USING CIRCUIT BOARD MATERIALS FOR THERMAL ......Account for them in the thermal design and rout them...

USING CIRCUIT BOARD MATERIALS FOR THERMAL CONTROL IN MEDICAL DIAGNOSTICS

USING CIRCUIT BOARD MATERIALS FOR THERMAL CONTROL IN MEDICAL DIAGNOSTICS KEYTECHINC.COM 2

Circuit board materials and structures can be cleverly adapted to become thermal control components for medical diagnostic systems. Printed circuit board (PCB) fabrication techniques simplify design by providing an easily customizable source from which heat (and cooling) can be generated, distributed, measured and dispersed. Through iterative simulation and physical prototyping, PCB techniques can rapidly advance designs for precision thermal management in the tight spaces and dynamic climates of microfluidic cartridge architectures. Conveniently, PCB fabrication techniques also easily scale from rapid prototyping to mass production, speeding design transfer. Key Tech has employed PCB techniques for thermal management whether or not there is already a need for electronics on a diagnostic cartridge, and these PCB-based thermal managers can live on the cartridge or near it on the instrument. We’ve dramatically shortened thermal design time using PCB-based modules, and solved some unique thermal management challenges along the way.


In Vitro Diagnostic (IVD) assays almost always require some type of thermal control of a fluidic sample. Heating may be needed to assist with lysis during sample prep, thermocycling is required to amplify DNA, and certain detection methods require the sample to be at a precise temperature when measurements are taken. Many IVD systems are also using microfluidic lab-on-a-chip technologies to increase speed, reduce device size, and bring these instruments out of the lab and closer to the point of care. Thermal control in these microfluidic applications can present many challenges, including:

• Maintaining highly precise temperatures

• Maintaining tight temperature uniformity, especially when there are multiple zones at different temperatures in close proximity

• Achieving fast rise and cool times, especially while maintaining temperature uniformity

• Accurately measuring temperature on a small scale where traditional sensors are too large and would influence the measurement

So how do you quickly and economically design thermal control for microfluidic IVD applications with these challenges? For macro heating applications, cartridge heaters embedded in blocks of metal contacting the vials or cartridges containing the sample will do the trick, but in small scale applications, the precise heat control required and the small space limitations usually make this approach impractical. Thin film heaters can work in some cases, but customizing these heaters to challenging applications can be tricky. Custom heating elements built into the microfluidic cartridges can also be a promising option, balancing heat management requirements with cartridge cost targets. We’ve worked on applications where traces of metals like platinum are bonded to microfluidic chips to create custom heating and temperature sensing elements. These implementations allow for total customization and may be required for demanding

applications with fast thermal cycle times, but they will increase design scope and the unit cost of cartridges.

At Key Tech, we have been leveraging Printed Circuit Board (PCB) fabrication techniques to create custom thermal control components, where the thermal masses and heat conduits are built right into the PCB. PCBs are constructed by alternating layers of electrically conductive copper traces and insulating substrates like FR4, a common class of glass fiber/epoxy composite used in PCB’s. These same materials that are used for their beneficial electrical properties are also good thermal conductors and insulators, so they are optimal for creating thermal control components. There are a number of benefits with using this approach:

• Leveraging a common fabrication technique allows for quick prototyping and also easily scales up to mass production.

• Thermal management components embedded in the PCB can be customized over a wide range of shapes and sizes.

• Multiple components and functions can be built into the same PCB (e.g., power and control circuitry, resistors for heat generation, thermistors or RTDs for temperature measurement, electrochemical detection circuitry, etc.).

• The low thermal mass compared to some alternative approaches allows for faster temperature cycling.

Using this approach can be especially beneficial if the microfluidic cartridge or device already requires a PCB for other functions. Because of the other benefits listed above, this approach can also be beneficial even if a PCB isn’t already required for other functions.

