Is my voltage reference design sensitive to humidity?Methods of Controlling Humidity and Performance of Precision Analog Systems

“From the perspective of the entire PCB manufacturing process, this article explores how a design engineer or PCB assembly engineer can keep the system from being affected by the external environment while ensuring the simulation performance of the system.

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By Paul Perrault and Robert Kiely

Introduction

Voltage references play a critical role in precision analog systems and are often used to set lower noise/resolution limits in analog-to-digital converters (ADCs) for applications such as instrumentation, test and measurement, and energy metering Precision measurement system in . For design engineers, the product portfolio offered by a vendor can be dizzying with a dizzying array of chips to choose from. However, using a variety of voltage reference specifications (voltage noise, accuracy, drift, quiescent current, series and shunt, etc.) and their packaging options (hermetic ceramic, plastic, bare die), it is possible to evaluate whether the final Electronic product will perform as expected Great performance, well worth it. There are many pitfalls in your design that can silently keep you from reaching your desired μV or nV noise accuracy goals. From the perspective of the entire PCB manufacturing process, this article explores how a design engineer or PCB assembly engineer can keep the system from being affected by the external environment while ensuring the simulation performance of the system.

background knowledge

While every electronic design will compromise to varying degrees in performance, a typical analog signal chain will have some form of analog input signal conditioning, such as ADCs and voltage references. To help illustrate the main idea of ​​this article, we will take a mid-speed 100 kSPS, 16-bit analog sensor signal input design as an example, as shown in Figure 1. For more information on some design tradeoffs and design choices for this signal chain, see the CN-0255 circuit note.

Figure 1. 16-bit signal chain functional block diagram.

The 2.5 V voltage reference used in this application is the ADR4525 in the ADR45xx plastic packaged voltage reference family, which provides high accuracy, low power, low noise with ±0.01% (±100ppm) initial accuracy, excellent temperature stability and Low output noise. The low thermally induced output voltage hysteresis and low long-term output voltage drift of the ADR4525 improve system performance. A maximum operating current of 950 μA and a low dropout voltage (max) of 500 mV make this device ideal for portable equipment.

After you’ve selected the components to use in the precision analog signal chain, it’s time for the PCB assembly team to produce a reproducible system that uses the printed circuit board as the substrate for the electronic design. Anyone who has worked with precision electronics knows that board-level mechanical stress manifests itself in the form of DC bias in precision circuit designs or MEMS-based sensor designs. The verification method is simple, you just need to press the plastic package of the voltage reference and you can see the change in the output voltage or sensor output. Environmental factors such as moisture and temperature can affect electronic device performance due to differential stress caused by moisture/humidity/temperature. Due to the different coefficients of thermal expansion of the materials from which the package and board are made, temperature can cause mechanical stress to the package and board. Since both plastic and circuit boards absorb moisture and expand, moisture can cause mechanical stress to the package and circuit board. Mechanical stress due to environmental factors tends to manifest itself as a drift over temperature/time in a plastic packaged voltage reference, and as an increased offset in a plastic packaged MEMS accelerometer. For plastic packages, the mechanical stress caused by humidity is quite significant, and one of the ways to control this humidity effect is to encapsulate the integrated circuit in a ceramic or hermetic package. While this approach addresses a number of humidity-related challenges, this solution adds additional packaging costs and typically results in larger component sizes.

Conformal coating options

Another way to isolate these stresses from the reference voltage is to use a conformal coating during PCB manufacturing so that any mechanical stress on the board will have less of an effect on the reference voltage. In this case, applying a thin composite coating on the voltage reference and the corresponding PCB can ensure that the stress on the PCB due to moisture or temperature does not fully translate into differential stress on the reference voltage chip package, and drift. This also ensures that the effects of low temperature condensation moisture on the package are reduced.

HumiSeal^®is a specialty coatings manufacturer offering a variety of conformal coatings including acrylics, polyurethanes, silicones, epoxies, and waterborne coatings for protecting sensitive components in PCB production. The water vapour permeability (MVP) parameter, which is the rate at which water vapour passes through the coating, can determine whether a coating is suitable for use. This is quite important as we are trying to keep the PCB from being affected by humidity.

