Wednesday, 29 July, 2015
Designing and manufacturing PCBs is a complex process that involves many steps and considerations. This is the first article in a three-part series on PCB design and manufacturing. This article details the types of PCBs and explains why it’s important for designers to understand manufacturers’ capabilities.
Printed circuit boards (PCBs) are made from fibreglass material, commonly known as copper clad laminate, with copper tracks in place of wires. Copper tracks are chemically etched ie, printed — this connects different components on the PCB. Through-hole components are fixed in position by drilling holes and then soldering them on other side of the PCB. The different types of PCBs that are in common use these days include:
The substrate used to manufacture a PCB is purely rigid in nature and therefore it is called a rigid PCB. Rigid PCBs are mainly divided into 3 categories — single-layer (or single-sided), double-layer (or double-sided) and multilayer PCBs.
Single-layer PCBs — The most basic PCBs have the components mounted on one side of the board and the conductor pattern on the other side. Because there is only a conductor pattern on one side, this type of PCB is called ‘single-sided’ or ‘single-layer’. This type of circuit board is suitable for simple circuits only. Because there is only one side, no wires can cross and they have to be routed around each other.
Double-layer PCBs — Double-sided or double-layer PCBs are better suited to complex circuits as they have twice the area for the conductor pattern compared to single-sided PCBs. Double-sided PCBs have a conductor pattern on both sides of the board. Having two separate conductor patterns requires an electrical connection between them. Such electrical ‘bridges’ are called ‘vias’. A via is simply a hole in the PCB that is filled or plated with metal and touches the conductor pattern on both sides.
Multilayer PCBs — Multilayer PCBs have one or more conductor pattern inside the board to increase the area available for the wiring. This is achieved by gluing (laminating) several double-sided circuit boards together with insulating layers in between. The number of layers is referred to as the number of separate conductor patterns — usually even and including the two outer layers. Most boards have between four and 10 layers, but PCBs with almost 100 layers can be made.
Advanced electronics demands efficient dissipation of heat away from the components. Thermally conductive dielectric material like aluminium or copper alloy can help achieve this more effectively than standard FR-4 material. The major advantages of metal core PCBs are heat dissipation, thermal expansion and dimensional stability.
Flexible and rigid-flex PCBs
Flexible printed circuits (FPCs) are made with a photolithographic technology. A flex circuit or flex PCB is a patterned arrangement of printed circuitry and components that utilises flexible-based material with or without flexible coverlay.
FPCs offer the same advantages of a printed circuit board: repeatability, reliability and high density, but with the added ‘twist’ of flexibility and vibration resistance. The most important attribute compelling designers to adopt flex circuit technology is the capability of the flex circuit to assume three-dimensional configurations. Rigid-flexible circuit boards are a combination of rigid and flexible PCBs. Rigid circuits (FR-4 PCB) are connected with single or multiple flex through PTH (via), inside or outside of flex circuit layer.
Manufacturing capability statements
It’s crucial for designers to know PCB manufacturers’ capabilities. Imagine a situation where you have designed a complex circuit board and sent to the manufacturer for a quote. Instead of receiving a quote, you are asked to make few design changes to suit their manufacturing capability. This unnecessarily lengthens the design process and disturbs the project plan. Below are some tips on understanding manufacturers’ capability statements and using that information during the design process.
1. If a manufacturer has a separate capability statement for prototype and production manufacturing, ask for a copy and refer the correct one. It is possible to manufacture small volume PCBs with tighter tolerances so if PCBs are required in small numbers, one may choose their tightest capability statement.
2. Three basic limitations to know about the manufacturer before starting the design process include:
(a) Etching limitation (ie, minimum track width/spacing) — For 1 oz standard finish copper thickness PCBs, 0.127-0.152 mm is the minimum track width and spacing requirement for most manufacturers. With special attention, 0.089-0.102 mm can be achieved on small- to medium-volume production. If your design requires thicker copper then minimum track width/spacing can increase up to 0.305-0.330 mm, depending on the required thickness of the copper.
(b) Hole size limitation — Most manufacturers accept finish hole sizes from 0.3 mm and higher as standard. Some manufacturers charge extra for 0.25 mm finish holes (standard for QualiEco Circuits). A few manufacturers accept 0.2 mm finish holes by charging extra. In some special cases, a 0.15 mm finish hole size can also be acceptable using mechanical drilling process. Any less than 0.15 mm can be achieved by laser drilling, which is an expensive process.
(c) Bonding and drilling limitation (for multilayer PCBs only) — The most important criterion to keep in mind while designing a multilayer PCB is the gap between the edge of the hole and the nearest copper area (track/pad/pour) in inner layers. For multilayer PCBs, drilling is performed after bonding so this criterion is extremely critical for PCB manufacturers.
It is important to know whether a manufacturer has added plating tolerance while mentioning minimum isolation in their technical capability statement. Most manufacturers need to add approximately 0.1 mm plating tolerance for via holes. Some manufacturers display minimum isolation sizes without adding plating tolerance while some do add them. Unfortunately, there is no specific guideline for displaying this criterion.
3. Another important factor to consider in your design is the minimum copper and solder mask pad size around holes.
- Copper pad size — Most manufacturers define copper pads around holes as ‘annular ring’. There is a minimum annular ring size you need to maintain everywhere in the design.
- Solder mask pad size — Solder mask is nothing but an opening to prevent solder bridges between adjacent pads and traces during the soldering process.
The next article in this series will provide insights on PCB design software packages and choosing the right PCB manufacturer.
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