Testing of Current Carrying Capacity of Conducting Tracks in HighPower Flexible Printed Circuit Boards

Remesh Kumar K R1& K. Shreekrishna Kumar1

Received: 31 May 2018 /Accepted: 14 February 2019 /Published online: 27 February 2019# Springer Science+Business Media, LLC, part of Springer Nature 2019

AbstractIn high power electronic applications, the current carrying conducting tracks of the printed circuit board (PCB) and the compo-nents generate heat. For the proper working of the circuit, this heat must be expelled to the surroundings. In flexible PCBs, heatsinks cannot be used for reducing the temperature. We developed a multilayer flexible PCB structure that includes a heatconducting thick layer and electrically non-conducting thin film layer. In this paper, we have tested a flexible PCB for highpower applications and evaluated its performance when the current flows through the conductive tracks in it. The heat generatedby current flowing through the copper conducting tracks of the PCB at different current conditions and its convection is analyzedin relation to board dimensions and structure. The ANSYSWorkbench is used to test the temperature and current capabilities of thePCB. The test results show that the temperature of the flexible PCB gets reduced as the area of convection increases. In addition,the current carrying capacity of tracks in the flexible PCB is also increased. The high-power handling flexible PCB will help theheavy power electronics boards to reduce size and give PCB designers more freedom to design a layout fitting any shape of thecabinet of the device.

Keywords Flexible PCB . Power electronics . High power flexible PCB . Computational fluid dynamics . ANSYS workbench .

Electro-thermal analysis

1 Introduction

The printed circuit board (PCB) [1, 2] plays a vital role inalmost all areas of electronics industry [3]. A PCB intercon-nects electronic components through conducting tracks(traces). Also, a PCB handles both the weight of the compo-nents and the heat generated by them. Therefore, a PCB musthave a good mechanical strength, proper heat handling capac-ity, dielectric strength and insulation to high voltages and cur-rent. The specification for a good PCB is closely realized inthe current PCB fabrication methods [1, 4–7]. When the ma-terial used for manufacturing these PCBs has better electricaland mechanical properties; the cost of fabrication is lower.

However, the PCBs used in most of the electronic devicesare rigid and not flexible. Most of the industries do not con-sider this problem because only flexible devices need flexiblePCBs [8–14]. At present most of the flexible PCB develop-ments aims at small portable devices that can be flex, such asflexible displays. If we can use flexible PCBs for applicationssuch as power electronic devices [15–17], it will help reducethe size of the device drastically and single design (layout) ofPCB can suit any shape of device cabinets. Very limited re-search has been done investigating the application of flexiblePCBs for bigger devices. The main problem of existing flex-ible PCB is their low power handling capability and lack oftemperature management solution for embedded components.So, it is necessary to improve the thermal management system[18–21] and the current rating of the flexible PCB.

Researchers have tried to reduce the size of power electron-ic devices by using more advanced, compact and efficientcomponents and new thermal management systems. But onlya few researchers have attempted to reduce the size by chang-ing the way of interconnecting components, that is, by chang-ing the technology of PCB inside the device. Erik et al. [22]introduced a method of reducing the size of the power elec-tronics convertor by combining rigid and flexible PCB in a 3-

Responsible Editor: H. Manhaeve

* Remesh Kumar K Rremeshlogo@gmail.com

K. Shreekrishna Kumarkshreekk@gmail.com

1 School of Technology and Applied Sciences, Pullarikunnu Campus,Mahatma Gandhi University, Kottayam, Kerala, India

Journal of Electronic Testing (2019) 35:131–143https://doi.org/10.1007/s10836-019-05782-3

