PCB Fabrication

Printed Circuit Board Layers

    There are several layers even in a simple PCB or Printed Circuit Board. As implied by the name, a printed circuit board has an electrical circuit imprinted on it in the form of traces. The traces connect electronic components that the assembler will be placing on it. Usually, a PCB constitutes four layer-types, at least one layer each:

    • Substrate layer
    • Copper layer
    • Solder-mask layer
    • Silkscreen layer

    Each layer above functions differently.

    Medical PCBs almost always relate to a human life at some point or the other. Therefore, safety is the most significant point. The medical instrument needs to be precise, and for this, medical PCBs need signal integrity, high number of interconnections, and lead-free joints. Reliability, accuracy, and repeatability are equally important over the lifetime of the device—requiring use of high-quality components and materials in the medical PCB.

    Healthcare involves a vast range of devices covering small wearables to large full-body imaging systems. While wearables promote wellness, larger systems can analyze the health of internal organs. Whatever the size of the device, it depends on a medical PCB for its functionality. In fact, one can find a medical PCB in heart monitors, MRIs, CT scanners, pacemakers, defibrillators, temperature monitors, electrical muscle stimulators, blood glucose monitors, and many more.

    With so much electronics in use in monitoring health conditions, it is imperative that medical electronics conform to several safety and quality standards. Several agencies such as Federal Communications Commission or FCC, US Food and Drug Administration or FDA, International Standards Organization or ISO, International Electrotechnical Commission or IEC, and others publish regulations and standards to which medical devices must conform and comply.

    Geopolitics controls these standards to a large extent, as each country has its own standards and governing bodies. Although abiding by standards such as ISO and IEC cover almost all countries, there are specific regulations for special medical devices. This is necessary as medical devices vary widely to be suitable for use in different areas of application. This makes it nearly impossible to compile a complete listing of regulations. Moreover, publishing such listings makes them immediately outdated due to the accelerating innovations in medical technologies.

    Functionality of Layers
    Medical devices may range from standard equipment to very complex systems. It is usual to group them into three different categories:

    Substrate: The PCB gets its strength from its substrate layer. Apart from rigidity, this layer also forms a firm level surface for positioning electronic components. Manufacturers create the substrate layer with woven fiberglass cloth dipped in epoxy resin. Together, they form a strong and flat base layer. The electronic industry calls this combination as Glass Epoxy FR4. The FR is for its Fire-Retardant qualities, while 4 denotes the extent of the fire-retardant properties of the substrate material.

    Copper: By bonding a copper layer on one side of the substrate, manufacturers create a copper clad substrate, or more commonly a copper clad. When creating a PCB, a fabricator transfers the circuit pattern on to the copper surface, and removes the unwanted copper. The remaining copper on the board functions as traces or connections. They interconnect the many electronic components soldered on to the board.

    Solder-mask: If left uncovered, copper traces will oxidize slowly and become electrical open circuits. This may also make the PCB non-functional. To prevent this, fabricators cover all copper traces with a solder-mask layer, leaving only the pads exposed. As the copper traces are now no longer exposed to air, they remain protected and this extends the life of the board. Although a green solder-mask is common for PCBs, manufacturers may use a blue, red, or black thin epoxy layer for the solder-mask.

    Silkscreen: Identifying all components on the board requires numbering their positions with their BOM reference numbers. A silkscreen layer carries these numbers. Apart from the reference numbers, the silkscreen layer also depicts polarity of components and mounting instructions important to assemblers. For transferring the information in the layer on to the solder-mask surface on the PCB, fabricators typically use an epoxy ink that is in contrast with the solder-mask color.

    Quality of Layers
    The quality of the individual layers adds up to the total quality of the printed circuit board. The glass transition temperature of the substrate depends on the type of epoxy present. If the ambient temperature rises beyond this glass transition temperature, the board may lose its rigidity and become deformed. Therefore, it is very important the temperature of the application is always lower than the glass transition temperature of the substrate.

