# Electrical Characterization of Solderless Interconnections Dedicated to Pressure Assembly Power Modules

Paul Bruyere, Eric Vagnon, Yvan Avenas

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Paul Bruyere, Eric Vagnon, Yvan Avenas. Electrical Characterization of Solderless Interconnections Dedicated to Pressure Assembly Power Modules. 2025 IEEE International Workshop on Integrated Power Packaging (IWIPP), Apr 2025, Tuscaloosa, AL, United States. 10.1109/IWIPP61784.2025.10971546. hal-05048788

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Submitted on 28 Apr 2025

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# Electrical Characterization of Solderless Interconnections Dedicated to Pressure Assembly Power Modules

Paul Bruyere<fnref n="1" />,<fnref n="2" />, Eric Vagnon<fnref n="2" />, Yvan Avenas<fnref n="1" />

***Abstract***— This paper presents the electrical characterization of solderless interconnection for the development of modular power modules with pressure assembly thanks to soft materials like metallic foams. This type of material can be used to connect two Printed Circuit Boards (PCB), and allows disassembling and changing the PCB if necessary. Electrical resistances of these soft materials are measured as a function of pressure and compared to classical interconnections like copper plates. With differences higher than 70% compared to copper plates, soft materials seem interesting for low resistance packaging interconnections.

***Keywords***—Modularity, Packaging, Copper plate, Metal foams.

## I. INTRODUCTION

Currently, power module designs are increasingly oriented to modular criteria [1] in order to support disassembly or replacement of power module parts. The objective of this modularity is to integrate the semi-conductors at a lower granularity than the classical integration technique named as Multi-Chip Module (MCM) [2]. By considering these modularity criteria, the integration technique can shift from MCM to Single Chip Module (SCM) [3]. This latter integration method lies on the development of a single prepackage containing the semi-conductor [4], a classical switching cell can then be obtained by the assembly of two prepackages. This assembly can finally be pressure-based to enable the disassembly of the active parts. Using a clamping force to form tight physical connections can also reduce stress and deal with the Coefficient of Thermal Expansion (CTE) mismatch [5]. A possible structure with Electrical Interface Materials (EIM) which meets the above criteria is shown in Fig. 1.

<figure><img src="image_2.png" /><figcaption>Fig. 1: Example of pressure-based power module assembly based on the SCM integration technique and EIM interconnection.</figcaption></figure>

A switching cell is therefore based on the assembly of prepackages with different Printed Circuit Board (PCB) named as motherboards. Electrical contacts between the prepackages and the PCB motherboards are made through the EIMs. These latter can hold on different mechanical structure types, e.g. hard or soft materials. The first one can be represented as a dry contact between two conductive parts as we can see in press-pack power modules [6], and the second one can be represented by a porous material. Whatever the type of material, a solderless interconnection is mandatory in order to enable an easily disassembly as mentioned before.

EIM based on soft materials, i.e. porous structures, can be made by compressible metallic foams. These foams were used in the past as die contact [7] or as thermal solutions [8]. They need to be investigated as power electrical interconnection. By analogy with Thermal Interface Materials (TIM), the foams allow absorbing the possible thicknesses mismatch in the power module assembly, and improving the contact quality. Indeed, as depicted in Fig. 2, thanks to the porous structure of the foam and the given mechanical deformation possibility under the pressure, the electrical contact is improved with the correction of surface roughness or planarity. A gap resulting from this roughness can be filled by the foam. Like the heat transfer can be improved with the TIM, the current repartition, i.e. the electrical contact, could be improved thanks to EIM.

<figure><img src="image_3.png" /><figcaption>Fig. 2: Comparison of hard and soft interconnections behavior against pressure in case of surface defects between two faces.</figcaption></figure>

For power electronics application, the electrical resistance of the current path must be as low as possible to reduce the overall power module conduction losses. These EIM materials being investigated as power electrical interconnection, the

<footnote n="1">¹Univ. Grenoble Alpes, CNRS, Grenoble INP, G2Elab, Grenoble, France</footnote>

<footnote n="2">²Ecole Centrale de Lyon, INSA Lyon, Université Claude Bernard Lyon 1, CNRS, Ampère, UMR5005, 69130 Ecully, France</footnote>
knowledge of the electrical properties of different types of EIM is necessary.

