A schematic is an abstraction, a representation of an ideal circuit. A PCB Assembly, on the other hand, is a complex mechanical assembly. All the components in the circuit design must fit within the physical boundary of the PCB. The designer must locate and place hundreds of components subject both to the mechanical constraints of the product design and the manufacturing process while not introducing electrical noise into the circuit.
It can seem overwhelming to someone new to the PCB layout process. Fortunately, there are some simple guidelines used by the designers at PNC that will help break the layout task into manageable pieces.
Start by identifying the mechanical constraints on the component location. The Printed Circuit Board size and shape will be defined by the overall product configuration. Typically, the product’s mechanical design will define the location of the connectors, the keep-out zones and the locations of the heat sinks. The initial mechanical layout of the PCB is often an iterative process defined during design and early prototyping.
One recommended keep-out area is the perimeter of the board. The perimeter should be kept free to give the manufacturer a place to grip the PCB during assembly process, and room to score smaller boards to allow them to be removed from a larger multi-arrayed panel.
PNC recommends that all components be kept .050” from the PCB edge. Taller and more fragile components like capacitors should be kept .125” from the edge if possible, to allow room for tooling access to prevent damage when the PC Board is routed or scored from the panel.
The next step is to divide the PCB into functional modules to simplify routing and grounding. This will help minimize noise between power components, high speed digital components and analog components. Switching power supplies are particularly noisy, so the power supplies and their associated components should be grouped together as far as possible from noise sensitive sections of the circuit. While defining the location of the modules, be aware that high power components will need heat sinks, which may limit placement options.
Once the edge components are placed and functional areas of the Printed Circuit Board are defined, the next step is to locate the BGAs, quad packs and other large high pin count components. The room needed to fan out their pin counts is going to drive the locations of all the components around them.
In general, try to align the active components ICs with the #1 pin or A1 pin in the same orientation. While this may not have any utility in the actual circuit, it will make inspection and debugging of the initial prototypes far less frustrating.
As a follow on to locating the active components, the bypass or decoupling capacitors on the power pins should be located as close to the IC as practical to minimize the parasitic inductance. If more than one capacitor is being used PNC recommends that the lowest value capacitor should place closest to the power pin. If components will be located on both sides of the PC Board, the bypass capacitors are often located on the opposite side, directly under the power pin. If possible, each power pin should have its own bypass capacitor.
Polarized components such as electrolytic capacitors and diodes should be arranged so they are all facing the same way. During debugging, it is much easier to spot the one diode that is not like the others rather than having to look at the silkscreen of each diode to determine if it is installed correctly.
Pull-up resistors and other groups of identical components are often grouped and lined up to simplify layout, debugging and inspection. Consider using resistor arrays when possible to save board space and reduce component count.
Inductors break this placement guideline. Because inductors generate magnetic fields, placing them too close together, particularly end to end, can cause inductive coupling, changing the value of the inductors.
Double sided PCBs, PCBs with components on both sides, are expensive to produce, but may be necessary when real estate on the PCB design is tight, and when the overall product form factor is more important than PCB cost. A double-sided PCB is more expensive because it will need to pass though the pick and place machine and reflow oven twice.
Since the double-sided PCB will need to pass through the reflow oven twice, PNC recommends that the lighter and more heat resistant components be placed on the bottom of the board. Larger components on the bottom of the board will be glued to keep them from falling off, the smaller passive components cannot be glued, but the surface tension of the solder will hold them in place during the second pass through the oven.
These are some general guidelines to aid in component placement used by the PCB designers at PNC. However, they are just guidelines. The best way to ensure that the printed circuit board assembly can be manufactured reliably is to have the layout reviewed by the people who will manufacture it. PNC’s designers can review your design to help reduce production cost and improve yield and reliability. Contact PNC today to schedule a review.