Thermal degradation and decomposition of FR4 laminate PCB substrates joined by Friction Riveting

doi:10.21203/rs.3.rs-4930131/v1

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Thermal degradation and decomposition of FR4 laminate PCB substrates joined by Friction Riveting

https://doi.org/10.21203/rs.3.rs-4930131/v1

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Glass fiber-reinforced epoxy laminates (FR4), widely used in printed circuit board (PCB) fabrication, face challenges in joining processes due to their susceptibility to thermal degradation. Friction Riveting, a promising technique for joining FR4 substrates, offers advantages over traditional methods, but might induce thermal damage at elevated temperatures. This study investigates the thermal degradation mechanisms of FR4 laminates joined by Friction Riveting, focusing on the influence of process parameters and copper layer configuration. Microscopic cross-sectional analysis, differential scanning calorimetry (DSC), thermogravimetric analysis, Raman spectroscopy, and energy-dispersive X-ray spectroscopy (EDX) were employed to assess thermal degradation under different joining conditions. The results revealed that higher process temperatures led to increased rivet deformation and charring of FR4, especially in laminates with double copper layers. The presence of copper layers influenced the decomposition behavior, exhibiting a complex interaction between enhanced thermal stability and accelerated degradation due to increased thermal conductivity and friction. These findings highlight the importance of optimizing process parameters to mitigate thermal damage and ensure the reliability of friction-riveted FR4 joints in PCB assemblies.

thermal decomposition

epoxy resin

temperature

PCB

Friction Riveting

Glass fiber-reinforced epoxy laminates (FR4) are essential components of modern printed circuit boards (PCBs), providing the electrical insulation [1], mechanical strength, and heat resistance [2] required for a wide range of electronic devices [3]. Named by the National Electrical Manufacturers Association (NEMA, USA), FR consists of multiple layers of woven glass fabric bonded by a thermosetting epoxy resin, often incorporating bromine-based flame retardants for enhanced safety [4, 5].

Despite their numerous advantages, FR4 materials face challenges related to thermal degradation, particularly during their assembly or operation at elevated temperatures. Polanský et al. [6] highlighted that the thermal decomposition of FR4 typically occurs within the temperature range of 324°C to 400°C, leading to irreversible structural changes through the breaking of chemical bonds [7]. This process results in significant weight loss and formation of carbonized products, accompanied by the release of volatile compounds. These changes can negatively affect the physical and chemical properties of the material [8, 9]. Although the addition of halogenated flame-retardants (containing bromine or chlorine) to the FR4 epoxy matrix has been shown to mitigate some of these effects without compromising the physical properties [10], the thermal stability of FR4 remains a critical concern.

Traditional joining methods for FR4, such as soldering and press fitting, have limitations that hinder their applicability in certain scenarios [11, 12]. Although widely used, soldering can introduce thermal stresses that may damage heat-sensitive components or lead to failures in solder joints [13, 14]. Press fitting, however, requires precise tolerances and may not be reliable for joining dissimilar materials or creating complex geometries [15]. These limitations have encouraged the development of alternative joining techniques.

Friction Riveting has emerged as an attractive alternative for joining FR4 substrates in electronics because of its advantages over traditional methods, such as soldering and press fitting [14, 15]. It eliminates the need for additional materials (e.g., solder), generates less heat, and is well-suited for automated assembly of complex assemblies. However, although Friction Riveting offers significant benefits, the high temperatures involved can still pose challenges, particularly for FR4 laminates, which are susceptible to thermal degradation. These temperatures, typically between 60% and 90% of the rivet's melting point [16], have been shown to induce microstructural changes in FR4 substrates, including thermal oxidation and degradation [17, 18]. Studies have found that temperatures exceeding 300°C during Friction Riveting can negatively affect the anchoring efficiency of FR4 joints [19]. Downsizing the Friction Riveting process using thin PCBs and smaller rivet diameters has been demonstrated to alleviate thermal stress [20]. Thus, a thorough understanding of FR4's thermal behavior during Friction Riveting is essential for optimizing process parameters and ensuring the reliability and performance of such joints.

This study aimed to elucidate the thermal degradation mechanisms and stability of FR4 laminates under Friction Riveting conditions. Joints formed with FR4 and AA 2024-T3 were the focus of the present investigation. The FR4 base material and the resulting friction-riveted joints were characterized to identify degradation products and assess the extent of thermal damage at various stages of the process. These findings will contribute to the development of Friction Riveting for FR4 laminates, leading to more reliable and durable electronic products.

2.1 PCB substrates

Commercially available FR4 laminates (Shenkai Electronics, China) were used to create friction-riveted joints with AA 2024-T351 rivets (4 mm diameter). Two common FR4 laminate configurations were selected: FR4-I Cu with a single copper layer and FR4-II Cu with double copper layers, both 1.5 mm thick[1].

