Dr. Mike Randall, Venkel LTd. has released whitepaper “The Definitive Guide to SMD Surface Mount Chip Resistor Selection” that assist with selection guide of SMD resistors.
Surface mount chip resistors are ubiquitous. Hundreds of billions of these devices are sold every year into myriad applications from handheld devices to precision lab test equipment to aerospace electronics and others.
Resistors impede current flow, causing a voltage drop when placed in an electrical circuit. Both alternating and direct currents are impeded by perfect resistors. The unit for resistance is Ohms (Ω), named after German physicist Georg Ohm. An Ohm is defined as the amount of resistance required to create a voltage drop of 1 volt (V), when the current flow is 1 Ampere (A). From a dimensional standpoint, an Ohm is defined as:
m is meter
Kg is Kilograms is second
C is Coulomb
J is Joule
S is Siemens
F is Farad
W is Watt
It is evident from the above that the Ohm may be described in many different terms including time, distance, mass, charge, energy, capacitance and power and conductance. As illustrated in Figure 1, the resistance to current flow between two planes (i.e., plane 1 and plane 2 in Figure 1) of cross sectional area within a conductor is found by the relation:
ρ is the resistivity of the material through which the current traverses (units, Ω-m)
L is the length that the current traverses between planes 1 and 2 (units, m)
A is the cross-sectional area of the conductor through which the current traverses (the area of either plane 1 or plane 2 (units, m2 )
This is bulk resistance, and the above relation can be further simplified if the conductor is broken into square segments (i.e., if W = L) as shown below. In that case, resistance simplifies to:
T is the thickness of the conductor through which the current traverses (units, m)
In the above case, resistance simplifies to a value having units of Ohms per square (Ω/h), which is typically called “sheet resistance.” Sheet resistance is a simplification of resistance that is useful to chip designers as it greatly simplifies the process of resistor design.
The chip resistor device designed will typically have at least one resistor element. The element is usually constant in thickness (T) with a geometry comprised of squares. The width and thickness of the trace helps establish power rating and the number of squares is utilized to determine the resistance of the device. Thus, it is important to maximize the number of squares in the design when it is desirable to maximize resistance within a small case size device. Thicker and wider squares typically result in the ability to carry more current and to handle more power, but the number of squares (and the resulting resistance per unit length) is reduced, limiting the maximum resistance possible within a given case size device.
During the chip resistor design process, the designer picks a material having a specific Ω/square value in order to enable the intended nominal resistance within the given package size. The designer will also utilize a serpentine pattern of interconnected squares in order to maximize resistance within the case size if needed, as a serpentine pattern of squares enables more resistance (i.e., squares) to be packed into a smaller area, thereby making the best of circuit board “real estate.” An example of this is illustrated in Figure 2. Use of a serpentine pattern of squares, in this case, enables almost 2X the resistance in the same lineal distance.
The resistor pattern is deposited onto a substrate, that is typically comprised of an alumina-based ceramic (typically Al2 O3 with from 1 w% to 10 w% glass as a sintering aid). However, other materials, such as silicon carbide (SiC), etc., may be used for high power applications or other application needs. The resistor patterns are typically deposited, many at a time onto a large substrate that is singulated into individual devices, later in the manufacturing process, in order to enable cost effective mass manufacture.
The resistor pattern is connected to two terminals that are also deposited on the substrate as well as around the edges of the substrate in order to form surface mount terminals, typically one on each end of the device, or in multiple stripes along the long sides of the device in the case of a resistor network. These exterior terminals or terminations enable connection of the chip resistor device with the circuit board. The resistor trace is trimmed to meet nominal resistance within the specification range for the device as necessary, and the resistor trace is over-coated with an electrically insulating material. After curing, the over-coat material is marked and each device is tested in order to create the finished chip resistor product which is then packaged (typically in tape and reel form) for storage, shipping, delivery, and placing or mounting with proper orientation.
