Types of Film Resistor in Electronics

Thin and thick film resistors are the most common types in the market. Film resistor is an important current-limiting element in electronic circuits, which has the function of dividing voltage and shunting current. According to different use environments and circuit requirements, it can be divided into two types.

What is Film Resistor?

Generally speaking, the substrate material of film resistors is insulating materials such as ceramics, glass, graphite, etc., while the conductive materials are metals, alloys, carbon, etc., which are thin films formed on the substrate.

The formation of thin film is essentially a process of gas-solid transformation and crystal formation, which can be roughly divided into the following main steps:

1) Atoms or molecules hit the surface of the solid.

2) They are absorbed by the atoms on the solid surface or directly reflected back to the space.

3) The adsorbed particles migrate or diffuse on the solid surface to move to the appropriate lattice position on the surface and enter the lattice.

Resistive films require heat treatment. The purpose is to form a certain amount of insulating phase to improve the temperature performance of the film and the long-term stability of the film. The manufacture of resistive thin films is often a high-temperature short-term process, so a large number of non-equilibrium defects and some metastable structures are retained, resulting in the gradual disappearance of these defects and the gradual transformation of the metastable state during the long-term work of the film. Through heat treatment, some dislocations generated during the deposition process can be moved to the surface and disappear by themselves, and the number of grain boundary defects is reduced. In this way, the structure of the film is greatly improved, the film changes from a mass state to a stable state, and the film performance tends to be stable. In addition, since the film and the substrate are often different materials, interdiffusion and chemical reactions inevitably occur at their interface, which causes gradual changes in the properties of the resistive film. Similarly, at the surface of the film, the properties of the film change over time due to diffusion and reaction.

The performance of the resistance film without heat treatment is not stable enough, so it is necessary to choose the appropriate heat treatment conditions, such as temperature, time and atmosphere. Generally speaking, high temperature should be selected, because only at high temperature can multiple processes in the film be completed within a limited time. Except that, to form a dense protective layer on the surface of the film, the heat treatment atmosphere must also be selected. For example, use an oxidizing or nitriding atmosphere. The insulating phase formed in this way can achieve an extremely fine dispersion structure, such as the fine dispersion of the molecular line, so that the stability and temperature performance of the film can be optimized.

Marking Convention

The marking method of the resistance value and error of the film resistors:

(1) Direct labeling method - mark the main parameters and technical performance of the resistor directly on the resistor body with numbers or letters.

(2) Text symbol method - combine the text and numbers regularly to represent the main parameters of the resistor.

(3) Resistor color codes - use different color bands to indicate the resistance value and error level of the resistor. Ordinary resistors are generally represented by 4 bands, and precision resistors use 5 bands.

(4) Digital method: Use three digits to represent the nominal value of the component. From left to right, the first two digits represent significant digits, and the third digit represents 10^n (n=0～8). When n=9, it is a special case, which means 10^(-1), and the resistance value of 0-10 ohms with decimal point is expressed as XRX and RXX.

Thin Film Resistor

Thin film resistor is made by vacuum evaporation, magnetron sputtering and other processes, evaporating the resistivity material (chromium nickel) on the surface of the base insulating material (alumina ceramics, silicon or glass), with a thickness of about 0.1um, and then forms a pattern through etching process.

Features:

1) The material is relatively more uniform (to facilitate the design of the resistance value), and the manufacturing process is more controllable. It can also be modified by photolithography or laser to generate a pattern to increase and calibrate the resistance value, so the accuracy of the thin film resistance can be 0.1% or even 0.01%.

2) A conductive material with a very low temperature coefficient (TCR) can be selected, so that the resistance values change very little with temperatures.

3) Thin film resistors are generally used in precision applications: various instruments and meters, medical equipment, power supplies, electrical equipment, electronic digital products, etc.

Thick Film Resistor

Thick film resistor refers to resistors that are printed using a thick film process (a paste of ruthenium oxide or other alloys), generally using a screen printing or stencil printing process.