Key Tech has designed a number of thermal control components from PCBs over the years. Below is a guideline for designing these types of applications based on our experience:

INTRODUCTION


Generating heat is one of the most basic functions of thermal control. The simplest way to generate heat on a PCB is by soldering a power resistor to it. Resistors generate plenty of heat, but because they are a point source, the PCB design must sufficiently spread the heat out. Otherwise a metal heat spreader may need to be attached. Custom elements like metal heat spreaders are typically more useful in an instrument than on a microfluidic cartridge. Figure 1 shows a cross section of a PCB illustrating how this approach can be implemented. A power resistor is placed on the bottom of a PCB and then Vertical Interconnect Access holes (VIAs) transfer the heat to the top side where a copper plane spreads the heat out over the desired area to maintain a uniform temperature. Figure 2 shows a PCB where this approach was implemented to allow for 64 independently heated zones.

Another approach is to use copper traces to create a heating element inside the PCB. Even though copper is very conductive, it still has some resistance, so a long thin trace (e.g., 10-15cm) can be used to form a heating element within the PCB (Figure 3). This approach may not generate as much heat as a dedicated resistor, but it can provide a more uniform heated area without the need for a separate heat spreader. Additionally, since the trace that creates the heat is formed as part of board fabrication and no additional components are needed, the heating element comes “free” as part of the design. This is very appealing for on-cartridge heating applications, especially if a PCB is already needed in the cartridge for other functions. Even if the cartridge doesn’t already require a PCB for other functions, for demanding applications it may still be worth considering integrating a PCB into the microfluidic cartridge just to implement this approach. It may be more cost effective than designing custom heating elements onto the cartridge (e.g., layering platinum heaters onto the plastic cartridge components).

Peltier elements can create heat and can also cool, so if active cooling is also needed, a Peltier can be connected to a PCB. More details on Peltier elements are discussed in the cooling section below.

Figure 1: Construction of PCB with a resistive heater on the bottom and copper top plane to spread heat on the temperature controlled top side

Figure 2: Thermal Management PCB with 64 independently controlled temperature zones heated by resistors attached to the underside of the PCB

Figure 3: Trace layout of a heater made with a thin winding trace of copper

Copper

FR-4

Resistor

Copper plane on top surface to spread heat

Vias for heat transferthrough the PCB

Ground plane alsofacilitates heat transfer

V+ for resistor

Zones formed by copper planesisolated by FR4

HEAT GENERATION

Winding a trace of copper across the heated zone allows for a long trace length


In order to provide accurate closed-loop temperature control, the temperature of the thermal control component must be measured. A thermistor soldered onto a PCB is one

of the simplest methods for measuring temperature.

Thermistors can provide precise measurement at

temperatures up to 150C. Like a power resistor, a

thermistor provides a point-source measurement,

so the design must accurately relate the thermistor

temperature to the temperature of interest. The goal in

diagnostics is to control the fluid sample as it proceeds

through assay steps on the cartridge, so the thermistor

must measure a temperature as close as possible to

the fluid sample temperature. Offset temperatures

between thermistors and the fluid sample can

sometimes be successfully characterized (e.g., sample

temperature is 1C less than measured thermistor

temperature at target fluid temperature of 95C). This

requires iterative modeling, design and testing to

generate sufficient data for a repeatable and accurate

offset relationship.

Another common temperature measurement

challenge is verifying during development that the

temperature being measured is accurate within the

required tolerances. This requires an independent

measurement outside the design, and you can

paradoxically influence the temperature you are trying

to verify. Miniature calibrated thermocouples can

provide measurements in the small spaces available

in a diagnostic system, and perform well over a wide

range of temperatures. It is nearly impossible to

eliminate the effect of the measurement technique

influencing the measurement, but the small size of

miniature thermocouples helps reduce the effect as

much as possible. The low thermal mass of miniature

thermocouples can also allow for fast response times,

which can be especially helpful when measuring

transient applications.

Another challenge with verifying temperature

performance during development is to obtain accurate

surface temperature measurements. Non-contact

infrared (IR) measurement can be a helpful tool; while

IR cannot give a highly accurate absolute temperature

measurement (+/- 2C typically), it can provide

very accurate differential measurement of surface

temperature (within a few tenths of a degree C). For

this reason IR sensors can be especially helpful

when verifying the temperature uniformity of a heated

surface. When using the IR approach you need line-of-

sight to the surface being measured, and the emissivity

should be relatively constant over the surface being

measured. Changes in materials, coatings and surface

textures over the surface of interest can change the

emissivity so that a reliable differential measurement

can’t be obtained. However, some forethought in the

design to ensure constant emissivity over surfaces

of interest could allow IR sensing to be used and

simplify verification. To obtain line of sight to the

surface, certain components that aren’t critical to the

thermal functions of the device could be designed to be

removable. Alternatively a microfluidic cartridge can be

designed such that a “thermal verification” configuration

can be made with portions cut away to allow a view of

internal surfaces where thermal uniformity is critical.

