The good performance of a product is dependent on the way the multilayer PCB installed inside is designed. Correct and seamless flow of current through PCB and management of heat and other obstacles is essential for ensuring the flawless working of the device it is lodged on. This is possible to achieve only with fool-proof planning. A good quality substrate is the guarantee of a high performing device. Thus, it needs to be designed meticulously keeping all the deterrent factors in mind. EMI reduction, reducing crosstalk and maintain impedance at optimal levels are some of the challenges one needs to meet while designing a multilayer PCB.
So, first let’s start with how to determine the layer count. The number of PCB layers is determined by finding the largest trace, vial and clearance allowed. The most promising arrangement is found in 4/4 MIL (trace/clearance) and 20/8 MIL vias clearance that are characteristic of a high speed design. The number of layers also depends upon how complex you need the design to be. The number of layers is also determined on the basis of current required and the desired impedance level.
Once the crucial part comprising of selection of number of PCB layers is done, here are some of the useful tips for taking the multilayer PCB designing to a fruitful end.
- Ensure to space the signal layers and the planes as close as possible (< 10 MIL), then include a large core measuring approx 40 MIL between the ground and power plane; the resultant substrate should be approx 62 MIL in thickness. Coupling of trace with plane minimizes the cross talk common among the traces and helps achieve acceptable level of impedance. This tip is quite useful when you are designing a 4-layer stack up.
- Always keep signal layer adjacent to the plane. Use power and ground plane as returning path for the signal.
- While placing the surface mount components on a multilayer advanced PCB, control ground plane should be placed on the inner layer so that it can shield the power circuits from the control circuits.
- Grounding plays crucial role in high speed circuit designs. Ground, as such, is not restricted to one point only in the circuit. In fact, whenever the current flows uninterruptedly through traces, ground path witnesses the differences in voltage at various points. Thus, a ground plane is needed to be included for the control circuit. The correct way of doing this is using vias to make ground connections to the ground plane instead of routing them through the PC traces.
- All lengths and areas of the loops through which high frequency switching current is to be flown should be kept minimum in dimensions.
- All capacitors should be placed closer to the corresponding pins. These capacitors do the job of bypassing bias supply voltages of ICs; ideally, a large capacitor (10 µF – 100 µF) and a smaller ceramic capacitor (0.1 µF – 1.0 µF) should be used in combination. Filter capacitors should be placed such that their leads are directed right into the printed circuit board traces where these filter the mainstream of the current.
- There are certain accepted standards such as IPC2152 that lay guidelines for the heat generation when the current passes through the traces. The width of PCB traces is to be chosen according to the temperature rise these can withstand. Trace also requires prevention from fusing at a short circuit current that may develop before the activation of an electric protection. Width of PCB trace is also supposed to handle an accepted level of DC and AC impedances.
- While choosing the trace spacing in multilayer PCBs, it is necessary to manage creepage and clearance efficiently. As per UL/IEC 60950, the minimum trace spacing should be 6.4mm. Standard practices take into account the amount of current to be flown. If the signal is to be flown, the width requirement is different from what is required for current flow. For instance, 0.010” width is recommended for low current signals of both digital and analog type, whereas when the current of 0.3 amps is required to be flown, trace has to be wider. PCB trace width is determined keeping factors like purpose, location, length, thickness and amperage in consideration. Trace location is chosen so as to ensure trapping of optimal heat. Thickness of the trace is chosen keeping the thickness of copper layer in mind. All these rules are helpful in designing traces on multilayer PCBs. In case of using power semiconductors in parallel, it is advisable to implement symmetrical routing where every paralleled device is provided with equal conductor impedance. This helps keep the impedance at optimal levels, and therefore, is necessary for optimizing the PCB for high speed transmission.
- The selection of components has to be done keeping their thermal resistance ratings in mind. The safe heat resistance rated components enable easy management of temperature preventing undue heating of the PCB eventually.
- PCB multilayer design requires controlling of heat production. It is done effectively with the help of thermal reliefs. The addition of thermal reliefs is considered critical for the wave soldering application on multilayer PCBs. Through-hole components should also be supplemented with thermal reliefs so that the rate of heat singling can be slowed down.
Thus, all tips pertaining to multilayer PCB manufacturing suggest that there should be proper provisions for easy soldering and mounting of components. The heat should be maintained at optimal levels, and the impedance levels too should not go beyond permissible limits. All such factors contribute to device-friendly, high performance PCB.