Features:

1) Thick-film resistors are printed on ceramic substrates and fired by mixing paste substances of alloy conductive materials and insulator materials; the conductive materials after firing are similar to glass, and it is difficult to cut and control resistance, so their accuracy is poor (10%, 5%, and 1% are common accuracy). In addition, print the resistive layer on the substrate at 850°C and burn the paste to become glassy, making it well moisture-proof.

2) The conductive material of thick film resistors is a cake-like substance, and its temperature coefficient (TCR) is difficult to control and is generally large. —TCR cannot be ignored, and the TCR of 1% of ordinary resistors is several thousand ppm /°C range. And the change in overall resistance is related to the material, actual power, and physical size of the resistor.

3) Thick film resistors are relatively low-cost and are by far the most used resistors in electrical and electronic equipment. Package for chip resistors such as 0402/0603/0805/1206 are commonly used on single boards are generally thick film resistors.

Thin Film Resistors vs Thick Film Resistors

The film thickness of thin film resistors is more accurate, and the coating is easy to cut to facilitate the manufacture of high-precision resistors; while the distribution of metal particles in the resistive material of thick film resistors is difficult to make uniformly, and the control of printing thickness is relatively more difficult, and finally It is difficult to make a glassy crystal during manufacturing processing. In short, these factors mentioned make the accuracy of the thick film resistor relatively low.

The specific analysis of thin film and thick film resistors is as follows:

1) The resistance layer of thin film resistors is more uniform than that of thick film resistors, and it is easy to make high-precision and stable resistors.

2) The resistance layer of a thin film resistor has its optimum thickness requirements. If the resistance layer of a thin film resistor is too thin, it will be more likely to be oxidized and seriously affect its temperature coefficient (TCR). Also the requirements of the resistance layer thickness of a thin film resistor have seriously affected the resistance range, and the degradation rate of large resistance thin film resistors is also very high.

3) Thick film resistors rely on the contact between metal particles in the glass matrix to form resistance, and these contacts form a complete resistance. In addition, the thermal strain during work will interrupt the contact, and the resistance value will continue to increase with time and temperature, and its stability is poor.

4) The series of charge movement in the thick film resistor structure, and the granular structure makes the thick film resistor generate higher current noise. Under the same size, the higher the resistance value (less metal composition), the higher the noise and the worse the stability.

Because the free electrons in the thick film resistor are under the action of the electric field force, the probability of collision during the movement (more impurity particles) is much higher than that of the thin film resistor, so the electron movement is more chaotic and the noise is louder.

5) The glass component in the thick film resistor structure forms a glass phase protective layer during the resistance process, so the moisture resistance of the thick film resistor is higher than that of the thin film resistor, and it is less likely to be oxidized.

Negative Etching

At present, the production of many thin film resistors will adopt this method. In the manufacturing process, photolithography and etching are first carried out to obtain the overall pattern of electrodes and resistors, and then the nested resistor mask is used as a selective mask during this process. The biggest problem with this approach is the narrow registration window. Since the "alignment mark" is limited to a specific structure, usually the edge portion of the ceramic sheet, it is not guaranteed to be registered anywhere. Because registration will be affected by lens abnormalities, abnormal clamping of ceramic sheets, misalignment of mask patterns, and deformation of ceramic sheets themselves.

Therefore, it is necessary to pay attention to aligning the "alignment mark" on the nested mask with the microstructure pattern on the upper layer, and then observe whether the patterns on the two are aligned. If there is a misalignment, fine-tune it, and perform follow-up work such as exposure after the graphics are completely aligned. The mask used in this method is a negative film, that is, most of the mask is a light-shielding area, and only a small part is transparent. The large shadow area on the mask makes the boundaries of the microstructure graphics not clear enough, the graphics registration operation is relatively difficult, and the misplacement of the graphics is prone to occur.

