Where precision resistors are concerned, a small size may not always be the perfect answer and SMT has its trade-offs. Power density in surface-mount chip resistors is higher than in through-hole parts, which results in chip resistors running hotter. Surface area in the case of through-hole resistors is higher, allowing them to dissipate internal heat to the surrounding air, while SMT devices mostly dissipate through the PCB. That leads to heat build-up in the system, affecting all the components. Due to this excess heat, long-term stability of resistors is degraded, especially when operating at higher temperatures.
Configuration considerations also affect SMT components. The length vs. width of the chip is an important parameter. If this aspect ratio is beyond a limit prescribed by reliability studies, generally ~2:1, board flex stresses may cause the chip to de-laminate from the board or to crack. Widening the chip does not help to eliminate the stress – rather that makes it harder to remove solvents and resins from under the chip.
Therefore, the best choice of a resistor for high-precision application would be one specifically designed to provide higher resistance value, dissipate higher power, be available in tighter tolerances and provide better long-term stability. It must also use less board space and allow easier cleaning of resins and solvents. All this is possible when precision resistors are configured in metal hermetic cans or in molded rectangular blocks, with through-hole leads extending from the bottom surface. The construction also prevents the resistor from being subject to thermo-mechanic stresses from the PCB.
Such constructions, which include stand-offs, allow reliable cleaning from under the component and the approach also minimizes the required board space. However, on some occasions, the only option available for design is to use an SMT. In such cases, using a surface-mount device with flexible terminations will be most useful.
The main idea behind use of SMTs is miniaturization. However, a tightly packed board may not always be a good idea, specifically for precision applications. For example, a mounted resistive element placed parallel to the PCB is susceptible to vibrational movement, resulting in parasitic microphonic noise. This is one reason designers do not prefer using SMT components in the feedback path of circuits. A better choice here would be to use vertically oriented through-hole parts. The legs help to absorb the deflections from the PCB.
For a precision resistor to remain stable over long periods, its temperature must remain within limits specified by the manufacturer. Heat from adjacent components and changes in the ambient temperatures affect the stability. This is defined by the TCR or temperature coefficient of resistance. Self-heating (because of load), is another factor and this is defined by the Power TCR or power coefficient of resistance.
Specific equipment such as medical instrumentation using precision resistors is highly dependent on these performance characteristics. Designers prefer to use Bulk Metal Foil resistors to deliver proven stability and reliability performance. Bulk Metal Foil resistors perform superbly even when exposed to unstable levels of humidity and temperature including other harsh environmental conditions. Foil resistors also feature non-capacitive and non-inductive designs.