dimensional manner. They developed a transformer using aflexible PCB and other components placed on a rigid PCB;further these are combined together to provide folding ability.So, this circuit can be folded and inserted into a small space.Also, they addressed the thermal management system for thistype of PCB usage. This technique achieved 66%more powerdensity than the conventional PCB. Leong et al. [23] studiedthe application of a flexible PCB serving as a computer moth-erboard and its mechanical performance. They used computa-tional fluid dynamic (CFD) tools for studying deflection ofmotherboard with various conditions. Their study includesthe deflection of flexible PCB by the weight of the compo-nents, by force induced due to flow of air, by different fasten-ing options and by different component layouts. From thiswork, it is evident that the component layouts, the velocityof air coming from the cooling system and the weight of thecomponent should be considered while designing a flexiblePCB for applications like computer motherboards. Von bank[24] studied the problems of thermal management in flexiblePCBs. He addressed the problems of flexible PCBsmade withthermally insulating polyimide material and examined the dif-ficulties of heat removal from the components in the flexiblePCB. He created some simulation models of a flexible PCBhaving a heat generating chip in combination with traditionalheat removing mechanisms like natural convection, radiativeheat transfer or flow-over heat transfer (forced convection).From this work, it is clear that the traditional heat removalmethods are not enough for removing heat from polyimidebased flexible PCB. It is clear that the flexible PCB basedpower electronics device will be smaller in size because ofbetter space utilization. The flexible PCB handles heaviercomponents without any mechanical deformations. Also, thetraditional thermal management system is not compatible withflexible PCB and so additional features are necessary.

Power electronic circuits have high current, elevated volt-age and heavier components, and they generate more heat.Therefore, proper thermal management and weight handlingstrength are necessary for power electronics PCBs. Most pow-er electronics uses rigid PCBs, which needs to change to flex-ible PCBs. We cannot convert a rigid PCB directly in to aflexible PCB using the existing technology because the ther-mal management system in the rigid PCB is bulky, hard anddistributed along the circuit. In addition, an existing flexiblePCBs will not handle the weight of the components. To over-come this issue, a strong flexible PCB is needed for handlingheavy components and the generated heat must be carriedaway from the components using a heat sink. In the flexiblePCB, a traditional heatsink may not be used because it wouldaffect the freedom of flexibility. Therefore, our aim is to makea flexible PCB that can itself act as a heat sink and dissipatethe generated heat. For developing a thermally conductiveflexible PCB, a thin film [25, 26] of electrically nonconduct-ing material is coated on a thermally conductive copper sheet.

Conductive tracks of the circuit are laid out on the thin film ofnonconductive coating. Figure 1 shows the proposedmodel ofthe PCB. Here, a large bending cycle is not needed becausethe power electronic application requires fewer bending cy-cles. This type of flexible PCB is generally known as staticflexible PCB [1]. Copper is selected as the base because it hascertain flexibility, has high thermal conductivity and is rela-tively inexpensive. For testing, the properties of the noncon-ductive layer are assumed to be the same as those of polyimide[27].

In this paper, we introduce a solution serving to reduce thetemperature of flexible PCB due to heat generated by thecurrent flow in tracks (considered as electronic components)and to increase the current handling capacity of the PCB.Computational Fluid Dynamic [28–30] simulation softwareis used to analyze the track and study the thermal and electricalproperties of the PCB models. The ANSYS design modeler isused to model the PCB. ANSYS workbench’s thermal – elec-trical analysis is used to analyze the temperature handlingability of PCB and the voltage droop in the conducting trackdue to current flow. We measured the temperature and voltagedroop of the conducting tracks with change in convection [31]of the PCB and the current passing through the conductor indifferent models of PCB. Then, we determine which model ofthe PCB shows a better performance in terms of temperaturehandling and voltage drop.

2 Methodology

2.1 Experimental Setup

& We developed 3 different sized PCBCADmodels withoutfins and 3 other PCB models with different fin shapes.

& Each model of PCB contains a conductive track for testing.& For comparing the differentmodels of the PCBs, the dimen-

sion of the conductive track is kept the same in all models.& For testing and analyzing the PCB, we assigned thermal

and electrical properties of copper to the conductive trackand base sheet. For the thin layer, we used the electricaland thermal properties of polyimide material.

& The ANSYS design modeler was set up for CAD model-ing of PCB and the ANSYS thermal electric analyzer wasused for testing, measuring and analysing the electricaland thermal performance of models.

& The current carrying capacity of the conductive track wasdetermined on the basis of generated heat and voltage dropacross it.

& The tests were repeated with different convection ratesbecause flexing will affect the convection characteristicof the flexible PCB.