When ordinary contact or proximity lithography machines are used to socket resistors, once the patterns are misaligned, a chain reaction will be caused, resulting in corrosion and dislocation of the resistor patterns. If it is light, the deviation of the resistance value will be large, and if it is heavy, it will cause damage to the microwave circuit pattern, which will directly affect the yield of the product. Therefore, it is generally used to rework in time when the overlay is found to be misplaced during the microscope inspection, but it is laborious and wasteful, and the production efficiency is not high.

Thin film resistors are chromium nickel films formed by vacuum deposition on alumina ceramic substrates, usually only 0.1 μm thick, only one-thousandth of thick film resistors, and then etched into a certain shape. The advanced photolithography process is very precise and can form complex shapes, so the performance of thin film capacitors can be well controlled.

Thin film resistors have an optimal temperature-sensitive deposit thickness, but this also severely limits the range of possible resistance values. Therefore, the thickness of the deposited layer can represent different resistance value ranges. In addition, the aging process of resistors will also vary due to the thickness of the film. This chemical/mechanical aging also includes high-temperature oxidation of the resistance alloy. And thinner deposited layers are easier to oxidize, so the degradation rate of high-resistance thin-film resistors is very high. In addition, changing the optimal film thickness can seriously affect TCR.

The following figure shows that the TCR characteristics of thin film resistors are better than thick film resistors.

Thick film resistors rely on the contact between particles in a glass matrix to form a resistor. These contacts form a complete resistance, but thermal strain in operation interrupts the contacts. Due to parallel connection in most cases, thick film resistors will not open circuit, but the resistance value will continue to increase with time and temperature. Therefore, thick film resistors have poor stability (time, temperature and power) compared to other resistors. The physical structure of a thin film resistor determines its current characteristics.

Granular structures can also make thick film resistors very noisy due to the movement of charges in clusters within the structure. For a given size, the higher the resistance value, the less metal content, the higher the noise, and the worse the stability. The glass component in the thick film resistor structure forms a glass phase protective layer during the resistor processing, so the moisture resistance of the thick film resistor is higher than that of the thin film resistor.

Types of Thin Resistor

The following introduces several thin film resistors:

Metal film resistors are the most widely used thin film resistors so far, with high precision, stable performance, simple and light structure. It adopts high-temperature vacuum coating technology to tightly adhere nickel-chromium or similar alloys to the surface of the ceramic rod to form a film. After cutting and adjusting the resistance value to achieve the final required precision resistance value, it is cut with appropriate joints and coated on its surface, then it is sealed and protected by epoxy resin. The resistance value can be controlled by grooving and changing the thickness of the metal film, so its manufacturing process is more flexible. Not only can its material composition and film thickness be adjusted, but also the resistance value can be adjusted by grooving, so it can be made into a kind of resistor with good performance and a wide range of values.

Carbon film resistors are also a type of film resistors. It adopts high-temperature vacuum coating technology to attach carbon tightly to the surface of the ceramic rod to form a carbon film, then add appropriate joints to cut, and coat the surface with epoxy resin for sealing protection. Its surface is often coated with green protective paint. The thickness of the carbon film determines the size of the resistance value, so the thickness of the control film and the groove are usually used to control the resistor. This resistor is also known as "thermal decomposition carbon film resistor". So it is also known as the "thermal decomposition of carbon film resistor". And it has many advantages including low price, stable performance, wide resistance value and power range.

Metal oxide film resistor is a device that uses special metals or alloys as substrate materials, and uses vacuum evaporation or sputtering to form an oxidized film layer on the basis of ceramics or glass. During process, the alloy is heated in a vacuum, and evaporates, so that a layer of conductive metal film is formed on the surface of the ceramic rod. Grooving and changing the thickness of the metal film can control the resistance. Its heat resistance, noise potential, temperature coefficient, voltage coefficient and other electrical properties are better than carbon film resistors. And its manufacturing process is relatively flexible. Not only can its material composition and film thickness be adjusted, but also the resistance value can be adjusted by grooving. This is the commonality of thin film resistors mentioned before.

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