label:printed circuit board,PC motherboard market,PCB fabrication
Engineers' traditional rational for printed circuit boards (PCBs) was to save cost over hand-soldered wiring. The earliest PCBs were made from paper saturated with phenolic resin. This is now known as FR-2 (flame-retardant #2). Since phenolic resin is a thermoset resin like epoxy or Bakelite, it has good structural properties. Unfortunately, the paper foundation of the board is subject to cracking and tears when you subject it to high loads. In 1974, GMC Truck mounted ignition ballast resistors on paper phenolic boards. The board worked fine under the normal design mechanical and thermal loads. What the designers did not foresee was the force auto assemblers used when they had to push a wire harness connector onto the board. That act, before the vehicle was even competed, broke the PCBs and caused immediate warranty problems.
The structural limitations of paper-phenolic boards were not confined to General Motors. In 1980 the least reliable part in a Ford vehicle was the ignition module. The module was a 5 x 8-inch zinc die casting with a paper phenolic board holding the timing, logic, and drive circuits to fire the spark plugs. The modules worked fine in the lab, but in a real production environment, the paper phenolic board would warp under temperature variation and the circuit traces would crack. This would cause a catastrophic failure of the ignition system, stranding the car in or alongside the road. Ford management considered the cost of switching to a fiberglass-based FR-4 (flame retardant #4) board to be prohibitive. Instead, they had the division making the module partially fill it with sand before potting the paper phenolic board. This increased the thermal mass and provided a cushioning effect that reduced the failure rates to fewer than 5% in the warranty period. It was still the worst reliability Ford part of the era.
Now that telecommunications, automotive, PC, and portable electronics markets have all adopted the FR-4 board, the cost differential with paper phenolic boards is not as high as it was in the 1980s. What is most remarkable about FR-4 PCBs is that they have gone from single or dual layer to 8, 12, and even 26 layers. The PC motherboard market helped drive the demand for these high-layer-count boards, although the integration of chip sets and PC vendor’s adoption of serial buses like PCI Express means that a modern motherboard may only have six or eight layers compared to five years ago when you might have seen more layers. In addition to all these layers for your ground and power planes, the line widths you can routinely have are 4 mil (thousandths of an inch), and with special processing, 2-mil lines and spaces. The most advanced fab houses will offer 1.25-mil lines.
Although PCs have room on the PCB, consumer electronics, servers, and cell phone base stations still need high layer counts. That might be old news to seasoned electronics professionals, but there are many processing options that have made PCBs truly a hot technology. Blind and buried vias is one technology. When a via only goes down several layers but does not penetrate the whole board, it leaves room on other layers for more traces.
Not only can you use buried and blind vias, the size of the vias can be much smaller. Modern processing lets you have laser-drilled holes as small as 0.0025 in. Some vendors, such as Proto-Express, can offer mechanically drilled holes as small as 0.004 in. In addition to small vias, PCB fab vendors can fill vias with conductive or non-conductive material. The conductive material draws away heat. You might fill vias with non-conductive material to keep solder from wicking down the vias and interfering with proper reflow operations.
Another new aspect of circuit board technology is how thick the copper can be on your board. The need for lower voltages for FPGAs and microprocessors brings with it a need for greater currents. This has driven PCB technology to the point where you can have 25oz (ounces per square inch) copper laminate on your PCB. Now, if you understand the practical limitations of etching traces, you will know you can’t have 2-mil traces that are made in 25oz copper. Different PCB fabrication houses will have different design rules, but very often you can specify a board stack-up that will obviate the need for additional bus bars since the copper in the board is thick enough to carry the current by itself.
A dozen layers, thick copper, fine lines, and buried vias are just the processing side of the modern high-tech PCB. The substrates themselves are now high tech. You could always specify Teflon or polyamide substrates for high-speed circuits. These materials have a lower dielectric constant and are less lossy when you transmit high-frequency signals across them. But companies like Sanmina SCI can make you a top-layer FR-4 substrate that is 4 or even 2 mils thick. This means that you can have a ground plane 4 mils away from your signal traces. The close proximity means that the signal traces can be much narrower and still meet 50-ohm impedance specifications. With FR-4’s dielectric constant of 4, a 50-ohm trace is twice as wide as the distance to the ground plane. This means that a 4-mil thick substrate can let you use 8-mil traces to get 50-ohm impedance.
Many PCB vendors can also imbed decoupling capacitors and ICs inside the PCB, keeping valuable real estate open on the top and bottom of the board. Texas Instruments uses such a technology on its TPS82671EVM (TPS82671EVM datasheet) power supply boards (Figure 1).
A major high-tech push has been metal-core PCBs. Metal-core PCBs dissipate heat nine times better than FR4 PCBs. These substrates have flourished with the advent of LED lighting. The metal core provides a solid structural mount while taking away the heat from the LED. The core of the PCB can be copper, aluminum, or steel. You mount and route traces to your components on a thin dielectric layer. PCB fab houses can form and shape the substrates to your needs (Figure 2).
Vendors can provide metal cores with thickness between 30 and 125 mil. Thicker or thinner substrates are available but not as common. Fab houses can accommodate your copper thickness specification between 1 to 10 oz. The metal-core PCB combined with high-tech processing can solve problems in applications as diverse as automotive, solar power, motor control, power supply, and streetlights. Cell phone base station designers are putting the power amplifier on the antenna mast where there is no fan cooling. Metal-core substrates will help carry the heat away.
Be sure to look at the capabilities of a modern PCB processor 1. In addition to the old high tech like flex circuits, there are a host of improvements that make a whole new set of high-tech PCB designs that are truly a hot technology.
Source: //www.seekic.com