& The performances of different models of PCBs were com-pared by analyzing the values from simulation results.

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& The contour of the temperature distribution was taken intoconsideration for the placement of track and temperaturesensitive components during PCB assembly.

In the ANSYS modeling tool, we developed 100 by100 mm, 150 by 150 mm and 200 by 200 mm copper sheetmodels. When the thickness is increased, the copper sheet willno longer flex and will become rigid. As a result, it cannot beused as a flexible PCB. If the thickness of the copper sheet isreduced, its strength [32] will also reduce. So, the thickness ofthe copper sheet model is kept at 0.24 mm. A 500 nm non-conducting layer is created over the copper sheet. It has thesame dimension as that of the copper layer. On top of thislayer, a conducting track [33] made of copper is placed. Acopper track with a length of 100 mm, a width of 5 mm anda thickness of 0.1 mm is created. Figure 2 shows a 100 by100 mm PCB with the copper track, Fig. 3 shows a 150 by150 mm PCB with copper track and Fig. 4 shows a 200 by200 mm PCB with the copper track.

Increasing the size of a PCB may not be appropriate inmost case, because there is a limitation to the maximum al-lowable size for the circuit board in the cabinet or casing.Therefore, we tried to increase the convective surface areawithout increasing the size of PCB. In an ordinary heat sink,fins are used to increase the convective surface area. Here, ifwe try to use the same technique, that will cause a problem. Asthe height of the fin increases the convection also increases.But, the increased height of the fin limits the bending capabil-ity of the PCB. Besides, high-density placement of fins alsoaffects the bending [34]. Therefore, the height is kept small;here it is 10 mm. The fin shape also affects the convection.Thus three types of fins are used, circular [35–37], spherical[38] and rectangular [39, 40]. Figure 5 shows 8 by 8 circularfins with 200 by 200 mm PCB, Fig. 6 shows 8 by 8 sphericalfins with 200 by 200 mm PCB and Fig. 7 shows 8 by 8 squarefins with 200 by 200 mm PCB. Fins are also made of copperand are attached to the copper sheet.

Fig. 2 100 × 100 mm PCB model Fig. 3 150 × 150 mm PCB model

Fig. 1 3D model of our proposed PCB: conductive copper track, nonconductive coating and copper base sheet are shown

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3 Results and Discussion

3.1 Theoretical Explanation

Temperature, current, voltage and resistance are inter-dependent quantities. In general, according to Ohm’s law,the voltage is directly proportional to the current flowingthrough a conductor at constant temperature. So, if there isany change in temperature, then the proportional factor (beingthe resistance) used in the application of Ohm’s law willchange. In our work, the conductor (tracks) is not at constanttemperature because of the temperature rise during the opera-tion. There are some general equations that express the rela-tionship between current, voltage and resistance under varyingtemperature conditions. In our work, we need to findthe temperature and voltage droop across a conductorfor various current flows and convective heat losses.The temperature rise of the conductor is due to the heatgeneration in it. This heat generation depends on currentflow and the resistance of the conductor. Also, the

resistance of the conductor is dependent on the temperature.Thus, temperature and resistance are inter-dependent.

In our approach a conductor (thin film of copper track) isplaced on the PCB that is capable of removing heat from thisconductor. So, most of the heat generated by the conductor isremoved through PCB. This will prevent temperature rise andfurther resistance variation. For better understanding, we willuse some general formulas relating heat, current andresistance.

As per the following equation, it is clear that the tempera-ture increases due to the heat generation in the conductor.

Tfinal ¼ Tinitial þ Q1=mc ð1Þ

Here, Tfinalis the final temperature (temperature of theconductor), Tinitial, the initial temperature of the conductor,Q1, the generated heat from the conductor, m, the mass ofthe conductor and c, the specific heat capacity of the material(assuming, copper is the material). The heat generation is

Fig. 7 8 × 8 Square fins with 200 × 200 mm PCB

Fig. 6 8 × 8 Spherical fins with 200 × 200 mm PCB

Fig. 5 8 × 8 Circular fins with 200 × 200 mm PCB

Fig. 4 200 × 200 mm PCB model

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caused by the resistance of the copper and current flow in theconducting track. According to the Joule’s heat equation(Eq. 2), heat generation will increase when current increases.

