MELF Resistor

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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 are more expensive than the more widely used SMD resistors, but they are used in applications where the additional performance is needed.

MELF resistors are able to provide higher levels of performance in terms of long term stability, moisture resistance, reliability, and the resistance to the effects of temperature cycling experienced int he soldering processes used today.

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.

MELF resistors are manufactured by depositing a homogeneous film of metal alloy onto a high-grade ceramic body. The ceramic rod or body is typically 85% alumina.

The next stage in the process is to press nickel plated steel termination caps onto the metallised rods. Once this has been done a specialised laser cuts a helical grove in the resistive layer to bring the resistor to the required value of resistance. This does mean that the MELF resistor does have some inductance, but this is not an issue for many applications.

The next stage is for the resistor elements to be covered by a protective coating. This coating is designed to provide protection against a variety of electrical, mechanical, and environmental conditions.

Finally the terminations are given a pure tin plating to ensure optimum solderability with whatever solder process is used.

MELF resistor sizes and outlines

In view of get usage of MELF resistors in many high reliability applications, they have been included in various standards. EN 140401-803 and DO-213 describe multiple MELF resistor sizes and outlines.

• MELF (MMB) 0207   L: 5.8 mm, Ø: 2.2 mm 1.0 W, 500 V
• MiniMELF (MMA) 0204   L: 3.6 mm, Ø: 1.4 mm 0.25 W, 200 V
• MicroMELF (MMU) 0102   L: 2.2 mm, Ø: 1.1 mm 0.2 W, 100 V

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

Overall the cylindrical construction of MELF resistors provides an optimal power rating and pulse load capability related to the mounting space.

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:

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