Electronic
Is my voltage reference design sensitive to humidity?Methods of controlling humidity and performance of precision simulation systems

Is my voltage reference design sensitive to humidity?Methods of controlling humidity and performance of precision simulation systems

Voltage reference plays a vital role in precision analog systems. It is usually used to set the lower limit of noise/resolution in the analog-to-digital converter (ADC). It is suitable for applications such as instrumentation, test and measurement, and energy metering. Precision measurement system in the. For the design engineer, the product portfolio provided by the supplier may include many chips to choose from, which is dizzying.However, using various voltage reference specifications (voltage noise, accuracy, drift, quiescent current, series and shunt, etc.) and its packaging options (hermetic ceramic, plastic, bare chip packaging), it is possible to evaluate whether the final Electronic product can achieve the expected Excellent performance, this

Author: Paul Perrault and Robert Kiely ADI

Introduction

Voltage reference plays a vital role in precision analog systems. It is usually used to set the lower limit of noise/resolution in the analog-to-digital converter (ADC). It is suitable for applications such as instrumentation, test and measurement, and energy metering. Precision measurement system in the. For the design engineer, the product portfolio provided by the supplier may include many chips to choose from, which is dizzying. However, using various voltage reference specifications (voltage noise, accuracy, drift, quiescent current, series and shunt, etc.) and its packaging options (hermetic ceramic, plastic, bare chip packaging), it is possible to evaluate whether the final electronic product can achieve the expected Excellent performance, this is very worthwhile. There are many misunderstandings in the design, and they may quietly prevent you from achieving the desired μV or nV noise accuracy target. From the perspective of the entire PCB manufacturing process, this article discusses how design engineers or PCB assembly engineers can ensure system simulation performance while protecting the system from the external environment.

background knowledge

Although each electronic design will make different levels of compromises in terms of performance, the general analog signal chain will perform analog input signal conditioning in some form, such as ADC and voltage reference. In order to help explain the main point of this article, we will take a medium-speed 100 kSPS, 16-bit analog sensor signal input design as an example, as shown in Figure 1. For more information about some of the design trade-offs and design choices of this signal chain, please refer to the CN-0255 Circuit Note.

The 2.5 V voltage reference used in this application is the ADR4525 in the ADR45xx plastic package voltage reference series, which can provide high accuracy, low power consumption, low noise, and has an initial accuracy of ±0.01% (±100ppm), excellent temperature stability and Low output noise. The ADR4525’s low thermally induced output voltage hysteresis and low long-term output voltage drift improve system performance. The maximum operating current of 950 μA and the low dropout voltage (maximum value) of 500 mV make this device ideal for portable equipment.

Is my voltage reference design sensitive to humidity?Methods of controlling humidity and performance of precision simulation systems

Figure 1. The functional block diagram of the 16-bit signal chain.

After you have selected the components to be used in the precision analog signal chain, it is up to the PCB assembly team to produce a reproducible system. They use the printed circuit board as the substrate for the electronic design. Anyone who has worked in precision electronics knows that board-level mechanical stress will manifest itself in the form of DC bias in precision circuit design or MEMS-based sensor design. The verification method is very simple. You only need to press the plastic package of the voltage reference to see the change in the output voltage or sensor output. Since moisture/humidity/temperature will cause differential stress, environmental factors such as moisture and temperature will affect the performance of electronic devices. Due to the different thermal expansion coefficients of the materials used to make the package and the circuit board, the temperature will cause mechanical stress on the package and the circuit board. Since both plastics and circuit boards absorb moisture and expand, moisture can cause mechanical stress on the package and circuit board. In the plastic package voltage reference, the mechanical stress due to environmental reasons is often expressed as drift with temperature/time changes, and in the plastic package MEMS accelerometer, it is expressed as an increase in offset. For plastic packaging, the mechanical stress caused by humidity is quite significant. To control this humidity effect, one of the methods is to package the integrated circuit in a ceramic or hermetic package. Although this method can solve a large number of humidity-related challenges, this solution will add additional packaging costs and often leads to larger component sizes.

Conformal coating options

Another way to separate these stresses from the reference voltage is to use a conformal coating during the PCB manufacturing process, so that any mechanical stress on the circuit board will have a smaller 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 will not be completely converted into the differential stress on the reference voltage chip package. And drift. This can also ensure that the impact of low-temperature condensed moisture on the package is reduced.

HumiSeal® is a professional coating manufacturer that provides a variety of conformal coatings, including acrylic, polyurethane, silicone, epoxy, and water-based coatings used to protect sensitive devices in PCB production. The water vapor permeability (MVP) parameter can determine whether the selected coating is suitable. This parameter is the rate at which water vapor passes through the coating. This is very important because we are working hard to keep the PCB from being affected by humidity.