H ¼ I2Rt ð2Þ

In Eq. (2), H represents Joule’s heat, I, the current flowingthrough the conductor, R, the resistance of the conductor and t,the time interval of current flow. If the Joule’s heat is the onlyheat source for the conductor, thenQ1 in Eq. (1) is the same asJoule’s heat H. But the heat due to the current flow is reducedby the convective heat transfer of the PCB. The generated heatH from the conducting track is transferred to the copper basesheet via nonconductive layer.

Q2 ¼ −kdTdx

� A ð3Þ

In Eq. (3), Q2 is the heat transfer due to conduction, k, thethermal conductivity of material, dTdx, the temperature gradient(x is the thickness of the layer and T is the temperature differ-ence of the heat conductive layer’s walls) and A, the area ofheat transfer. In Eq. (3), the thickness of the nonconductivelayer is in the range of nanometres. So, there is a very smallheat block in this layer. Hence, the heat generated from thetrack is almost completely transferred to the copper sheet.Therefore, only the thickness of the copper sheet issignificant in Eq. (3). The copper sheet is in contactwith the surrounding air so the heat is transferred tothe surrounding air. Equation (4) gives the amount ofheat transferred to the air.

Q3 ¼ h Ts−T f� �� A ð4Þ

In Eq. (4), Q3 is the convective heat transfer, h, the heattransfer coefficient, Ts, the temperature of the copper sheetssurface, Tf, the temperature of the surrounding air and A, thearea of the convective surface of copper. The heat due toconduction (Q2) is almost completely transferred to air viacopper sheet, which means Q2 is the same as Q3. So, we needto consider only Q3 as the removed heat. Due to this reason,the actual affected heat (Q1) of the conducting track is thedifference between the generated heat (H) and the heat trans-ferred to the air (Q3).

In Eq. (2), the resistance R is almost constant, expect for asmall variation in its value because of the variation in theresistance of the copper track with temperature. The resistanceversus temperature relation is given by the following equation.

R ¼ Rref 1þ α T−Tref� �� � ð5Þ

Here, R is the conductor resistance at temperature T, Rref,the conductor resistance at reference temperature Tref, α, the

temperature coefficient of the conductor material and T, theconductor temperature. Equation (5) shows that the resistancedepends on temperature. Temperature rise due to current flowwill be limited by the PCB’s heat transfer capability.

In this analysis, there are two conditions that are not con-sidered. First is the heat removal by radiation. The radiationbased heat loss is significant only in vacuum and it needssome more conditions. Thus, in our case, the radiative heatloss is negligible and hence not considered. Second, convec-tive heat removal from the track side (component side) of thePCB (see Fig. 1 for structural clarification) is not considered.This transfer will not occur after the final implementationbecause the PCB would then contain a thick coating neededfor protecting the copper track. It will block the heat transfer tosurrounding air. Also, components placed on that side willalso block heat transfer to surrounding air. In this paper, theterm Bconvection rate^ refers to the value of a convectioncoefficient. From Eq. (4), the convective heat transfer dependson the surrounding temperature and surface area. In reality wecannot predict surrounding temperature inside the cabin. Itdepends on the flow of air and ventilations in the device cabin.Also, the area for convective surface after installation cannotbe predicted at the time of the development of PCB. The onlyway to change the convective heat transfer is by changing theconvection coefficient in the software. So, during the tests,we change convection coefficient for changing the rateof convective heat transfer.

3.2 Results

For the proposedmodel, we have done thermal-electrical anal-ysis in ANSYS. The voltage droop across the conductor andthe temperature rise of the conductor due to various magni-tudes of current flow are measured and analyzed. In addition,the effect of convection in the temperature generation and thevoltage droop in the conductor are also analyzed. In the PCBmodels shown in the Figs. 2, 3, 4, 5, 6 and 7, three sizes ofPCBs and three types of fin-based PCBs are included. First, a100 mm by 100 mm, 150 mm by 150 mm and 200 mm by200 mm PCB model without fins and 200 mm by 200 mmcircular finned, spherical finned and square finned PCBs aretested and analyzed for finding the temperature variation dueto current flow in the conducting track. Second, PCB modelsare tested with different convection rates to measure the tem-perature and voltage drop of the conducting track. A graphicalrepresentation of the test results is provided for analyzing in-dividual PCB models. Tables are provided for comparing themaximum and minimum temperature occurrences of differentPCB models. Also, the temperature and voltage contours areprovided for analyzing the temperature distribution and volt-age drop in different lengths of the conductor. In the contour,blue color represents the minimum value and red color, themaximum value.