An experimental characterization of the electrical resistance of different types of solderless interconnections under pressure is therefore proposed in this paper. In the following section, the overall electrical characterization methodology is described with the presentation of a dedicated test bench and the different solderless interconnections. Then, experimental results are presented and analyzed in a final discussion section.

## II. ELECTRICAL CHARACTERIZATION METHODOLOGY

This section presents the overall methodology used to characterize the electrical resistance of different solderless interconnections as a function of the pressure. The first part will present the experimental test bench developed for this characterization. Then, the different solderless interconnections characterized in these works will be presented with their main characteristics.

### A. Experimental test bench

An experimental test bench developed for the electrical characterization of the interconnections is depicted in Fig. 3. This bench is composed of two different parts from the top to the bottom which represent the pressure system and the test vessel. The last part on the right-hand side of Fig. 3 represents the measurement setup used for this characterization.

<figure><img src="image_4.png" /><figcaption>Fig. 3: Synoptic diagram of the experimental test bench developed for the electrical characterization of solderless interconnection as a function of pressure.</figcaption></figure>

The pressure system is made with a screw and a spring. By tightening the screw, and thanks to the mechanical guide and the spring, a pressure can be applied on the test vessel depicted in Fig. 4. This latter is composed of two metallic jaws and the Device Under Test (DUT), i.e. the electrical interconnection material. These aluminium jaws are referred as electrodes for the electrical measurement. Thanks to their good electrical properties with low resistivity material, the electrical resistances of these jaws are in the range of few microOhms. Due to the low density of aluminium, the top jaw pressure applied on the DUT due to its own weight is negligible. The shape of the upper jaw is defined so that it can press against the DUT. These parts are aligned with a guide screwed to the lower jaw. A cavity in this guide allows the DUT to be placed with an area of approximately 1cm². Finally, the lower jaw is a simple

aluminium bloc of 50mmx40mmx30mm which is also aligned to the pressure system.

The electrical resistance of the DUT is measured thanks to a 4-wire resistance measurement system (C.A 6240 from Chauvin Arnoux) connected to both electrodes. A current injected through the DUT and the voltage drop measurement between jaws allow to obtain the electrical resistance. With this setup, the measured resistance is the sum of two contact resistances between the DUT faces and the jaws, and the bulk resistance of the DUT. Finally, the pressure applied on the test vessel is measured thanks to a strain gauge. This latter is placed between the mechanical guide of the pressure system and the upper jaw. The measurement range of this strain gauge is between 0N and 1000N.

<figure><img src="image_5.png" /><figcaption>Fig. 4: Synoptic diagram of the test vessel developed for the characterization.</figcaption></figure>

### B. Solderless interconnection materials

The different solderless interconnections characterized in this work are presented in Table 1 with their main properties. Two categories of materials can be identified, i.e. hard and soft materials. The principal difference between these two categories lies on the mechanical deformation possibility of the soft materials. In fact, due to their porous structure, these materials can be compressed under pressure. On the contrary, the only possible mechanical deformation of hard material is at its surface with the correction of surface defects, i.e. roughness and planarity.

<table><caption>Table 1: Main characteristics of solderless interconnections.</caption><thead><tr><th>Material Type</th><th>Name</th><th>Material</th><th>Initial Area [cm²]</th><th>Initial Thickness [mm]</th></tr></thead><tbody><tr><td rowspan="6">Soft</td><td>Cu Foam</td><td>Cu</td><td>1</td><td>2.5</td></tr><tr><td>Ni Foam</td><td>Ni</td><td>1</td><td>3</td></tr><tr><td>Nickel Chrome (NC) Foam</td><td>NC</td><td>1</td><td>3</td></tr><tr><td>Fuzz Button®</td><td>Au/BeCu</td><td>0.12</td><td>1</td></tr><tr><td>Thermal Spring</td><td>Sn</td><td>1</td><td>0.25</td></tr><tr><td>Hard</td><td>Cu Plate</td><td>Cu</td><td>1</td><td>1.6</td></tr></tbody></table>
Illustrations of the different interconnections are depicted in Fig. 5. All the different foams are manufactured by Recemat. Their grade, i.e. the number of Pore Per Inch (PPI), is between 47 PPI and 53 PPI with an average pore diameter of 0.4mm. The porosity of these foams, i.e. the ratio between the volume of metal and air, are 95%, 90% and 93% for respectively Ni, Nickel Chrome (NC) and Cu foams. These properties being out of the scope of these works, the specified data come from the manufacturer and are not experimentally characterized. Finally, the mechanical structures of these foams are stochastics and they are supplied in sheet form. The different 1cm² DUT patterns are therefore obtained by manual cutting.

