Soldering

A soldering kit does not have to be very expensive. The tools shown in the above photo are what I could to any soldering job with. You do not need them all and we will discuss the important ones below.

Soldering Iron

For capacitor removal on motherboards it is important to have a reasonably powerful soldering iron. A 40w is the minimum that you would require. Actually a 60w is preferred and an 80w is what many of the professionals use. It must be a grounded soldering iron.


Soldering Stand with Sponge


Soldering Irons are available in corded models or as Soldering Stations which are Analog or Digital. For desoldering capacitors from motherboards it is not absolutely necessary to get a soldering station and definitely not necessary to get a digital one because you will be desoldering capacitors at the maximum temperature (450oC or more).

It is important to get a stand for safety reasons though and most important to have a wet sponge on the stand so that you can clean the iron of solder periodically in order to do the best soldering job.

Soldering Iron Tips

You might think that a very thin tip is required for removing capacitors from motherboards. This is incorrect. A thin tip will not get hot enough and transfer enough heat to the board. The best tip to get is a chisel tip. Around 2mm is the best size.

Solder

Solder is a matter of personal preference. Standard 60/40 solder is fine but preferably around 0.8mm diameter.

Clippers

Large clippers are no use for cutting the capacitor leads. You need small lead clippers.

Solder Sucker

A pneumatic solder sucker is a type of desoldering tool. It is useful for removing excess solder or desoldering damaged connectors. It is not recommended to use it to clear the thru-holes in the board because the pressure is excessive and there is a chance that when it recoils it may hit the board and damage a trace. If you insist on using it to clear thru-holes of solder then use it half-cocked.

Antistatic Wrist Strap

It is not necessary to buy an antistatic soldering station as you can just use an antistatic wrist strap. It is important to have a grounded soldering iron though.

Flux Remover Spray

If you are not using no-clean flux solder then you must clean the flux from the board. This is necessary because some kind of fluxes are slightly corrosive. It also is important for cosmetic reasons if you are fixing someone elses board, when the flux is cleaned the job looks more professional. You can use a flux remover spray for this. I use Flux Off from Cramolin. Alternatively you can use isoprpyl alcohol and a toothbrush.

Canned Air

When you have finished the soldering job there may be some debris on the board like bits of clipped leads or solder flakes. It is a good idea to use some canned air to make sure they dont remain and cause a short.

Bipolar Junction Transistors

The bipolar junction transistor (BJT) was the first solid-state amplifier element and started the solid-state electronics revolution. Bardeen, Brattain and Shockley, while at Bell Laboratories, invented it in 1948 as part of a post-war effort to replace vacuum tubes with solid-state devices. Solid-state rectifiers were already in use at the time and were preferred over vacuum diodes because of their smaller size, lower weight and higher reliability. A solid-state replacement for a vacuum triode was expected to yield similar advantages. The work at Bell Laboratories was highly successful and culminated in Bardeen, Brattain and Shockley receiving the Nobel Prize in 1956.

Their work led them first to the point-contact transistor and then to the bipolar junction transistor. They used germanium as the semiconductor of choice because it was possible to obtain high purity material. The extraordinarily large diffusion length of minority carriers in germanium provided functional structures despite the large dimensions of the early devices.

Since then, the technology has progressed rapidly. The development of a planar process yielded the first circuits on a chip and for a decade, bipolar transistor operational amplifiers, like the 741, and digital TTL circuits were for a long time the workhorses of any circuit designer.

The spectacular rise of the MOSFET market share during the last decade has completely removed the bipolar transistor from center stage. Almost all logic circuits, microprocessor and memory chips contain exclusively MOSFETs.

Nevertheless, bipolar transistors remain important devices for ultra-high-speed discrete logic circuits such as emitter coupled logic (ECL), power-switching applications and in microwave power amplifiers. Heterojunction bipolar transistors (HBTs) have emerged as the device of choice for cell phone amplifiers and other demanding applications.

In this chapter we first present the structure of the bipolar transistor and show how a three-layer structure with alternating n-type and p-type regions can provide current and voltage amplification. We then present the ideal transistor model and derive an expression for the current gain in the forward active mode of operation. Next, we discuss the non-ideal effects, the modulation of the base width and recombination in the depletion region of the base-emitter junction. A discussion of transit time effects, BJT circuit models, HBTs, BJT technology and bipolar power devices completes this chapter.

MOS Field-Effect-Transistors

The n-type Metal-Oxide-Semiconductor Field-Effect-Transistor (nMOSFET) consists of a source and a drain, two highly conducting n-type semiconductor regions, which are isolated from the p-type substrate by reversed-biased p-n diodes. A metal or poly-crystalline gate covers the region between source and drain. The gate is separated from the semiconductor by the gate oxide. The basic structure of an n-type MOSFET and the corresponding circuit symbol are shown in FigureFigure . 1.0.0.1 Cross-section and circuit symbol of an n-type Metal-Oxide-Semiconductor-Field-Effect-Transistor (MOSFET).

As can be seen on the figure the source and drain regions are identical. It is the applied voltages, which determine which n-type region provides the electrons and becomes the source, while the other n-type region receives the electrons and becomes the drain. The voltages applied to the drain and gate electrode as well as to the substrate, by means of a back contact, are referred to the source potential, as also indicated Figure 1.0.0.1


A conceptually similar structure was proposed and patented independently by Lilienfeld and Heil in 1930, but the MOSFET was not successfully demonstrated until 1960. The main technological problem was the control and reduction of the surface states at the oxide-semiconductor interface.

Initially, it was only possible to deplete an existing n-type channel by applying a negative voltage to the gate. Such devices have a conducting channel between source and drain even when no gate voltage is applied. They are called "depletion-mode" devices.

A reduction of the surface states enabled the fabrication of devices, which do not have a conducting channel unless a positive voltage is applied. Such devices are referred to as "enhancement-mode" devices. The electrons at the oxide-semiconductor interface are concentrated in a thin (~10 nm thick) "inversion" layer. By now, most MOSFETs are "enhancement-mode" devices.

While a minimum requirement for amplification of electrical signals is power gain, one finds that a device with both voltage and current gain is a highly desirable circuit element. The MOSFET provides current and voltage gain yielding an output current into an external load, which exceeds the input current, and an output voltage across that external load which exceeds the input voltage.

The current gain capability of a Field-Effect-Transistor (FET) is easily explained by the fact that no gate current is required to maintain the inversion layer and the resulting current between drain and source. The device has therefore an infinite current gain in dc. The current gain is inversely proportional to the signal frequency, reaching unity current gain at the transit frequency.

The voltage gain of the MOSFET is caused by the current saturation at higher drain-source voltages, so that a small drain-current variation can cause a large drain voltage variation.