TEMPERATURE MEASUREMENT


Managing heat transfer is another critical function. Proper heat transfer management ensures that thermal zones will be sufficiently uniform and also sufficiently isolated from each other. In a PCB, you can manage heat transfer by using its thermally conductive copper layers to facilitate heat transfer, and its insulating substrate layers to prevent it. Table 1 below lists approximate thermal conductivities of components used in PCB fabrication to manage thermal control. Copper and aluminum can be used to facilitate heat transfer and FR4 can be used to prevent it (Figure 4). Thermal grease and epoxy can be used to improve heatflow between the PCB and external components like aluminum heat spreaders The thermal conductivities of grease and epoxy may seem low, but significantly reduce the effect of the thermal contact resistance.

Lateral heat transfer in the x and y directions can be controlled by the design of the copper layers in the PCB. Large planes of copper can be used to spread heat out to create areas of uniform temperature, and traces of copper can be designed to direct heat from one location to another on the PCB. Remember, however, that the traces used for a PCB’s first purpose, electrical routing, will also conduct heat. Account for them in the thermal design and rout them in a way that contributes to the desired thermal profile. For example, if a PCB has multiple thermal zones that must be kept at separate temperatures (e.g., for PCR), electrical traces for resistors, thermistors and other features coming from each zone should avoid crossing into the other zones if possible to minimize the thermal cross talk between zones.Heat transfer in the z direction through the thickness of the PCB is facilitated by adding VIAs, which are the same metal plated holes commonly used to electrically connect different layers of a PCB. A number of methods can be used to improve the heat transfer of VIAs, including using arrays of densely packed VIAs, maximizing the VIA plating thickness,

and specifying that the hollow cores of the VIAs be filled with solder.

Thicknesses of the copper and insulating layers can also be varied to achieve the desired goal. Thicker copper layers will increase lateral heat flow, while thicker insulating layers will increase thermal resistance through the thickness of the PCB in areas where VIAs are not present. There is a practical limit to the copper thickness that can be specified when using standard fabrication techniques (typically the max is 4mils, or 0.1mm). If the thermal uniformity still isn’t sufficient with the maximum practical copper thickness, bonding metal heat spreaders onto the PCB can work in some applications. It is important to ensure that a good thermal epoxy/grease is used or that the heat spreaders are soldered/brazed to the PCB to ensure good thermal contact. Otherwise varying thermal contact resistances may result in inconsistent performance. Figure 5 below illustrates how a heat spreader can be integrated with a PCB to provide uniform heating when using copper on the PCB alone will not provide sufficient thermal uniformity.

Table 1 : Typical Thermal Conductivities of Common Materials

Figure 5: PCB with Resistive heaters and thermistors embedded in aluminum heat spreaders

Figure 4: Cross Section of a PCB with VIAs and internal traces

MATERIAL

Copper 400

150

1-8

0.25-0.35 - through plane (z)0.8-1.0 - in plane (x-y)

Aluminum

FR4

ThermalGrease/Epoxy

THERMAL CONDUCTIVITY(W/m*K)

HEAT TRANSFER

PCB

Aluminum HeatSpreaderThermistor Resistor


Achieving precision thermal performance requires iterative analysis to predict performance and determine the effects of changing design variables as the design matures. Analyzing the design of PCB-fabricated thermal components is fairly straightforward, because heat transfer within the PCB is governed by conduction. For simple components, hand calculations suffice for calculating heat transfer and temperatures. Using the thermal circuit approach, each direction of the PCB can be considered a “composite wall” consisting of copper and the insulating substrate. Specific thermal resistances can be calculated for the x, y and z directions based on the area of copper and area of insulating substrate in the cross section of the board perpendicular to each direction. The calculated thermal resistances can then be used in thermal circuit hand calculations to estimate heat flow and general temperature differences between different sides of the PCB or different zones on the PCB. These quick, rough calculations can be helpful early on in the design process when creating and iterating on simple breadboard prototypes.

