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, they 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. Microhm HPMRY series are typical MELF resistor.
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.
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.
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
Microhm HPMRY series has the size of 0102, 0204, 0207, 0411 and 0617.
