Tag Archives: PCB

What is PCB Prototyping?

Each electronic piece of equipment has at least one printed circuit board or PCB. The PCB holds electronic components in place, interconnects them appropriately, and allows them to function in the manner that the designer has intended. This allows the equipment to perform according to its specifications.

A designer lays out the printed circuit board carefully following the schematic diagram and other rules before sending it for manufacturing, assembling, and using in the final product. However, it is possible to overlook small mistakes and incorrect connections during the design. Often, only when the PCB is in the final product, is it observed to be not working properly.

Sometimes, things can go wrong during the routing and layout phase. Two of the most common issues are shorting or opens. A short is an unintentional electric connection between two metallic entities, while an open is an unintentional disconnection between two points. A short or an open can prevent the printed circuit board from performing as intended.

To overcome this issue, designers prefer to generate a netlist, preferably in an IPC-356 format, that they send to their PCB manufacturer along with the other Gerber files. The netlist is a database of electrical connections that confirms and maps that the layout in the Gerber files is correct, and will work as intended. The manufacturer loads the netlist along with the Gerber files into the CAM program before verifying the correctness of the design.

The manufacturer can compare the netlist file to the data for finding shorts or opens within the routed file. On discovering an open or short in a PCB, the designer must scrap or redesign the board. If the discovery of the error is at a late stage, the designer has no alternative but to scrap the board. However, if the manufacturer discovers the error before assembly, it is possible to redesign the board rather than scrap it.

Prototyping a board is the process of manufacturing only a few numbers of the board initially. These boards undergo full assembly and then rigorous testing to weed out all errors in them. The testing stage makes a complete list of the errors, and the designer can go back to the design process to rectify the mistakes. Once the designer has addressed all the corrections, the board can proceed with production.

If the errors are of a minor nature, it may not be necessary for the designer to redo the design and layout. The manufacturer can suggest simple tweaks and the PCB engineers can accept them through an approval process. Manufacturers can easily handle a trace cut or add a thermal connection or a clearance that they can easily and cleanly complete.

Allowing the manufacturer to handle the required changes versus a complete revision by the designer is much more cost-effective, and faster. During the prototyping process, it is sufficient to document the process. Later, an ECN can fix the data set, create a completely new version or bump the revision as necessary. This process is inexpensive and accurate.

What are Ball Grid Arrays?

Initially, surface mount devices, especially ICs, came as perimeter-only packages, with pins for soldering placed along the edge of the device. As ICs became more complex, they needed more pins for external interfacing, which made the packages larger. Manufacturers soon realized there was a large unused real estate that lay just under the package. Therefore, they made the ball grid array (BGA) packaging, which, in place of pins, had solder balls aligned in a grid under the device. Soldering BGAs involves melting these solder balls onto pads on the PCB.

Using BGAs leaves a considerably larger area free on the PCB. Compared to mounting a package with pins on its perimeter, BGAs offer better thermal and electrical properties, and this has made the format popular, following the continued miniaturization of electronics.

Since their introduction, although their basic concept has remained the same, BGAs have changed in dimension and now come with far smaller pitches and smaller outlines. There are varieties as well, with some packages having connections only on the periphery and none at the center, while others have the connections distributed evenly across the bottom of the package.

For simpler BGAs, routing traces on the PCB is simple as the balls are placed well apart or there is space in the middle of the device. However, with increasing pin counts and decreasing pitches, routing between the pins becomes more difficult, resulting in increasing the layers of the board, thereby increasing the cost and reliability concerns.

As BGAs become increasingly more complex, designers have to depend on vias to connect the BGA with the rest of the circuitry on the PCB. Vias are small holes drilled through the multilayer PCB and plated with copper to provide connection between pads and traces on different layers. Some vias are through-hole types, meaning they start and end on the two extreme layers of the PCB, and may connect to other layers in between. Other vias can be blind types, starting from one of the outermost layers and ending on an internal layer, possibly connecting other layers in between. Blind vias are not visible on the PCB surface as they start and end at different internal layers, and may connect other internal layers as well. However, all the above require great precision while manufacturing, and are expensive processes.

Ordinarily, PCB designers prefer not to use vias on a pad, as during soldering, vias can wick solder from the pads leaving the joint in a dry and unsoldered state. However, with BGA pitches getting increasingly smaller, designers do not have much choice, but tenting is offering a way out. Tenting allows filling the via hole with an insulating material and covering the top with a layer of copper, thereby preventing wicking.

As the BGA pins lie in between the device body and the PCB, traditional soldering methods such as hand soldering and wave soldering are no longer useful, and assemblers rely on infrared heating or reflow ovens to solder BGAs to a PCB. This requires a pick-and-place machine placing the BGA package precisely on the pads and uniformly heating the area to form the actual connections.