This paper was presented by Stephen Oxley, TT Electronics at the 3rdPCNS 7-10th September 2021, Milano, Italy as paper No.3.1. and voted by attendees as:
OUTSTANDING PAPER AWARD
The most cost effective and simplest way of converting a measured current to a voltage signal is to use a low ohmic value current sense resistor. The increase in products containing batteries, motors or actuators which call for current monitoring or control has led to huge growth in the market for current sense chip resistors with values below one ohm over the last two decades. But more recently, driven by power efficiency demands and enabled by low noise sense voltage amplifiers, the value range has been extended downwards from milliohms to hundreds of micro-ohms.
Such low ohmic values present challenges to the user at many stages in their design and manufacturing processes. This paper considers the nature of these challenges and suggests strategies to overcome them. The stages considered are component selection, PCB layout design, verification of the ohmic value of unmounted components, critical assembly processes, and expected ohmic values during product life. At each stage there are potential pitfalls but also opportunities to quantify and minimise error and variation.
Although sub-milliohm chip resistors are still just chip resistors, it is advisable to treat them as being a separate class of component, and to discover the particular considerations and techniques that enable their successful use.
The final consideration for a design engineer relying on a sub-milliohm resistor for accurate current measurements is how the ohmic value may change during the life of the product. These changes fall into two categories: reversible and irreversible.
Firstly, reversible changes occur because of the finite TCR. This is a combination of the resistance element TCR and the termination TCR and it is important not to rely too much on the element TCR stated on some datasheets, by wrongly assuming it relates to the whole product. As TCR of these parts is normally tested by manufacturers in the mounted state, it also includes the TCR for the solder between mounting pad and termination. In applications with minimal self-heating, the TCR simply reflects worst case value changes across the local ambient temperature range of the resistor. Where self-heating is greater, it becomes an issue of non-linearity since ohmic value becomes a function of current. This is sometimes expressed in terms of Power Coefficient of Resistance (PCR) which combines the temperature rise per watt with the TCR to indicate the sensitivity of ohmic value to power, measured in ppm/W. This is clearly a function of the mounting conditions as the temperature rise depends on the wider thermal environment.
Secondly, irreversible changes occur due to changes in the element resistance. Corrosion and oxidation can consume some of the resistance alloy, leading to upward value drift, whilst grain boundary changes within the alloy can cause resistivity changes which are accelerated by elevated temperatures.
A number of figures are given in the performance data of product datasheets to enable the designer to assess the maximum lifetime change in resistance value. In general, only one of these figures should be used; the one that most closely reflects operating conditions. A Shelf Life figure applies where loading is negligible, and the environment is benign. The Load or Endurance figure applies where power dissipation is the main factor, the Long-Term Damp Heat figure where humid environments may be encountered. In all these tests the majority of the value change happens within the period of the test, as the value will tend to stabilise. For example, the 1000-hour Load figure is a good guide to the change predicted over a longer period of service.
The figure of most interest to designers is the maximum total error in resistance value at the end of product life, or before scheduled re-calibration, if applicable. This is termed the total excursion and is the root of the sum of the squares of applicable, statistically independent short-term and long-term factors. This is best illustrated by an example.
An LRMAP2512 resistor with 1% tolerance and TCR of 50ppm/°C is to be soldered to a PCB for use in a laboratory-based power supply. The mean current through the resistor during operation will dissipate over 50% of the rated power. The operating temperature range inside the equipment is 20 to 50°C.
Tolerance ±1% TCR ±0.15% (±50ppm/°C x 30°C) Soldering ±0.3% typical
Load at Rated Power ±0.3% typical
√ (12 + 0.152 + 0.32 + 0.32) = 1.1
Total Excursion Estimate: ±1.1% typical
Clearly if initial calibration can be used, this can eliminate tolerance and soldering process induced errors, and this gives:
√ (0.152 + 0.32) = 0.34
Calibrated Total Excursion: ±0.34% typical
SUMMARY AND CONCLUSIONS
The growing use of sub-milliohm chip resistors for current sensing brings challenges for the designer and the process engineer at multiple stages which need to be well understood. The component format should first of all be selected to support the chosen thermal management approach, with metal element flat chip resistors having two terminals being the most cost-effective solution. Next, it is essential to design the PCB tracks and pads to meet the needs of Kelvin connection, heat dissipation, and avoidance of induced noise. Thirdly, if it is necessary to make accurate measurement of unmounted components’ ohmic value, great care is needed to set up fixturing according to the manufacturer’s guidelines and to establish a capable test system. Next the assembly processes of solder paste application and pick and place require close control to ensure consistent solder thickness and component location. And finally, how the ohmic value may change should be evaluated, in order to establish the expected range of mounted values across the product life.
 Optimize High-Current Sensing Accuracy by Improving Pad Layout of Low-Value Shunt Resistors; Marcus O’Sullivan; Analog Dialogue 46-06 Back Burner, June (2012); www.analog.com/analogdialogue
 The Truth about Placement Accuracy; Patrick Folmar; SST Semiconductor Digest; https://sst.semiconductor-digest.com/2000/04/the-truth-about-placement-accuracy/