<figure><img src="image_6.png" /><figcaption>Fig. 5: Illustrations of the different characterized interconnections (top view) and their implementation on the test vessel (front view).</figcaption></figure>

The Fuzz Button® is also considered as soft material. Its mechanical structure is composed of a Gold-Plated Beryllium Copper unique wire compressed into a cylindrical shape. Due to this final shape, the mechanical structure of the Fuzz Button® is also stochastic. This type of contact can be used as PCB interposer or packaging electrical connection. For example, Emon et al. developed a double-side SiC power module where topside die connection is made of 15 Fuzz Button®. In this work [9], the authors have not electrically characterized the resistance of this connection but used experimental data from the manufacturer. On top of that, the contacts are placed vertically to the die while in our case, the Fuzz Button® are placed horizontally to the surface of the jaws.

The Thermal Spring from Indium Corporation is a very thin sheet of tin originally developed for thermal applications. This type of contact is here investigated as electrical connection due to the quite good electrical properties of tin and mechanical structure of this sheet. Their waffle shape allows a possible mechanical deformation under the pressure. Finally, a copper plate is taken as the reference for this characterization. The contact between this plate and the jaws can be likened to a dry contact found in press-pack modules. Each copper plate used for characterization were pre-treated by sanding the plates with an abrasive sponge to remove any oxide layer.

Due to the stochastic structure of the soft materials and possible disparity between one contact to another, five different specimens are considered for each type of DUT. The experimental results of each interconnection are presented in the next section.

### III. ELECTRICAL RESISTANCE OF THE DIFFERENT SOLDERLESS INTERCONNECTIONS

#### A. Comparison of hard and soft copper interconnections

Hard and soft copper interconnections are firstly characterized with eight different force values for each DUT. For each force point, five different resistance measurements are carried out. The results obtained for these DUTs are depicted in Fig. 6.a and Fig. 6.b for respectively Cu foam and Cu plate.

<figure><img src="image_7.png" /><figcaption>Fig. 6: Experimental results of five specimens for (a) Cu foam and (b) Cu plate as DUT.</figcaption></figure>

For these two interconnections some differences between specimens can be pointed out on the entire force range. These differences can be attributed to the non-controlled mechanical structure of the specimens with different surface aspects. Taking this into account, a global average on all specimens can be computed for each force point. The result of this simplification is depicted in Fig. 7. Each point represents therefore the average of five different specimens with their associated standard deviation.
<figure><img src="image_8.png" /><figcaption>Fig. 7: Global average results on five specimens for Cu foam and Cu plate as DUT.</figcaption></figure>

The different average results $\mu$ and associated standard deviations $\sigma$ are summarized in Table 2 for both interconnections. With a ratio between standard deviation and average result of 77% and 42% for respectively Cu foam and Cu plate for the lowest force, the previous comment on the high disparity between specimens is confirmed. These ratios are reduced to 19% and 36% for the highest force. The reduction of these ratios can underline a possible modification of the contact resistances for both interconnections as the force increases.