How to test MVP: Take dry cups and coat them accordingly, place them in a constant temperature chamber with different humidity levels, and then weigh the cups periodically to assess how much moisture is getting through the coating into the dry cups. A week-long test showed that the coatings were effective at slowing the passage of moisture.

Table 1 shows the respective MVP nominal values ​​and material thicknesses for the various conformal coatings selected.

Looking at the data in the table shows that in all cases (except for very thick UV-cured coating materials such as UV40) these coatings show some degree of moisture penetration over time. This is based on measurements of the water penetration weight of coatings over a given surface area over a given time period; in these measurements, the time period is seven days. A selection of the commonly used 1A33 coating, an easy-to-use, cost-effective polyurethane coating, showed that the coating was more than 10 times more effective at slowing the rate of water vapour absorption than a rubber-based 1B51 coating of the same thickness. However, the important conclusion to draw from this table is that these coatings cannot completely isolate moisture penetration when left in a high humidity environment for a sufficient period of time.

This does not negate the use of conformal coatings. Instead, it can help understand the environment in which the electronic device is located. Do exposed electronics only experience high water vapor penetration for short periods of time? Will the packaging/container of the electronic device block the penetration of water vapour? Is it as useful to have a conformal coating as wearing both a belt and a harness? The environment in which electronic devices are exposed to changes so frequently, is conformal coating used just to make electronic devices perform better when the environment changes too quickly? It is important for product owners to understand all of these issues before starting to adopt conformal coatings.

One point to consider before discussing actual data is that the use of conformal coatings can increase mechanical stress in some cases. This is because the coating can increase package stress if not applied properly. For example, during the PCB manufacturing stage, if the surface of the voltage reference package contains moisture before coating, it is almost certain that this moisture will penetrate into the hydrophilic plastic package. From the 1A33 product data sheet: “The cleanliness of the substrate itself is critical to the successful application of a conformal coating. The substrate surface must be free of moisture, dirt, wax, grease, flux residues and all other Contaminants. Contaminants under the coating can cause problems that can lead to assembly failure.” This is a must-see for anyone thinking of conformal coatings.

Data and discussion: Is it humidity sensitive?

To evaluate the effect of the conformal coating, Analog Devices built a set of test panels. Each test board features 27 identical high-performance voltage references soldered to the PCB using the recommended J-STD-020 reflow method. The boards were placed in a humidity chamber and measured using a Keysight 3458A 8.5-digit digital multimeter (Model 002), verified to achieve 4 ppm/year drift using an LTZ1000. The humidity chamber maintains a constant temperature and humidity so that the board remains stable. The boards are placed in a humidity cabinet for a week, after which the temperature is kept constant and the humidity is increased. We employed two different conformal coating processes on a plastic package voltage reference to evaluate the effect of humidity on the coating.

Figure 2. Voltage reference in the ADR4525 ceramic package.

Taking the ADR4525 in a ceramic package as a reference (Figure 2) and placing it in a 70% humidity environment for 100 hours, the results show that the change in output voltage is about 3 ppm, or 0.075 ppm/%RH, which indicates that the ceramic package has excellent performance. stability. The data first peaked when the temperature jumped due to a sudden change in humidity. As can be seen from the data, the temperature in the humidity chamber slowly returned to 25°C. In contrast, when a voltage reference chip in a plastic package is placed under the same environmental and test conditions, its voltage output varies by about 150 ppm, as shown in Figure 3. Normalizing the data in Figure 3 to a 60% RH drift shows that the output drifts around 2.5 ppm/% RH without the conformal coating. Furthermore, it is evident that the drift did not stop completely after placing the board in a high humidity environment for 168 hours.

Figure 3. The ADR4525 reference voltage in a plastic package is affected by 20% to 80% humidity.

The HumiSeal 1B73 acrylic coating was next tested and the data are shown in Figure 4. The application steps are as follows: first wash and dry the board (quickly dip the board in 75% isopropyl alcohol and 25% deionized water several times, brush lightly by hand, then bake at 150°F for 2 hours), then spray the specified thickness of 1B73 coating. Except for the edge connectors, the entire board is covered with a coating, and the board must be clean in order to measure the output voltage.

Figure 4. The HumiSeal 1B73 acrylic coating was applied to the ADR45xx voltage reference surface by spraying.