For a more precise analysis of temperature uniformity, or when performing transient analysis, it is usually preferable to use FEA (Finite Element Analysis) modeling. Because most of the heat transfer within the PCB is governed by conduction, complex CFD (Computational Fluid Dynamics) modeling isn’t usually needed and standard FEA using SolidWorks Simulation or similar software can provide good results. For a quick simple model, specific thermal resistances for the x, y and z directions can be calculated by hand as described above and a custom non-isotropic material can be created in the FEA model with the unique thermal resistances specified for each direction. The PCB can then be modeled as a simple rectangle and an FEA can be quickly performed. If you have access to FEA software and understand the fundamentals well enough to establish an accurate yet simple model, it is usually faster and more thorough to go right to simple FEA modeling rather than performing hand calculations.

For PCBs with complex trace routing and multiple temperature zones, the simplified modeling approach may be too crude, and better results can be had by more fully detailing the model with the geometry of the copper traces. It can take some time to fully create a model of the PCB, but FEA best practices can be used to simplify the modeling and analysis (e.g. use symmetry, simplify geometry, ensure proper mesh size and refinement, use appropriate boundary conditions etc.). Additionally, a number of Electrical CAD (ECAD) to Mechanical CAD (MCAD) connectors can

allow for more detailed exports of PCBs from ECAD software to MCAD software so the work of recreating the details of the PCB design in MCAD software for FEA analysis can be simplified. Figure 6 illustrates a hybrid approach where a detailed PCB was exported into MCAD, but some of the internal traces were combined into a single central custom non-isotropic material to simplify the model. This particular FEA is analyzing the temperature uniformity of a central cold zone when all surrounding zones are heated.

While FEA can be used to inform the design, confirmatory testing should be done to verify that the predictions match reality. There are many vendors that offer prototyping and quick turn PCB fabrication services, which allow for rapid design iterations. An FEA can be performed to help generate an initial design, and a week later the design is fabricated and back in-house being tested. The FEA can then be refined to match the real world performance of the initial prototype and changes to improve performance can be evaluated with a more accurate FEA model that’s informed by the testing. The cycle can then be continued with iterations in quick succession until the performance meets the requirements (Figure 7).

DESIGN ANALYSIS

Figure 6: FEA model of a 9-zone PCB based thermal control component using 2-plane symmetry for simplicity

Figure 7: Thermal component development cycle using FEA modeling

Power Resistor

Symmetry Planes

Central Cold Zone

Heated Outer Zones


DIFFERENTIAL THERMAL EXPANSION

COOLING

Another important consideration to keep in mind is the effect of differences in coefficient of thermal expansion (CTE) of the materials being used. FR4, a common

PCB substrate material, has a similar in-plane (x-y)

expansion coefficient to that of copper. However, as

seen in Table 2, it is an anisotropic material due to its

glass fiber/epoxy resin composite, so it has a higher

through-plane (in the z-direction) expansion coefficient

than copper. In some cases, this could affect heat

transfer characteristics, but typically the bigger

concern is that the stresses imparted could damage

the PCB (e.g., break traces) if it must operate over

extreme temperature ranges or if it must withstand a

high number of cycles. Thermal expansion must also

be considered when bonding components like heat

spreaders onto a PCB. The coefficient of expansion

of the heat spreader material as well as any type of

epoxy used to bond the spreader to the PCB must be

considered, as significant mismatches could cause

the bondline to break and result in inconsistent heat

transfer. If analysis shows that the materials or

temperatures required are pushing the limits of CTE

mismatch, thermal grease may be a better choice

than bonding with epoxy.

Additional fastening features will be needed to

secure the PCB to the heat spreader, but because

the thermal grease remains fluid it can be more

tolerant of the expansion and contraction of the PCB

and heat spreader materials. Care should be

taken to ensure that the grease selected will not

migrate over time and result in increased thermal

resistance. This can be prevented by selecting

grease of the proper viscosity, ensuring that liquid assay

ingredients can’t contaminate the grease, and properly

containing the grease in the assembly.

