- details of the MELF resistor a surface mount device, SMD used to provide superior performance over SMT resistors in certain applications.
Surface mount technology, SMT includes:• SMT overview • SMD component packages • SMD resistor • SMD resistor markings • MELF SMD resistor • SMD capacitor • Quad Flat Package, QFP • BGA, Ball Grid Array • SMD PLCC
Another form of SMD resistor that can be used is known as the MELF resistor - Metal Electrode Leadless Face. These resistors are not nearly as widely used as the standard SMD resistors, but in some instances they provide advantages and can be used.
MELF resistor basics & construction
The MELF resistor is cylindrical in shape and have metallisation on both ends. Land pattern sizes for MELF resistors are the same as SMD chip resistors.
The manufacture of MELF resistors is more complicated than the more standard thick film SMD resistors. A metal film is deposited onto a high dissipation ceramic former. To make the terminations tin plated terminating caps are fitted. The resistor is then adjusted to the correct value by producing a helical cut in the film. The body of the MELF resistor is finally protected by a lacquer coating.
MELF Resistor Outline
The MELF SMD resistors are used for a number of reasons:
- MELF resistors provide a high level of reliability.
- A MELF resistor has a more predictable pulse handling capacity than other SMD resistors
- MELF resistors can be manufactured with tolerances as tight as 0.1%
- They can be manufactured with very low levels of temperature coefficient, sometimes as low as 5 ppm/°C
Although the standard flat chip resistors are cheaper and much easier to handle during manufacture, the performance of MELF resistor can be an overriding factor making them a cost effective solution
MELF resistors in electronics manufacture
While MELF resistors provide some significant and compelling technical advantages for use in certain applications, they are not always the easiest to handle in manufacture.
The most common form of SMD resistor by far is the flat or cuboid format. These require one form of nozzle on a pick and place machine, however MELF resistors require a different one that allows the cylindrical shape of the MELF resistor to be accommodated. They also require a higher level of vacuum on the pick and place machine.
MELF SMD resistor markings
MELF SMD resistors are used on occasions in some designs. These resistors are cylindrical and do not lend themselves to characters being printed on the surface, although coloured bands are easy to use. As such the MELF SMD resistor marking code is effectively the same as that used for leaded resistors.
There are three variations used:
- Four band code: This system is used for resistors with tolerances up to 5% using the E24 resistor series. The first two bands provide the significant digits. The third band provides the multiplier and the fourth, normally wider, provides the tolerance.
MELF Resistor 4 band code
Sometimes an alternative colour banding system may be used where the bands are all grouped towards one end of the MELF resistor rather than having a wider band at the far end.
Alternative MELF Resistor 4 band code
- Five band code: This system is used for higher tolerance resistors typically better than 1% that use the E48, E96 or E192 series values. The first three bands provide the significant figures. The fourth band gives the multiplier and the fifth band gives the tolerance.
MELF Resistor 5 band code
- Six band code: This code provides space for a temperature coefficient marking. As with the four band code the first three bands give the significant figures. Next is the tolerance band, and finally the fifth band provides the tolerance. .
MELF Resistor 6 band code
Tables showing the various colours and figures are given below:
By Ian Poole
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