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Figure 8 shows the graphs of the maximum and minimumtemperature of various PCB models including copper track indifferent current flows. From these graphs, we see that, thetemperature increases as the current increases. The graphs(Fig. 8a–f) show the maximum and minimum temperatureof the flat PCBs. The maximum temperature occurs inthe conductor and minimum temperature occurs inPCB’s boundary areas. The maximum temperature isnecessary for analysis.

Table 1 shows maximum and minimum temperatures at30A current flow in different types of PCBs. Here, type ofthe convection is set as natural convective heat transfer; thatmeans convection rate of 5 W/m2 0C is set for the copper sideof PCB. From table, the maximum temperature of the PCBdecreases with increase in dimension. Also, finned PCB haslower temperature than the PCB without fins. From these re-sults, it is clear that, as the area of the PCB increases, the

temperature of the conductor decreases for the same current.Temperature reduction is due to the increase in the area ofconvective surface. The circular finned and square finnedPCBs show the best performance, because of their larger sur-face area than the normal flat PCBs. All finned PCBs containan equal number of fins and the same height. In sphericalfinned PCBs, the height and the radius of the fins are the same.Therefore, as the height increases, the size of the fin alsoincreases. It will cause the fin density in the PCB to increaseand it will further affect the bending ability of the PCB. So, theheights of 3 types of fins are to be suitably determined.Spherical fins possess lower surface area than other types offins. Therefore, a circular finned PCB has relatively lowerperformance than others.

Figure 9 shows the temperature contour of PCB. The tem-perature of the PCB is maximum in the copper conductivetrack and the neighboring area and the temperature is

Fig. 8 Temperature variationwith increase in current in PCBsof (a) 100mm× 100mm, (b)150mm× 150mm, (c) 200mm×200mm, (d) 200mm× 200mmcircular finned, (e) 200mm×200mm spherical finned, and (f)200 mm× 200mm square finned

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minimum at the boundary (outlying area) of the PCB. Byconsidering the temperature contour, the minimum value oftemperature should be small because the temperature must notaffect other tracks or components. As per Table 1, the circularand square finned PCBs have the lowest minimumtemperature.

Figure 10 is a graphical representation of voltage variationdue to the current changes in the track for different models ofPCBs. As per Ohm’s law, the voltage is directly proportionalto the current through the conductor at a constant temperature.But here, Ohm’s law cannot be applied because of the temper-ature variation. From the graphs, the voltage variation is al-most linear except for minor variations in lower sized PCBs.The non-linear variation of voltage is maximum in 100 mm by100 mm PCB (Fig. 10a). This non-linearity occurs due to thehigher temperature variation of the PCB. For 150 mm by150 mm PCB and above (Fig. 10b–f), the temperature is de-creased and stabilized due to the increase in the board dimen-sion (area of the PCB) and fin shape. So, the voltage versuscurrent graph almost obeys Ohm’s law due to the temperaturestability. From the Table 2, we see that the maximum voltagedrop across the full-length conductor decreases with an in-crease in area of the PCB. Circular and square finned PCBshave better surface area and so the voltage drop is minimumfor those types of PCBs.

From the voltage contour (Fig. 11), we see that the voltagedrop increases with increase in length (bottom to top) ofthe conductor. This is because the resistance of the con-ductor increases with increase in the length of the con-ductor. The minimum value of voltage is indicated inthe lower region (or shortest length of the conductor)and the rest of the PCB.

