all through-hole components on the top side. 

Power Delivery: Develop a plan for delivery of power to all of the necessary parts.  It is a good practice to draw power from a plane or a bus.  You never want to simply run the power from one component to the next, that practice can really cause lots of issues with your designs.  Also, make sure to use bypass capacitors and low pass filters to ensure that there are no unwanted spikes or outages to critical components. 

Verify Components Footprints: The sixth consideration as a designer is to verify (either yourself or through a library process) that the electronic version of all components match the physical.  The best way to accomplish this is through the datasheet.  Verify that the pad size and pitch are what is documented, and any hole size in the footprint not only matches but also takes into account maximum material condition (everything has a tolerance so use the worst case for establishing the maximum material condition).  There is nothing worse than getting your boards from the manufacturer and beginning to assemble and realizing that either the part in your hand will not fit onto the spot on the board or that the leads for the part will not fit into the hole.  The adage “measure twice, cut once” applies in practice as well.

PCB Layout is not a simple process but by following a logical process and planning for the inevitable you can increase the likelihood that your prototype will come out as you had designed.  If there is a tip that you feel is missing from this list that would benefit others please feel free to let me know and we will make sure that others know about it.