<table><caption>Table 2: Global average results and associated standard deviations for each applied force considered Cu foam and Cu plate as DUT.</caption><thead><tr><th></th><th colspan="8">Cu Foam</th></tr></thead><tbody><tr><th>F [N]</th><td>20.0</td><td>65.0</td><td>130</td><td>220</td><td>320</td><td>500</td><td>750</td><td>970</td></tr><tr><th>μ [mΩ]</th><td>10</td><td>5.1</td><td>2.6</td><td>1.5</td><td>0.92</td><td>0.44</td><td>0.27</td><td>0.23</td></tr><tr><th>σ [mΩ]</th><td>7.7</td><td>4.0</td><td>0.90</td><td>0.36</td><td>0.21</td><td>0.078</td><td>0.035</td><td>0.038</td></tr></tbody><thead><tr><th></th><th colspan="8">Cu Plate</th></tr></thead><tbody><tr><th>F [N]</th><td>27.0</td><td>84.0</td><td>130</td><td>260</td><td>360</td><td>510</td><td>750</td><td>1020</td></tr><tr><th>μ [mΩ]</th><td>8.8</td><td>4.4</td><td>3.4</td><td>2.0</td><td>1.6</td><td>1.3</td><td>1.0</td><td>0.76</td></tr><tr><th>σ [mΩ]</th><td>3.7</td><td>1.7</td><td>1.6</td><td>1.0</td><td>0.85</td><td>0.56</td><td>0.39</td><td>0.27</td></tr></tbody></table>

Although the disparities between specimens, experimental results of Fig. 7 exhibit a decrease of the DUT resistances when the force increases. With maximum and minimum results respectively equal to 10mΩ and 0.23mΩ, a reduction of -98% can be achieved between the extreme applied forces for Cu foam resistances. The same behavior is obtained for the Cu plate with a reduction of -91%.

A comparison of these results highlights the advantage of soft interconnection over hard interconnection. Indeed, despite equivalent results near 0N, i.e. a difference of 12% for the Cu plate compared to the Cu foam, an increase of the force applied to the DUT allows to change the paradigm with a difference of

70% for the Cu foam compared to the Cu plate near 1000N. This stronger decrease of the Cu foam can be explained by the porosity of this latter with a modification of both contact and bulk resistances of this type of interconnection. More explanations about this phenomenon will be given in the discussion section.

## B. Material type modification for the soft interconnections

This subsection aims to study other types of materials and solutions for the soft interconnections. These new configurations correspond to the remaining solutions in Table 1. The global average results are given in Fig. 8 without the standard deviations for clarity of the results. All the maximum and minimum resistances of each DUT are summarized in Table 3.

<figure><img src="image_9.png" /><figcaption>Fig. 8: Global average results for all DUT.</figcaption></figure>

<table><caption>Table 3: Maximum and minimum resistances results of each DUT on the entire applied force range and associated reduction.</caption><thead><tr><th>DUT</th><th>Ni Foam</th><th>NC Foam</th><th>Thermal Spring</th><th>Fuzz Button®</th><th>Cu Plate</th><th>Cu Foam</th></tr></thead><tbody><tr><th>R<sub>max</sub> [mΩ]</th><td>1.4</td><td>13</td><td>17</td><td>1.1</td><td>8.8</td><td>10</td></tr><tr><th>R<sub>min</sub> [mΩ]</th><td>0.069</td><td>1.2</td><td>0.90</td><td>0.090</td><td>0.76</td><td>0.23</td></tr><tr><th>Reduction</th><td>-95%</td><td>-91%</td><td>-95%</td><td>-92%</td><td>-91%</td><td>-98%</td></tr><tr><th>Difference against REF for R<sub>max</sub></th><td>-84%</td><td>+48%</td><td>+93%</td><td>-88%</td><td>REF</td><td>+14%</td></tr><tr><th>Difference against REF for R<sub>min</sub></th><td>-91%</td><td>+58%</td><td>+18%</td><td>-88%</td><td>REF</td><td>-70%</td></tr></tbody></table>

The reduction calculation between the maximum and minimum resistances of each DUT confirms the sharp decrease of the resistances as the force increases for all the interconnections. As the reductions in resistances are confirmed for each DUT when applied force increases, the differences
against the reference interconnection, i.e. the Cu plate, can be computed. In regard to the maximum resistances, NC foam and Thermal Spring results are higher than the reference with difference of respectively 48% and 93%. The best candidates without any applied force are Fuzz Button® and Ni foam with results lower than the reference and associated differences of respectively 88% and 84%. On the minimum resistances, NC foam and Thermal Spring still already higher than the reference. The other interconnections have better results than the Cu plate with differences between 70% and 91%.

These latter analyses entail to consider other types of materials or solutions than copper for soft interconnections. The electrical characterizations underline, for example, that Ni behaves better than Cu for the foams. This material choice needs however to be carefully made and underlined the necessity of experimental characterization before selecting the most suitable candidate for solderless interconnection.

