Talking about Non-Wirewound Resistors: Carbon, SMD and Network

23 Mar.,2023

 

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The metal foil pattern is etched chemically or patterned by means of an ion beam. The thin metal films are applied through a mask to the desired shape.

Linear PTC designs in metal glaze are marketed. Typical data are, for example, 10 kΩ and TCR +1500 ppm/°C. Metal film chips with resistance track in a serpentine pattern also exist. Typical EIA sizes are 0603, 0805 and 1206. Common TCRs range from +150 to +4500 ppm/°C while the corresponding maximum resistance values drop from 50 kΩ to 500 Ω. The higher TCR the lower the maximum resistance which depends on the film alloys that more and more are approaching the pure metal and its correspondingly lower resistivity.

Metal foil chips exist in two designs, partly as the wrap-around according to Figure 4, partly for use in integrated electronics (Figure 5). In the latter case, they are made with protective glass passivation and with bonding pads which implies separate encapsulation.

A variant of bulk metal chips is used as a current sensor. The resistance element consists of a metal plate that by laser cutting can be trimmed to tolerances of ±1% at resistance values in the range of 5 to 50 mΩ. The design and its tolerances require large chip dimensions (for example 0.4×0.2”).

Sometimes metal foil chips are delivered for special custom trimming by a scribed line.

Power chip resistors exist in thick film design. Common power ratings are 1 or 2 watts. The resistance range may cover 10 mΩ … 22 MΩ with tolerances between 1 and 10 %. The heat generation requires dimensions in the magnitude of 12 x 8 or 8 x 5 mm. The terminations usually consist of tinned compliant copper plates.

Sizes

With thick film chips, the main focus has been moved from the EIA size 0805 (2 x 1.25 mm) to sizes 0603 (1.6 x 0.8) and 0402 (0.8 x 0.5). Discrete thick film chips are marketed down to 0302 or 0201 (0.8 x 0.5 respectively 0.6 x 0.3 mm). The most common chip size in thin film technology is 20 x 20 mil (0.5 x 0.5 mm; EIA code 0202) but are also found conventional EIA sizes from 0201 to 2010.

Price comparison thick film – thin film

True price comparisons are difficult because of the difference between the characteristics of tolerances and TCRs that are situated in different ranges. In general, thick film is cheaper to make but consume more of the material.

MELF  

This component consists of a conventional resistor for hole mount, but in a form without leads. Instead one solder direct on the press-fitted naked end caps. At wave soldering the component body should be positioned with a glue lump (epoxy) to the substrate / printed circuit board.

Because the MELF design is a modified version of well-tried, well-established hole mount types, it in this SM component assimilates all the good characteristics that characterize the respective basic hole mount type. For metal film MELF, this means among other things a wide resistance range, low noise characteristics, fine tolerances and TCR values and good stability. With an extra thick metal film, one has in some cases improved heat dissipation and also pulse capability. A high-frequency capability is achieved by a so-called pulsed trim cut where the spiraled conductive pattern is broken in parts, thus reducing the inductive action from the helical trim cut and thus increasing the frequency range to several GHz.

MELF components are made in metal film, carbon film, and metal glaze types. Usually, they are supplied with end caps of conventional components but some manufacturers use metalized terminations.

Sizes

The MELF components were introduced as leadless metal film resistors in the DIN size 0207. Today 0402 (1.5 x 3.6 mm) is the most common, not least because of its smaller risk of open mode failure in the solder joints after frequent temperature changes. However, the size 0201 (1.1 x 2.2 mm) is already well established in the consumer market.

Price comparisons

Hole mount : MELF ≈ 2 : 3.

Cross over chips / “Jumpers”

Sometimes it’s necessary to complete existing application of printed circuits by separate short-circuiting between two points. Particular components are used, so-called cross overs, cross conductors or jumpers, with the lowest possible resistance. They exist both as rectangular thick film chips as well as metal film MELFs. Max resistance for the thick film chips usually is ≤50 mΩ at sizes between 0603 and 2010. Thin film MELFs in typical sizes of L x D ≈ 2 x 1.4 to 6 x 2.4 mm are specified for ≤10 mΩ.

As well as maximum resistance a maximum current is also specified for these components.

Mounting / Soldering / Gluing

It is important to remember that SM components, more than others, are influenced by temperature changes, especially for the larger component. Thus, dimensions and mounting directions are most important. See section M of ABC of CLR for SMD mounting guidelines: “Further, short and widebodies are preferable to long and thin ones, partly for strength reasons, partly because of the inductance and with that the frequency dependence will decrease”. Still, another reason is that differences in the coefficient of expansion between substrate and chip decreases in significance with shorter distances between terminations. One major manufacturer has taken this as a starting point when he has applied the terminations on the long sides of the chip. In addition, this approach results in a strongly improved thermal dissipation.

The thermal dissipation may push the temperature in the solder joints above the detrimental 110…115 °C temperature range before we attain the nominal power PR. Permissible maximum power of a MELF resistor can be calculated from the Equation below:

where:

  • Ta = ambient temperature in °C and
  • RthS = thermal resistance of the solder joint, i.e., its temperature rise in K/W
  • Pperm = permissible power.

Information about thermal resistance of the solder joint of standard components applies to solder conditions for normalized test board patterns.

Solderability is vital for a successful result. The user should use a wetting balance and international standard requirements at inspection. The end caps of MELFs sometimes have been difficult to solder. Metal glaze MELFs with metalized terminations generally have a better solderability.

At gluing or staking check that the termination metal really ”wets”.

Failure Modes

The weakest point of an SMD is the terminations and solder joints that break after sufficient temperature changes. Good components should stand at least 100 cycles between temperature limits. Applications like modern car electronics have forced the requirements up to some 1000…10 000 temperature cycles.

Except for failures resulting from the open mode in connections and solder joints there always are failure modes typical for corresponding hole mount types.

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