The method of testing MVP: take the dry cups and apply the corresponding coatings, place them in a constant temperature room with different humidity, and then weigh the cups regularly to assess how much water enters the dry cups through the coating. A week-long test showed that these coatings can effectively slow down the passage of water.

Table 1 shows their respective nominal MVP values ​​and material thicknesses when selecting various conformal coatings.

Table 1. Various HumiSeal coatings and their MVP

Material

Vapor penetration

((G/m2)/sky)

Standard steam penetration

((G/m2)/Day/mil)

thickness

(mil)

HumiSeal 1A33

9.18

0.315

29.13

HumiSeal 2A64

13.54

0.249

54.33

HumiSeal 1A20

21.89

0.492

44.49

HumiSeal UV40

0.83

0.024

35

HumiSeal UV40

Due to imperviousness, no data after one week of testing

Due to imperviousness, no data after one week of testing

61.41

HumiSeal UV40-250

9.1

0.156

58.26

HumiSeal 1B73

25.1

1.2

20.86

HumiSeal 1C49LV

60.14

2.22

27

HumiSeal 1B51

0.78

0.026

35

Looking at the data in the table, you can find that in all cases (except UV40, which is a very thick UV-curable coating material), over time, these coatings will have a certain degree of moisture penetration. This is based on measurements of the water penetration weight of the coating on a given surface area in a given time period; in these measurements, the time period is seven days. The results of choosing the commonly used 1A33 coating (a convenient, cost-effective and efficient polyurethane coating) showed that compared with the rubber-based 1B51 coating of the same thickness, the coating slowed down the water vapor absorption rate by more than 10 times. However, the important conclusion drawn from this table is that these coatings cannot completely isolate moisture penetration after being placed in a high-humidity environment for a long enough time.

This does not negate the use of conformal coatings. On the contrary, this can help understand the environment in which the electronic device is located. Will exposed electronic devices only experience high water vapor penetration for a short period of time? Will the packaging/container of electronic equipment block the penetration of water vapor? Using a conformal coating is as useful as wearing a waist belt and a suspender? The environment where electronic devices are changed so frequently, is the use of conformal coatings just to make electronic devices perform better when the environment changes too quickly? It is important for product owners to understand all these issues before starting to adopt conformal coatings.

Before discussing actual data, one issue that needs to be considered is that the use of conformal coatings can increase mechanical stress in some cases. This is because if applied improperly, the coating will increase package stress. For example, in 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. It can be seen from the data sheet of the 1A33 product: “The cleanliness of the substrate itself is critical to the successful application of the conformal coating. The surface of the substrate must be free of moisture, dirt, wax, grease, flux residues and all other Contaminants. Contaminants under the coating can cause problems and may lead to assembly failures.” For anyone who wants to use conformal coatings, this must be noted.


Data and discussion: Is it sensitive to humidity?

In order to evaluate the effect of the conformal coating, ADI has produced a set of test boards. Each test board has 27 identical high-performance voltage references, which are soldered to the PCB using the recommended J-STD-020 reflow method. Place these circuit boards in a humidity chamber, and then use Keysight 3458A 8.5-digit digital multimeter (002 model) to measure, and use LTZ1000 to verify that it reaches 4 ppm/year drift. The humidity box maintains a constant temperature and humidity so that the circuit board remains stable. The circuit board will be placed in the humidity cabinet for a week, after which, keep the temperature constant and increase the humidity. We use two different conformal coating processes on the plastic package voltage reference to evaluate the impact of humidity on the coating.

Is my voltage reference design sensitive to humidity?Methods of controlling humidity and performance of precision simulation systems

Figure 2. Voltage reference in the ADR4525 ceramic package.

Taking the ceramic package ADR4525 as a reference (Figure 2), placed in a 70% humidity environment for 100 hours, the results show that the output voltage change is about 3 ppm, or 0.075 ppm/% RH, which means that the ceramic package has excellent stability. The data peaked for the first time because the temperature jumped due to a sudden change in humidity. It can be seen from the data that the temperature of the humidity chamber slowly rises to 25°C. On the contrary, when the voltage reference chip in the plastic package is placed under the same environment and test conditions, its voltage output changes to about 150 ppm, as shown in Figure 3. The data in Figure 3 is normalized according to 60% RH drift, and the results show that without conformal coating, the output drift is about 2.5 ppm/% RH. In addition, it is obvious that after placing the circuit board in a high-humidity environment for 168 hours, the drift has not completely stopped.