## IV. DISCUSSIONS

The characterization of the electrical resistance of different solderless interconnections allow to underline a decrease of the resistance as the applied force increases for all the DUT. This decrease trend can be explained for the Cu plate by a correction of surface defects as the force increases, leading therefore to a decrease of the contact resistances, i.e. the overall resistance of the DUT. The bulk resistance is also faintly modified as the copper is pressed. The resistances decrease for the soft materials are explained by their porous structures with a compression of these latter under the pressure. Although the resistances of hard and soft materials decreased as the force increases, soft interconnections results are lower than the hard ones. This phenomenon can be explained on one hand by the compression of the material and on the other hand by the better contact resistances of the soft materials with gap-filling of surface defects as described in Fig. 2.

A comparison between the results of soft materials allows to underline a better behavior of certain DUT. Indeed, by comparing the results of foams, the Ni foam exhibits the lower resistance at the extreme applied forces. Two different aspects can explain the better results of this material. Firstly, the Ni foam contact resistances could be better that the Cu foam at the maximum force due to different mechanical properties. Secondly, the electrical conductivity of these two DUT could be different. Despite the fact that the resistivity of bulk Ni is larger than the Cu one, the conductivity of porous materials is difficult to estimate. In fact, this parameter depend of many quantities, e.g. grade, porosity, ligament size [10]. These two phenomena can therefore explain the observed differences between these two foam materials.

The Fuzz Button® seems to be an interesting solution for solderless packaging interconnection with results close to the Ni foam. This solution needs however to be carefully analyzed. In fact, due to its lower initial area compared to the others DUT, the force applied being the same, the applied pressure is however different. Then, one objective of the soft solderless interconnection being to fill the possible thicknesses mismatch in packaging assembly, this property is limited for the Fuzz Button® as a reason of the lower thickness of these contacts compared to the foams. On top of that, their initial area being

also limited, multiple contacts must be used in case of large conductive area, where foams can be cut to the desired shape and surface contact.

Finally, another aspect not detailed and deeply studied in this work is related to the aging of these solderless interconnections. For example, the different Cu plates considerate were characterized before the treatment against the oxidization. The experimental results of these initial plates are depicted in Fig. 9. The results comparison at the extreme applied forces entails to consider this phenomenon with a strong increase of the resistances when an oxide layer is developed on the contact.

<figure><img src="image_10.png" /><figcaption>Fig. 9: Global average result comparison of Cu plate before and after oxidization treatment.</figcaption></figure>

## V. CONCLUSION

This paper presents an electrical characterization of solderless interconnections dedicated to pressure-based assembly power modules. These interconnections allow to develop modular power modules with non-soldered interconnections supporting disassembly criteria. Two categories of materials are characterized, hard and soft materials. The first category is equivalent to a dry contact typically found in press-pack assemblies, while the second one is represented by deformable materials with porous mechanical structure. A dedicated test bench is developed for the electrical characterization with a pressure system made of a screw and a spring. A test vessel containing the interconnection allows the resistance measurement with a 4-wire electrical system.

For all the interconnections, a sharp decrease of the electrical resistance is observed as the applied force increases, even if strong disparities between the specimens of a same interconnection are underlined, as a result of the stochastic mechanical structure of the devices. For all the applied forces, the soft materials exhibit lower electrical resistance compared to the hard ones. Taking this into account, soft materials seem interesting for packaging electrical interconnection in case of pressure-based assembly power module with high-voltage (around 1.2kV) and low on-state resistances semi-conductors. By keeping a low resistance in the power module, thicknesses mismatches filling are also possible due to the deformation properties of these materials.
Other works need to be carried out with mechanical and
structural characterizations of these interconnections to better
understand and analyze the results. The electrical resistance
behavior as function of the temperature, as well as the ageing
of the contacts, could also be interesting in case of packaging
assembly to select the most suitable candidate for a specific
application.

ACKNOWLEDGMENT

The authors would like to thank Region Auvergne-Rhônes-Alpes (TAPIR project – Pack Ambition Recherche 2021) and ANR agency (ANR-21-CE05-0037) for the financial support of the project.

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