During the circuit assembly process, the resistor device is then removed from the tape and is deposited on the circuit board (PCB) using a pick and place machine. Each chip resistor is then physically connected to the circuit within the PCB at the assembly facility using a thermal heat treatment that reflows solder in order to physically, thermally and electrically interconnect the resistor chip and the PCB. The solder is typically applied to the PCB prior to the chip placement operation by stencil printer deposition of specialized solder paste and the solder reflow process is typically performed in a carefully controlled reflow oven.
The resistor pattern is typically established via one of two methods, either thick film deposition or thin film deposition. Other, much less prominent methods of manufacture are used as well for certain application specific devices. As a result, chip resistors are typically categorized as either thick film chip resistors or thin film chip resistors based upon the deposition method used in their associated manufacture.
Thick film manufacturing processes usually involve the precision deposition of particle loaded liquids (e.g. inks or pastes) onto a substrate using some type of printing process (e.g., screen printing, stencil printing, pad printing or the like). The printed inks or pastes are then dried and fired to a dense, conductive, patterned resistor trace. Because patterning of the resistor is done during the application of the thick film ink or paste, this is called an additive process. Thick film resistor technology benefits from relatively easy composition modification as modification of the resistor thick film “ink” (e.g., chemistry, glass content, dopants for TCR, etc. for the resistor trace) is relatively easy to accomplish. Thick film resistor materials are generally based upon ruthenium oxide (RuO2 ) or platinum (Pt) mixed with specialized glass formulations and other dopants in order to achieved desired properties during firing.
The thin film chip resistor manufacturing processes typically involves the precision deposition of an un-patterned film or material onto a substrate. The deposited material is usually applied utilizing either thermal deposition in a relatively “hard” vacuum, or by physical vapor deposition using a sputtering process in a “softer” vacuum (e.g., a vacuum backfilled with Argon or other gas to increase the pressure) in order to create a plasma. Thin film deposition techniques usually result in very thin, uniform films. While thin films may be patterned during the deposition process, they usually are not when manufacturing chip resistors. After the precision deposition of the film, the film is typically patterned, post deposition, using photolithography. Because of this, the patterns are formed by removing material and the process is called a subtractive process.
Thin film resistor compositions are generally based upon vapor deposited nickel-chromium metals, called “nichrome.” This is generally done using physical vapor deposition via a sputtering technique. The resulting resistor elements generally need not be fired to achieve desired properties using this technique. It is relatively difficult to change the composition of the resistor element using thin film technology. However, thin film technology typically benefits from better deposit uniformity and more accurate patterning than thick film technology, so both manufacturing methods for chip resistors have their associated advantages and disadvantages.
The general resistor manufacturing process involves designing the device to achieve a specified range around the resistance nominal while maintaining the power rating in the package size of interest. Next, the resistor material is deposited onto the substrate, which is selected for mechanical strength as well as for electrical and thermal properties. The resistor element is patterned either during deposition (additive, thick film) or after deposition (subtractive, thin film), then adjusted to nominal resistance as needed, then over-coated and the individual resistor chips are singulated, then terminated, tested and packaged. In the case of thick film resistors, the resistor trace chemistry is carefully selected to set Ω/square as well as to adjust temperature coefficient of resistance (TCR) and other key properties, and the material is deposited and patterned in one step using screen or stencil printing (additive). The thick film resistor deposit is then thermal treated to achieve the electrical properties desired. In the case of thin film resistors, the resistor material is first deposited to achieve a highly uniform thin film, and is then patterned using photolithographic technics.
In the case of both technologies, the deposit thickness is carefully controlled to achieve the desired Ω/square, and the pattern is further adjusted, typically via LASER ablation, to achieve the desired resistance (nominal). The resistor pattern may also be adjusted for high voltage applications, or other specialized applications. The thickness and the pattern uniformity of thick film resistor elements is typically much thicker and less uniform for thick film resistors in comparison to thin film resistors, making thin film resistors more desirable for certain applications (e.g., those involving, precision tolerances, high frequencies, etc.).