Many diagnostic applications also require active cooling to maintain proper temperature or to perform temperature cycling. Passive or forced air convection is the simplest approach for removing heat from PCB-fabricated thermal control systems. The fairly low thermal mass of these PCB-based systems can make this approach more feasible than for other approaches with larger thermal mass. Passive or forced air convection relies on ambient air temperature, so cooling performance changes with changing ambient temperature. However, you can never cool below the ambient temperature using this approach. So, for example, if the application requires cooling from 95-60C, passive or forced air convection may be practical, but if the temperature must be brought to 40C in an instrument enclosure, it would not.

When cooling requirements demand a solution beyond convection, a Peltier element (aka thermoelectric cooler, or TEC) can be employed. Peltier elements usually aren’t components that can

MATERIAL

Copper 1.7X10-5

2.2X10-5

2.0-5.0x10-5 (for epoxy)

7.0X10-5 - through plane (z)1.2-1.4X10-5 in plane (x-y)

Aluminum

FR4

ThermalGrease/Epoxy

Coefficient of Thermal Expansion (K-1)

Table 2: Thermal Expansion Coefficients of Common Materials.


be integrated directly onto a PCB, but a PCB can be integrated around a Peltier element to allow for active cooling as seen in Figure 8 and Figure 9. A Peltier element can cool below ambient and remove more heat than pure convective cooling may be able to do on its own. Additionally, a Peltier element can produce heat when the current flow is reversed. The ability to heat and cool can allow for higher precision temperature control and faster thermal cycling than a design that uses convective cooling with resistive heating. You pay for this benefit with added complexity. Space must be found to integrate the Peltier element, which can be difficult in miniaturized applications. Peltiers also require heat sinking and usually ducting and airflow to remove heat from the Peltier. Since Peltier elements are only 10-15% efficient, they generate a significant amount of heat on the hot side compared to the heat removed on the cold side. For these reasons, it is usually best to ensure that alternative approaches have been exhausted before going down the path of integrating Peltier elements. Sometimes, however, Peltiers are the only practical method if active cooling is needed.

When neither convective cooling nor a Peltier element seem ideal for the application, it may be worth considering alternative approaches that may not have as stringent cooling requirements. For example, when performing thermal cycling for DNA amplification, it

may be simpler to move the sample between different zones that are held at different constant temperatures vs. thermal cycling in place. This can be achieved through approaches like digital microfluidics or by forming microfluidic channels on the PCB surface. The latter approach can be performed by adhering a molded plastic part with channels to the PCB and then using common microfluidic pumping techniques to move the fluid between zones.

Thermal design for diagnostic systems can be challenging, particularly when assay design requires rapid thermal cycling, or when multiple proximate zones need to be maintained at different temperatures. Designs must simultaneously meet requirements for tight temperature precision, temperature uniformity across a sample fluid, real-time temperature measurement and more, all within the tight spaces defined by the cartridge and instrument architectures. Using PCB fabrication techniques can offer some significant benefits over other more traditional approaches. Key Tech has designed dozens of diagnostic instruments for clinical, industrial, and consumer applications, and we’ve learned how and when to use these PCB techniques, and when to use other solutions as well. Contact us with your thermal design challenges, especially the tough ones.

CONCLUSION

Figure 8: Cross section of Peltier element integrated with a PCB and heat spreader for active heating and cooling

Figure 9: Peltier module integrated with PCB mounted resistive heaters for active heating and cooling

Aluminum Heat Spreader

Peltierelement

Heat sink

PCB

Peltier

Resistors and thermistors on PCB embedded in heat spreader

40 EAST CROSS ST, BALTIMORE, MD 21230 | [email protected] | 443.610.2317

Since 1998, Key Tech has been transforming complex technologies into intuitive products. We design and develop medical, industrial and consumer products using novel sensors, wireless, ultrasound, microfluidics, optics and automation. Our uniquely personal approach attracts industry leading global companies, as well as innovative startups, to our Baltimore Headquarters. The Key Tech team of interdisciplinary scientists, engineers and designers take technologies into new applications, keeping your development pipeline fresh.

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USING CIRCUIT BOARD MATERIALS FOR THERMAL ......Account for them in the thermal design and rout them...

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