Next, we analyze the temperature for a constant currentflowwith different convection rates. This analysis is necessarybecause a flexible PCB can be folded in any direction. Thefolding angle and the radius depend upon the shape of thedevice casing. The shape of the device is not predictable atthe time of the design of the PCB. Therefore, we cannot pre-dict the folding angle of the PCB. With respect to convection,during or after flexing, the only thing happening to the PCB isa change in the convection rate. After flexing, the convectionrate of the board changes because the area of the convectivesurface of the PCB exposed to the air changes. The value ofconvection rate may increase or decrease depending upon theflexing and packaging. Here, natural convection is considered.So, the maximum convection coefficient (to air) is 5 W/m2 0Cand the minimum is 1 W/m2 0C. Figure 12 shows the temper-ature variation with convection rate in PCBs of different sizesand fin types. The minimum convection coefficient representsthe maximum flexing, that is, when the convection surface ofthe PCB has the least contact area with the surrounding air.The lower convection is the cause of the rising temperature ofthe PCB and track.

From the graphs (Fig. 12), we see that the temperaturedecreases with an increase in convection rate at a constantcurrent flow of 30A. Also, a temperature reduction is foundwhen the size and fin shape of the PCB vary. Table 3 repre-sents the maximum and the minimum occurrences of temper-ature at convection coefficient of 1 W/m2 0C. This conditionoccurs when the PCB is maximally flexed. There is a possi-bility of convection becoming zero but this condition is be-yond the capability of the testing software. So, we assume thatthis condition will destroy the track and PCB. The maximumand minimum temperature at minimum flexing (that meansthe condition corresponding to the maximum convection co-efficient) is same as that for a flat PCB (PCB without bend-ing). The results of this condition are already discussed inTable 1.

Finally, we examine the voltage droop across theconducting track with the changes in convection rate.Voltage analysis is critical because the circuit board handleshigh current with thin tracks. The thin track offers more resis-tance, and has a higher voltage drop than that of a thickconducting track. The convection change is due to the flexingof the circuit board. Figure 13 shows the voltage drop acrossthe conductor for different convection rates with a current of30A in different PCBs. The maximum convection rate is 5 W/m2 0C because the analysis assumes natural convection to air.

Table 1 Minimum and maximum temperatures of different models of PCB at current flow of 30A

PCB Type 100 × 100 mm 150 × 150 mm 200 × 200 mm Circular finned Spherical finned Square finned

Minimum temperature (°C) 102.26 51.849 36.812 32.263 33.863 31.925

Maximum temperature (°C) 107.06 57.882 44.189 34.566 39.528 34.568

Fig. 9 Temperature contour of PCB

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Flexing causes a decrease in the convection rate. In thisexperiment, when the convection rate increases, themaximum voltage drop across the conductor is de-creased (Fig. 13a–f). The reduction of voltage is dueto the change in the resistance of the copper conductingtrack. The variation of the resistance is due to the tem-perature reduction according to Eq. (5). Table 4 repre-sents the minimum and maximum voltage drop in theconductor at 1 W/m2 0C of convection rate. From

Table 4, the voltage drop across the conductor decreaseswith increase in PCB’s size and fin type. The maximumand minimum voltage at minimum flexing (that meansthe condition with maximum convection coefficient) issame as that for a flat PCB (PCB without bending). Theresults for that condition are already discussed inTable 2.

For analyzing the current handling capability of the pro-posed models of flexible PCBs, tests are carried out by

Fig. 10 Voltage variation withincrease in current in PCBs of (a)100mm× 100mm, (b) 150mm×150mm, (c) 200mm× 200mm,(d) 200mm× 200mm, circularfinned, (e) 200mm× 200mmspherical finned, and (f)200 mm× 200mm square finned

Table 2 Minimum and maximum voltage drop in different models of PCB at current flow of 30A

PCB type 100 × 100 mm 150 × 150 mm 200 × 200 mm Circular finned Spherical finned Square finned