The humidity stress of the oven used in this experiment was limited to 70% RH, and the normalized drift was around 100 ppm/40% RH or 2.5 ppm/% RH, which was not much different from when no coating was used. After consulting with HumiSeal, it is possible that the coating has not fully fused with the underside of the voltage reference package and the edge of the device. It’s also important to note here that 168 hours of testing in a high humidity environment may not be long enough, as the voltage reference doesn’t appear to have fully stabilized, similar to uncoated devices. It is worth noting, however, that the rate of change in the effect of humidity appears to have slowed, at least initially, providing a basis for the concept of moisture permeability, that the coating does not block moisture, but slows the rate of moisture penetration. speed.

The next test attempted the same conformal coating (HumiSeal 1B73), but with a deep immersion three-step coating process to ensure complete coverage of the entire board. The data is shown in Figure 5.

Figure 5. HumiSeal 1B73 acrylic coating was applied to the ADR45xx reference voltage surface using a deep immersion three-step coating process.

Due to oven problems, this test could not exceed 96 hours. The step of normalizing the data for the 30% RH to 70% RH range shows a drift of around 90 ppm or 2.3 ppm/% RH, which is not as much improvement as this application is trying to achieve, but there is a slight difference in the spray coating Improvements, of course, can also be said that if the test time is longer, these micro-improvements will disappear. Table 2 summarizes the three tests.

Future tests may employ other types of conformal coatings (silicone, rubber, etc.), with many variations in the application process. In addition, cross-sectional analysis after coating can also confirm that the applied coating thickness is up to the manufacturer’s required standard and that the coating is adequate at certain edge locations. Taken together, these experimental data demonstrate that ceramic hermetic packaging is an ideal defense against moisture ingress.

in conclusion

In designs with only 10-bit target accuracy (1/1000th type accuracy, or ±5 mV in a 5 V reference), various sources of error can silently affect accuracy. However, if your precision instrumentation system is targeting 16-bit or even 24-bit accuracy, then you must consider the entire system design, including PCB manufacturing, to ensure that accuracy is guaranteed throughout the life of the design. This article shows that the ideal way to ensure humidity performance is to use a hermetically sealed package, such as ceramic, and that conformal coatings can help slow down the exposure of precision analog electronics to humidity. When a design engineer’s design enters production, skills outside of the electronics domain are required, and coating companies need to be consulted to ensure that the product will perform well in challenging environments. The phrase “This argument holds water” usually means that your argument has merit and is correct. In this case, following good practices can ensure that your voltage reference itself is not eroded by water vapor, but rather keeps the water out, ensuring that your precision design maintains the performance you require. This design approach can be flawed, but your voltage reference won’t!

Reference circuit

ASTM E398-03, Standard Test Method for Water Vapor Permeability of Sheet Materials by Dynamic Relative Humidity Measurement. American Society for Testing and Materials, 2003.
Bryant, James. “Q&A for Applications Engineers – 11: How Accurate Must a Voltage Reference Be?” Analog Dialogue, January 1992.
HumiSeal 1A33 Polyurethane Conformal Coating Technical Data Sheet. HumiSeal, 2019.
“IPC-HDBK-830: Conformal Coating Design, Selection, and Application Guide.” IPC, October 2002.
“MT-087 Demonstration Tutorial: Voltage Reference.” Analog Devices, Inc., 2009.

author

Paul Perrault

Paul Perrault is a Senior Field Applications Engineer based in Calgary, Canada. He has worked at Analog Devices for 17 years and has been responsible for more than 100 CPU amplifier power supply designs as well as all current level designs for nA class sensor nodes and between nodes. He holds a Bachelor of Science degree in Electrical Engineering from the University of Saskatchewan, Canada and a Master of Science degree in Electrical Engineering from Portland State University. In his spare time, he enjoys skiing in the countryside, rock climbing on Rocky Mountain limestone, hiking in the local hills, and spending quality time outdoors with his young family.

Robert Kiely

Robert Kiely is a Senior Applications Engineer in the Linear and Precision Technology Group in San Jose, CA, USA. He joined Analog Devices in 2010. The focus is on precision signal chains and products, including sigma-delta ADCs, precision amplifiers, and voltage references. Rob holds a BSc in Electrical Engineering and an MSc in VLSI Systems Engineering from the University of Limerick, Ireland.