Is my voltage reference design sensitive to humidity?Methods of controlling humidity and performance of precision simulation systems

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

Next, the HumiSeal 1B73 acrylic coating was tested, and the data is shown in Figure 4. The application steps are as follows: first wash and dry the circuit board (quickly immerse the circuit board in 75% isopropanol and 25% deionized water several times, brush lightly by hand, and then bake at 150°F for 2 hours), and then spray the specified thickness 1B73 coating. Except for the edge connector, the entire circuit board is covered by the coating, and the circuit board must be clean before the output voltage can be measured.

Is my voltage reference design sensitive to humidity?Methods of controlling humidity and performance of precision simulation systems

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

The humidity stress of the oven used in this experiment is limited to 70% RH, and the normalized drift is about 100 ppm/40% RH or 2.5 ppm/% RH, which is not much different from when the coating is not used. After consulting with HumiSeal, it was learned that the coating may not be fully integrated with the bottom surface of the voltage reference package and the edge of the device. It should also be noted here that the test time of 168 hours in a high-humidity environment may not be long enough, because the voltage reference does not appear to be fully stabilized, similar to an uncoated device. However, it is worth noting that the rate of change in the influence of humidity seems to have slowed down, at least at the beginning. This provides a basis for the concept of moisture permeability, that is, the coating does not prevent moisture, but slows down the penetration of moisture. speed.

The next test tried using the same conformal coating (HumiSeal 1B73), but using a deep immersion three-step coating process to ensure that the coating completely covers the entire circuit board. The data is shown in Figure 5.

Is my voltage reference design sensitive to humidity?Methods of controlling humidity and performance of precision simulation systems

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

Due to the oven problem, this test cannot exceed 96 hours. The step of normalizing data in the range of 30% RH to 70% RH shows a drift of about 90 ppm or 2.3 ppm/% RH, which does not achieve the significant improvement effect that this application process wants to achieve, but the spray coating appears slightly Improvement, of course, can also be said that if the test time is longer, these minor improvements will disappear. Table 2 summarizes the 3 tests.

Table 2. Summary of humidity test with conformal coating

ADR4525 plastic package, uncoated

ADR45xx plastic package, 1B73 spray coating

ADR45xx plastic package, 1B73 deep immersion coating

ADR4525 ceramic package

Test duration

168

168

96

168

RH test method

20% RH to 80% RH

30% RH to 70% RH

30% RH to 70% RH

30% RH to 70% RH

Output drift result

2.5 ppm/% RH

2.5 ppm/% RH

2.3 ppm/% RH

0.075 ppm/% RH

Future tests may use other types of conformal coatings (silicone, rubber, etc.), and many changes will be made in the application process. In addition, cross-sectional analysis after coating can also confirm whether the applied coating thickness meets the standards required by the manufacturer, and whether the coating at certain edge locations is sufficient. In short, these experimental data show that ceramic hermetic packaging is the only ideal defense method to prevent moisture intrusion.

in conclusion

In designs that use only 10-bit target accuracy (1/1000 type accuracy, or ±5 mV in a 5 V reference), various error sources may quietly affect accuracy. However, if the target accuracy of your precision instrumentation system is 16-bit or even 24-bit, then you must consider the entire system design, including PCB manufacturing, to ensure that the accuracy is guaranteed during the entire life cycle of the design. This article shows that the ideal way to ensure humidity performance is to use hermetic packaging, such as ceramics. In addition, conformal coatings can help slow down the speed at which precision analog electronics are affected by humidity. When the design of the design engineer enters the production stage, it will need to use skills outside the electronic field, and need to consult the coating company to ensure that the product can achieve excellent performance in a challenging environment. The phrase “This argument holds water” usually means that your argument is valuable and correct. In this case, following best practices can ensure that your voltage reference itself will not be eroded by water vapor, but keep the water out, ensuring that your precise design can maintain the performance you need. This design method may be flawed, but your voltage reference will not!

Reference

ASTM E398-03, Standard Test Method for Determining Water Vapor Permeability of Sheet Materials by Dynamic Relative Humidity Measurement Method. American Society for Testing and Materials, 2003.

Bryant, James. “Q&A for Application Engineers-11: What accuracy must the voltage reference achieve?” “Analog Dialogue”, January 1992.

HumiSeal 1A33 Polyurethane Conformal Coating Technical Data Sheet. HumiSeal, 2019.

“IPC-HDBK-830: Conformal Coating Design, Selection and Application Guidelines”. IPC, October 2002.

“MT-087 Demo Tutorial: Voltage Reference.” Analog Devices, 2009.

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