Minimum voltage (v) 0 0 0 0 0 0

Maximum voltage (v) 0.13662 0.11782 0.11184 0.10783 0.10983 0.10785

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changing the current flow through the conductor in differentmodels of PCBs with natural convection. From results, it isclear that the temperature increases when the current flowincreases but the increased area of convective surface resultsin a decrease in temperature. This is due to the heat lossthrough the PCB. Also, the voltage droop across the conductor

is tested by changing the current through the conductor indifferent models of PCBs. The voltage droop across the con-ductor increases when the current flow increases. But as thearea of the PCB increases the maximum voltage droop isdecreased. The change in voltage droop is due to the variationin resistance of the track caused by the temperature change.For analysing the temperature handling capability of the PCBfor different convection rates in different models, the convec-tion rate is changed while the current flow is held constant andthe temperature is measured. The temperature decreases withincrease in convection rate. Also, the temperature varies withdifferent models. The voltage droop across the conductoralso changes with convection rate and models. This isbecause of the resistance variation of the conductor dueto the temperature reduction by convection. From theanalysis of the results the finned PCB especially circularand square finned performed best in different current flowsand convections.

Fig. 12 Temperature variationwith increase in convection inPCBs of (a) 100mm× 100mm,(b) 150mm× 150mm, (c)200mm× 200mm, (d) 200mm×200mm circular finned, (e)200mm× 200mm sphericalfinned, and (f) 200 mm× 200mmsquare finned

Fig. 11 Voltage contour of copper track in PCB

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3.3 Guidelines and Future Possibilities

From the test results and analysis, it is evident that we shouldfollow some guidelines (conditions) for the application levelusage of the heat conductive flexible PCB. The benefits ofusing these guidelines are listed below. Also, it includes thefuture possibilities related to this work.

& In this work, the performance of the PCB with only oneconductive track is analyzed. In a real PCB system, we

require a lot of tracks with different lengths. So, the heatgenerated by one conductor affects other conductors aswell as components on the PCB. For avoiding this prob-lem, the conductors need a minimum spacing. That isbased on their current and temperature. The temperaturecan be easily predicted with the resistance of the track andcurrent flow.

& Temperature contour of the PCB during current flowshows that the temperature of the nearby area of theconductive track is almost same as that of the

Fig. 13 Voltage variation withincrease in convection in PCBs,(a) 100 mm× 100mm, (b)150mm× 150mm, (c) 200mm×200mm, (d) 200mm× 200mmcircular finned, (e) 200mm×200mm spherical finned, and (f)200 mm× 200mm square finned

Table 3 Minimum and maximum temperature for different models of PCB at current flow of 30Awith convection coefficient of 1 W/m2 0C

PCB type 100 × 100 mm 150 × 150 mm 200 × 200 mm Circular finned Spherical finned Square finned

Minimum temperature (°C) 430.13 202.27 121.85 87.569 102.23 86.231

Maximum temperature (°C) 434.97 209.38 131.23 90.438 109.5 89.542

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conductor. However, the temperature is lower as wemove away from the track. So, the heat sensitivecomponents in the PCB should be placed away fromthe high current carrying conducting track becausethese tracks produce higher temperatures and transferheat to the nearby area on the PCB. For solving thisissue, an additional thick thermally insulative layeris placed between the temperature sensitive compo-nents and the PCB’s component side (track side).This additional layer blocks the heat transfer fromPCB to components.

& For protection of a copper track from surroundingenvironment, a thick layer is required as used innormal PCBs. But this coating also acts as a ther-mally insulating layer so that the heat generated bya component cannot be transferred to the copperbase of the PCB. For avoiding this condition, thearea where the heat generating component is placedmust be excluded from the coating. The componentand the track are on the same side, so we mustavoid printing tracks under the heat generating com-ponents. Also, it is necessary to use solder mask toavoid covering of solder pads with protectivecoating.

& Our proposed PCB has copper as the base materialand a thin film of insulating material on top of it.So, we cannot use through-hole components in thisPCB, because component leads will be shortcircuited. To avoid this condition, we should usesurface mount devices (SMD). Also, SMD compo-nents are lighter than through-hole components sothis gives more freedom of flexing to the PCB.The shape of the components also affects the free-dom of flexing. Also, the component redesign wouldgive more flexibility to the proposed PCB. This topicis beyond the scope of this paper.

