Base Electronics: Capacitors part III

By arag61 Mar 27th, 2009

In this article we will discuss about capacitors packaging and we will have a look to the SMD.

Packaging

With the package term, in electronics, we mean the container where some components typologies are included. The package shape and sizes can be very different each other, the material can be plastic, metal or ceramic. For capacitors, the main packaging features to take in account are the operating temperature, the lead types and the mounting options.

Lead Types
The lead types can be mainly divided in:

Axial leads: They prolong from the ends, along the axis or any other axial part of the capacitor. They do not prolong from the sides.

Axial leads Aluminium Electrolytic Capacitor - Courtesy of Tedss

Radial leads: They are the opposite of the axial leads. They prolong from the sides and not from the ends.

Radial leads capacitor - Courtesy of CKE

Flying leads: They project horizontally.

Flying leads capacitor - Courtesy of LCR

Tab leads: Tab lead terminals are designed to be directly mounted into a circuit board socket.

Tab lead configuration - Courtesy of WCI

Screw leads: In some capacitors the leads are designed to be fastened by screws.

Gull wing leads: They are widely used due to the form and mold low cost. They are flexible and can hold out the thermal expansion and contraction. Their integrity, to circuit board solder connections, is easy to be inspected.

Gull wing leads configuration _ Courtesy of WCI

J-leads : They are popular because they require less PCB material than gull wing leads, but their inspection and their forming is more difficult.

J- leads configuration - Courtesy of WCI

Surface Mount Technology (SMT): Capacitors, or devices generally, that use SMT do not have leads but are pad mounted.

Mounting Styles
The main mounting styles are:

Through Hole Technology (TH ): In the THT the components are mounted, on a printed circuit board (PCB), by the leads insertion through holes in the board itself and then welding the leads in the board opposite side. Even if the THT gives as result strong mechanical bonds compared to the SMT (Surface Mount Technology), the boards are more expensive due to required additional drillings. Moreover on multilayer boards, because of the holes must pass through all layers to opposite side, the routing area for signal traces is reduced in the layers below the top one. For these reasons, nowadays, the THT has been almost totally replaced from SMT, but it is still used for components like electrolytic capacitor or semiconductors that require a bigger mounting strength.

Surface Mount Technology: In the SMT components are mounted, on a PCB, by welding the component leads to the top board surface. SMT components have a flat surface that is welded to a flat pad on the PCB face. Generally the PCB pad is covered with a paste of solder and flux. SMT components are placed carefully on this paste and remain in position until very high temperatures melt the paste and weld the component leads to the PCB pads. In the lines production SMT components can be mounted, by pick and place equipments, quickly, with accuracy and cost effectively. For this reason the SMT has replaced the THT.

Bolt Mounted: The capacitor, in this case, is mounted by bolting in place. A nut is generally used to block the bolt.

Bracket Mount: Brackets, that are supports to hold something up, are necessary for capacitors mounting. The brackets can be added or the capacitors can be already equipped with them.

Pole Mount: Capacitors mount on poles and need special pole mounting brackets.

To conclude this article let's have a look to the SMD capacitors.

SMD Capacitors

After SMD resistors the SMD capacitors are the most used components. The main SMD capacitors are: the ceramic ones, the tantalum ones and the electrolytic ones.

Ceramic SMD capacitors

A ceramic SMD capacitor is a rectangular block of ceramic dielectric where several valuable electrodes are included. The inner electrodes are connected to the two terminations. The raw dielectric materials are mixed with very high accuracy, then heated to high temperatures, approximately 1100°C ÷ 1300°C, and finally other materials are added to get the wanted electric properties. The main ceramic SMD capacitors cases are: 0603, 0805,1206, 1210, 2010 and 2512. In the following table we summarize their features.

L = Length B = Width D = Thickness C = Width wrap around
N.B. L-B-D-C are in mm.

SMD multilayer ceramic capacitor - Courtesy of Trigon Components

Tantalum SMD capacitors

If higher levels of capacitance, compared to the ceramic SMD ones, are requested the tantalum SMD capacitors must be used. Due to the different construction process and and requested features they have different cases that are summarized in the following table:

L = Length B = Width D = Thickness

N.B. L-B-D-C are in mm.

SMD tantalum capacitor - Courtesy of Kexin Industrial

Electrolytic SMD capacitors

They are used in many areas because of their high capacitance level and low cost and are generally marked with value and working voltage. They are marked in different ways: a code system, for example, uses a letter followed by three digits. The letter indicates the voltage and the three digits indicate the capacitance (µFarad). The first two digits are the significant digits, the third one is the multiplier. In the below table there are the letters and their voltage value.