Estimated reading time: 8 minutes

Jack Olson: Component Placement Tutorial, Part II

Jack Olson has been putting together an introductory tutorial for new circuit board designers at http://howtopcb.net.

The entire site is not finished yet, but we can't resist making parts of it available to our readers. (Click around on his site--there's plenty to occupy your time when you should be working.)

In his last article, Jack laid out his approach to preparing for component placement. This week, we'll dive right into component placement.

By this point you should have mounting holes, tooling holes and fiducials placed, keepout areas defined and components with fixed locations placed and locked down. You've decided whether your design will be single-sided or double-sided.

Now, let's get started placing components!

EMC

It is generally a good idea to keep I/O lines interacting with the outside world away from internal signals. Most schematics with connectors also have some kind of passives (ferrite beads or caps or transorbs) that should be placed near the connector pins, and the traces connecting them should be relatively short to reduce the amount of noise coming in from the outside world.

Since we already have our connectors placed, it might be a good idea to go ahead and place the passives related to each node.

Figure 1 depicts a simple example of some passives that need to be close to the connector pin:

Figure 1: Passives that should be near the connector pin.

All components that touch the node connected to J1-13 should be placed close to J1-13 to keep that node area confined. Figure 2 shows one way of doing it for a single-sided assembly:

Figure 2: One approach to keeping relevant components near J1-13.Components on the other side of R24 and R25 (not shown) can be placed elsewhere. Continue placing components tied to connector pins until all I/O lines are done.

Floor Planning

Now its time to take a step back and look at the big picture. Since there are so many different types of board designs, it would be difficult to make specific statements about how you should proceed. But in a broader sense, electronic circuitry has some kind of flow, input and power comes in from somewhere, something happens to the input internally and some kind of result goes out.

Regardless of how the circuits are placed on the schematic pages, the floor-plan of the design should be somewhat logical. We want to keep functional groups of circuitry together, we don't want groups interfering with each other, and we don't want critical signals crossing back and forth over each other. Analog circuits should be separated from digital circuits. High frequency circuits should be isolated from low frequency circuits.

Try to get a mental picture of the best way to place the functional groups of components so the circuit flows logically.

If the design is very complex and you aren't sure how it should fit together, you can start working on the functional groups individually, and then shuffle the groups around later to make them fit together.

Functional GroupsHere is the general approach I use for almost every design I do. After performing the steps that have already been described, I:

• Scan through the schematic, mentally taking note of groups of components that look like they belong together, like four op amps that are packaged into the same IC.
• Look for circuits that aren't anything like the rest. For example, there may be a power supply with high current requirements and very large components, but everything else is smaller. One single circuit like this with special requirements probably needs to be done first using specific rules, and then moved into location in a particular area of the board, like near a power connector. If I see anything like this I will probably do it first and put it where it belongs on the board.
• Keep routing in mind. Although the focus of this tutorial is on placement, routing considerations can heavily influence placement decisions. If the design is primarily large digital devices, they will be arranged to optimize performance of clocks and buses (and possibly timing delays).
• Pay particular attention to repetitive chunks of circuitry. For example, there may be 12 channels of identical circuitry for a particular design. In this case I would do one "perfect" channel placement, taking extra time to massage the overall shape of the group into a rectangular arrangement, or some shape that will allow me to place the other 11 channels without wasting too much board space, and then copy the remaining channels to match.

Figure 3 is an example of duplicate circuitry and how I would place them as identical blocks on a two-sided assembly:

Figure 3: Placement of duplicate circuitry as identical blocks.

The top-side components are blue, and those on the bottom are red. I added short traces and vias to make sure the blocks would route, but removed them in the picture so you could see the components more clearly.

Figure 4: View showing top-side and bottom-side components.

For more complicated functional groups, I generally place decoupling caps first, then higher-frequency components like crystal oscillators and associated circuitry (which might be considered more important than decoupling, but they rarely interfere with each other), then feedback components or components in the "critical path," and then all the miscellaneous passives like pull-up or pull-down resistors.

As I go, I make sure to leave room for via fanout, quite often putting them in as I go. In fact, I usually do the very short routes from pin to pin WITHIN the group on surface layers, and then all the routing BETWEEN groups will be done on INNER layers after the groups have been shuffled into their final positions. If you use this method, make sure you add test point vias if ICT will be required.

Figure 5 illustrates one more functional block:

Figure 5: Yet another functional block.

As you place components, leave at least 2.5 mm between ICs for visual inspection of solder joints and possible rework, and keep at least 1 mm space between passive component bodies (land pattern edge-to-edge spacing may be less).

Figure 6: A view of the top-side and bottom-side components.

OK, that's enough examples. Keep sorting out groups until you have all your blocks done, or enough to start organizing them on the floor plan.

Even if large active devices don't seem to have an obvious group (or are connected to too many devices to manage easily), at least place all the required decoupling caps and other passives like pull-up and pull-down resistors. I usually try to put these underneath the large ICs so I can move and rotate them in convenient chunks.

When you have most of the groups sorted out, zoom back out to see the big picture again. You have functional blocks of circuitry placed, and now it's time to arrange them in some logical order. If you have been following the method outlined above, most of the short routes within the blocks are done, most of the fanout vias are in, and now it may be easier to see the netlines connecting the blocks together.

Power DistributionEven though you have made good progress on organizing components into groups, the remaining netlines may still look very complicated, and another way to help sort out the floor plan placement is by looking at the power requirements.

For example, many modern circuit board designs have several different voltage levels used by various sections. If there are many different voltages it will be impractical to devote entire planes to each one, so you may have to fit several voltages on a single layer by using split planes. Most of the time you will want to keep blocks using similar voltages together for the most efficient power distribution.

If your CAD system allows changing colors by net name or net type, try setting all nets to a dim neutral color, and then set each voltage level to a different bright color. By looking at the design in this way, you can sort out the blocks for different voltage levels quite easily. Quite often you might have a predominant voltage for the majority of the design (like 5V or 3.3V) and then miscellaneous smaller areas of other power requirements like +/-12V or 2.5V or 1.2V, etc.

I usually group the smaller voltage areas together first, along with any associated voltage regulators and bulk capacitance, and place them in a way that flows from one to the next. For example, a 12V power supply will feed any 12V circuits (so you will want to place them close by), but also provide the input to a +5V regulator, and then the +5V plane might supply power for a large amount of circuitry, but also provides the input for another regulator that drops down to 3.3V for that section. Try to place these cascading sections logically, and keep the routes for each section confined to that section as much as possible, without crossing over plane splits.

Final WordWithout knowing the particular characteristics of your design, it is difficult to say more about how you should proceed. Every design is different, and a strategy that worked well on one design may give poor results on the next. Learn from similar designs if possible. If the schedule allows, take a challenging circuit and try a few different approaches. You will get better and better at it the more you practice. Be alert to circuits that have been copied from previous designs. Look back to see how this was solved, and either improve upon it or consider saving it so it can be reused in the future to save time.

When all of the components are placed, review your original rules (height restrictions, keep-out areas, etc.) to make sure nothing was overlooked. You may also want to host a placement review before the final routing phase begins.

Jack Olson is currently designing circuit boards for Caterpillar. He can be reached at pcbjack@gmail.com.