& For the final preparation of the PCB after populatingthe components and connections, it is better to doconformal coating on the entire PCB excluding thebottom side. This will give more adhesive power toboth component and track to base because the PCBmay bend more than expected during installation ormaintenance. Also, it gives greater protection fromsurrounding environment.

& Installing the flexible PCB inside a cabin needssome precautions. From the results of our testingand analysis it is evident that the convection affectsthe temperature and voltage droop of the tracks. Itwill break the conducting traces in the PCB. So, wemust ensure that the PCB’s convective surface isexposed to air. But in our case, PCB may flex inany direction and this will affect the convection.During installation we must ensure that the bentflexible PCB is properly in contact with the sur-rounding air. To ensure the surrounding air flow,appropriate ventilation or other forced air-coolingsystem such as cooling fan must be provided inthe casing system.

& We recommend the use of thin film coating technol-ogy such as physical vapour deposition methods toimplement the flexible PCB because it will not useany hazardous chemical and it is an environmentfriendly fabrication process. Also, it is an additivemethod so material waste is the minimal and it willprovide better adhesive strength between the layers.Moreover, we can control the thickness of the trackin specific portions or in the entire circuit. Also, ifone or more traces are broken, we can recreate thatspecific traces using thin film coating. We have de-cided to implement this way so we can expect moreabout this in the very near future.

4 Conclusion

From the test results and analysis, the current handlingcapacity of copper conducting track is enhanced by in-creasing the area of a thermally conducting PCB. Thisis due to the increase in the convective surface area ofthe PCB in contact with air. This concept is demonstrat-ed by the testing of the PCB with varying convectionrate. For a constant current flow, the temperature of thetrack in the PCB is inversely proportional to the fold-ing. Adding fins to the PCB’s copper side increases thearea of convection and reduces temperature. This, inturn, avoids the variation in the resistance of the coppertrack and hence the voltage drop across the conductor iskept at a minimum. The copper - thin film dielectric

Table 4 Minimum and maximum voltage for different models of PCB at current flow of 30Awith convection coefficient of 1 W/m2 0C

100 × 100 mm 150 × 150 mm 200 × 200 mm Circular finned Spherical finned Square finned

Minimum voltage (v) 0 0 0 0 0 0

Maximum voltage (v) 0.13662 0.11785 0.11186 0.10785 0.10988 0.10788

J Electron Test (2019) 35:131–143 141

multilayer structure of the PCB increases the thermalconductivity and insulating layer improves the electricalperformance of copper tracks. Also, the temperature ca-pabilities of components on the PCB are improved. It isexpected that the forced convection would improve thethermal management system of the proposed models ofPCB more than the natural convection. In addition, thelow cost and eco-friendliness of the PCB materialsmake them more attractive to the industry. The physicalvapour deposition techniques for fabricating this PCBavoid the use of hazardous chemicals in the manufacturingprocess.

Publisher’s Note Springer Nature remains neutral with regard to juris-dictional claims in published maps and institutional affiliations.

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Remesh Kumar K R received B. Sc andM. Sc degree in electronics fromMahatma Gandhi University, Kottayam, India, in 2012 and 2014, respec-tively. He is now a Ph.D. candidate at Mahatma Gandhi University. Hisresearch interest includes thin film technology, printed circuit boards,power electronics, analog and digital circuit design, embedded systemdesign and computer application programming.

K. Shreekrishna Kumar is the Director of School of Technology andApplied Sciences, Mahatma Gandhi University, Kottayam, India since2005. He acquired his Bachelor and Post Graduate Degrees (MSc andMPhil) in Physics (Specialization Electronics) from MangaloreUniversity. He received his Ph. D in physics from Mahatma GandhiUniversity, Kottayam, India in 1993. He was appointed as the DirectorAll India Council for Technical Education by the Govt. of India in theyear 2010 for 4 years. His research areas are thin film science and tech-nology, solid electrolytes etc. He has authored a book on basic electronicsfor the engineering and science students of various universities in India.He was awarded (i) Eminent Educationist for the year2016 by INDO-USfoundation (ii) Best Civil Servant Golden Lotus Award for the year 2011,(iii) ANACON 98 award for the best research paper in 1998, and (iv)Merit Certificate awarded by the 1997 Indian Science Congress.

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