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Circuit Board Assembly is the process of installing the electrical components on a PCB, or a Printed Circuit Board. In through-hole types of circuit board assembly, the electronic components are placed in holes in the printed circuit board, and soldered in place to make the electronic connection. I surface mount construction, the electrical components are placed on pads on the printed circuit board. For circuit board assembly SMT applications, the solder is applied to the board using a stencil. The solder past goes over the area of the board that the component is placed on and the electrical component is then held in place by the solder paste until the solder is heated up and reflows to permanently bond the surface mount component to the printed circuit board. In both through-hole and SMT circuit board assembly, component leads are electrically and mechanically attached to the board by heating the solder up so it reflows and attaches the component to the board.

There are a few different soldering techniques used for Circuit Board Assembly. The goal in each different method is still the same – to attach an electrical component to the printed circuit board. The first method we will discuss is that used for high volume production. This is typically done with automated machine placement of the electrical components and bulk wave solder or reflow ovens. If very small parts are involved, then these are typically done with the help of a technician that will solder these very tiny parts by hand. To do this they use precision tweezers and a very fine tip soldering iron and will do this under high magnification by a microscope or other optical system. Some part cannot be soldered by hand because they have the solder pads underneath the chip where it is impossible for a person do get to. An example of a package that has this construction is the Ball Grid Array or BGA package. In this case, the circuit board assembly must be done by heating up the chip, either by a hand held device that blows extremely hot air above the chip or underneath below the printed circuit board or by a reflow oven that essentially heats up all around the assembly.

Many times there is a mix of Through-Hole and SMT Circuit Board Assembly. Often a circuit board assembly will involve both technologies of through-hole and surface mount technology (SMT). This is due to some parts only being available in either the through-hole or SMT versions. An example of this might be a power resistor used to sense the current through a feedback resistor. A five watt version of this is likely to be available in only through hole, as a SMT version would likely heat up the board significantly if it was attached to the board. The cost of the SMT resistor might also be considerably more expensive than its through-hole counterpart. In most cases, the circuit board assembly method does not dictate which technology is used for each part, but the economic and thermal characteristics take precedence over the circuit board assembly technique. Another factor to consider is the strength of the component. A through hold component can provide a lot more strength required in the application. The SMT components are usually preferred for smaller applications, as both the construction of the component and the mounting make it the superior choice for circuit board assembly applications that need to be as small as possible.

After the circuit board assembly is completed, it is time to test the board. That is the best stage to do the testing, because if there is a problem in the SMT programming or component value it should be caught right away so that subsequent boards can be fixed so that the error doesn’t propagate through all of the circuit board assemblies. The testing usually takes place in the following order: Visual Inspection, Power-Off Testing, and finally Power-On Testing.

The circuit board assembly is visually inspected for any obvious issues. The most common issues are a SMT component being in the tube backwards so that it is installed backwards on the board. This can be easily discovered as the printing on the component will be reversed from all of the other components. Another more sophisticated means of testing is by an automated optical inspection machine. This machine takes a snapshot of a known good circuit board assembly, and then uses this snapshot to compare it will all future builds. If the new circuit board assembly is different from the original in any way, it will catch the potential error and alert the operator of its findings. Other things that are checked with visual inspection of the circuit board assembly are soldering and general workmanship. If the solder joints do not look fluid, then there could be potential for cold solder joints where the lead is not actually connected to the pad correctly. This is one of the worst problems a circuit board assembly can have because it might pass all of the electrical tests fine, but after shipment or change in environmental conditions this ‘connection’ becomes disconnected, or simply build a large contact resistance between the two points so that the circuit no longer performs as it was designed to.

One of the tests commonly made is to test the board with a multimeter to measure certain values while the power is off. For instance, transistors have very unique characteristics that can be measured with a multimeter. There is a diode drop between two of the pins, an equivalent resistance between two of the pins, and a very high resistance between two of the other pin combinations. By doing a simple test between strategic points, the circuit board assembly can be validated that the components were placed correctly with all of the right values. One of the most important items to check prior to power on is to make sure that the power traces are not connected to the ground traces. A board with many integrated circuits, semiconductors, resistors, and capacitors will have a very similar resistance from +5vdc to 0vdc or ground for a given design. This equivalent resistance should be the same for identical circuit board assembly builds. So if the design comes out to 240 kohm on the design, and a technician measures 2k between these two points on a new board run, then there is likely an issue that should be investigated before powering up the board.

This type of testing on the circuit board assembly is done following the visual inspection and any power-off testing measurements are made. It is important to make these tests after this because when power is applied it is possible to completely destroy a board. For instance if there was a solder short between power and ground on the circuit board assembly, when power is applied all of the ICs could be backwards biased which could wipe out the entire board. At that point it would not be safe to replace the suspect components and the entire board would have to be scrapped.