    The quality of copper traces on the board depends on the etching process to a large extent. Although the thickness of the trace depends on the weight of copper of the copper clad, its width depends on the etching process. Over-etching can reduce the width from the specifications, thereby increasing the resistance of the trace. For traces that carry substantial amounts of current, the increase in resistance can melt it as the temperature rise of the track may be very high. Under-etching can leave a trace much wider than specifications, reducing the insulation between neighboring traces.

    The thickness of the solder-mask layer and its registration defines its quality. Registration of the solder-mask with respect to the circuit on the board is very important. The registration must be highly accurate, specifically for fine pitch components. A solder-mask finger must be present exactly midway between two close pads, otherwise, there can be solder shorts on some pads. Unless the mask is of uniform thickness across the board, there can be unwanted peeling off in areas where the layer is thin.

    The quality of the silkscreen must be high so that the lines in the layer are sharp and all characters are legible. If this is not so, assembly operators will have difficulty in placing components in their intended positions and the board may not function properly.

    Boards with Multiple Layers
    Printed circuit boards vary from being of a single-layer, two layers, to multiple layers. Fabricators usually make single-layer boards from single copper clad substrates. Double-layer boards usually use a substrate that has a copper layer on both sides. For easier understanding, one can consider a multi-layered board as made up of a double-layered board with added copper layers on both sides. Prepreg or insulating layers separate and insulate the different copper layers. During PCB fabrication the substrate is usually in the center, with the additional copper layers and prepregs equally distributed on both sides.

    The stack build-up or the total number of layers in the board depends on its overall size and its circuit complexity. The designer may need to use many copper layers to route all nets properly after finalizing the positioning of all components. However, while the solder-mask appears on two outermost copper layers, the silkscreen layer must appear at least on the top outer layer. The customer can reduce the number of copper layers by increasing the size of the board, or by changing the circuit to reduce the total number of components on the board.

    Materials for PCBs

    FR4: As a substrate, Glass Epoxy FR4 is one of the most popular materials. This is because FR4 has a glass transition temperature limit of 130 °C. Once the ambient temperature crosses this limit, the glass epoxy board starts to lose its electrical insulation properties and its mechanical rigidity.

    Hylam: Choice of the substrate material for a board depends on the ambient temperature of its application. PCB designers select a substrate with a glass transition temperature higher than the maximum temperature the application may experience. Designers often use cheaper material for the substrate, provided the application temperature is not very high. Hylam or Bakelite Hylam is one such cheaper material.

    With good electrical insulation and mechanical rigidity properties, Hylam is a suitable substrate material. However, unlike glass epoxy, hylam is not a fire retardant. That means, hylam continues to burn if it catches fire, whereas glass epoxy will not burn for long. Consequently, designers do not consider hylam to be reliable for use as a substrate material.

    Copper: Copper tracks connect components on the printed circuit board. There are several reasons why copper is a popular material for this task. For instance, copper exhibits:

    • High Electrical Conductivity — low resistance of traces
    • High Mechanical Ductility — easy control of copper weight on the copper clad
    • High Melting Point — copper traces have high current capacity
    • High Availability — PCB fabrication cost remains reasonable

    Individual properties of other metals are often better than those of copper. However, copper exhibits the most optimum combination of all the above properties. This makes copper as the best suited for use as copper clad for substrates.

    Prepreg: Fabricators use prepregs as insulators in multi-layered boards for gluing different layers of copper and the substrate. The prepreg or pre-impregnated layer is the material for holding the entire board together. Fabricators make prepreg of fiberglass by impregnating it with epoxy resin. They build up the board thickness by pressing the layers together at a high temperature.

    Aluminum Boards: Boards meant for mounting high-power LED lights often use an aluminum sheet as backing. Such metal-clad boards with aluminum backing help to dissipate the heat from LED chips mounted on the board.

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