2.3 Friction Riveting equipment and joining conditions

Friction-riveted joints were produced using a dedicated setup of Friction Riveting laboratory equipment (Loitz Robotic, Germany) integrated with a friction welding spindle RSM410 (Harms & Wende GmbH, Germany). A pneumatic clamping system (DZF-50-25-P-A-FESTO, Islandia, NY, USA) was used to fix different FR4 laminates in overlap configuration. Due to the limited thickness of the FR4 laminates, four layers were overlapped and fixed in the clamping system to prevent the rivet from welding to the equipment worktable. However, the primary objective was to analyze joints produced with a single FR4 laminate.

The friction-riveted joints investigated were obtained using process parameters that were systematically varied at three levels, each according to a Box-Behnken design. The parameters included rotational speeds (RS) of 3000, 5000, and 7000 rpm, displacements at friction (DaF) of 2.40, 2.70, and 3.00 mm; and friction forces (FF) of 2000, 3000, and 4000 N. This resulted in joints with process temperatures ranging from 250°C to 480°C, measured on the FR4 substrate using an infrared thermography camera (High-End Camera Series Image IR, Infratech GmbH, Germany) and IRBIS-3 software, as detailed in a previous publication [19].

Joints formed with FR4-I Cu at 250°C and 360°C, representing the lower and upper limits of the observed temperature range for this configuration, were selected for further analysis to assess the deformation, anchoring, and presence of degraded areas around the rivet-FR4 interface. In addition, a joint formed with FR4-II Cu at 480°C was selected to assess the extent of thermal degradation and potential failure mechanisms under extreme processing conditions. The joining conditions are listed in Table 1.

Table 1

– Joining parameters for FR4-I Cu and FR4-II Cu joints.
Joints	FR4 material	Process temperature [ºC]	Rotational speed, RS [rpm]	Displacement at friction, DaF [mm]	Forging Force, FF [N]
C_{250 ºC}	FR4-I Cu	250	3000	2.70	2000
C_{360 ºC}	FR4-I Cu	360	5000	2.40	4000
C_{480 ºC}	FR4-II Cu	480	7000	2.70	2000

2.2 Thermal Characterization (DSC and TGA)

Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) were employed to investigate the thermal stability and degradation behavior of the FR4-I Cu and FR4-II Cu base materials[2] within the temperature range relevant to the Friction Riveting process.

A NETZSCH F2 Maia DSC instrument (NETZSCH, Germany) was used to analyze samples weighing approximately 18 mg, placed in standard sealed aluminum crucibles, and heated from 25°C to 500°C at a heating rate of 20°C min⁻¹ under a constant nitrogen gas (N₂) atmosphere at a flow rate of 50 ml min⁻¹.

Additionally, a TG 209 F3 Tarsus instrument (NETZSCH, Germany) was used for TGA on samples weighing approximately 16 mg in open alumina crucibles under N₂ (flow rate 50 ml min⁻¹) atmosphere. Samples were heated from 24°C to 550°C at different heating rates of 5, 10, and 20°C min⁻¹ to assess the influence of heating rate on thermal degradation. Derivative thermogravimetric (DTG) curves were obtained to determine the rate of mass change with respect to temperature and reveal the thermal degradation profiles of FR4.

2.4 Chemical and microstructural characterization (Raman Spectroscopy)

Raman micro-spectroscopy (RMS) was employed to obtain detailed information about the thermal stability of FR4, focusing on the degradation products formed in the joints produced with FR4-I Cu. The RMS measurements were performed using SENTERRA equipment (Bruker Optik GmbH, Germany) with a laser incident on a polished surface of FR4, distant from any copper particles to avoid fluorescence effects. The scanned regions were obtained from the thermo-mechanically affected zones (TMAZ) of the FR4-I Cu joints produced under the two conditions specified for C250 (250°C) and C360 (360°C), as shown in Table 1.

RMS measurements were performed using a 785 nm wavelength laser source at 1 mW power. The spectra were recorded at room temperature using a 50x objective (confocal aperture of 2 mm), spectral resolution in the range of 3–5 cm⁻¹, and an acquisition range of 150 cm⁻¹ to 2000 cm⁻¹. This range was selected based on the chemical characteristics of the Raman bands of the epoxy resin and its supplementary products, resulting from thermal decomposition [21]. Raman signals were acquired at a 1000 ms acquisition time. Instrument calibration was performed using a silicon wafer prior to the data acquisition. All measurements were performed in the absence of light to avoid interference. The spectra were plotted using OriginLab 2020b software package. The degradation process of the FR4 laminates was analyzed by comparing the changes in the intensities and positions of the peak spectra.

The Raman spectrum of FR4-I Cu in its neat state (i.e. the base material, as shown in Fig. 1) was used as the baseline for comparison. The Raman band assignments for the epoxy resin in this spectrum are listed in Table 2.

Table 2

–Raman band assignments for epoxy resins [22].
Raman band [cm^− 1]	Assignment
400, 640	C-O-C bending vibration of epoxy group
917, 1046, 1080	C-O- stretching of the epoxy group
1440–1460	CH₂ of CH₂ or CH₃ deformation
1610, 1649	C = C stretching vibration in the aromatic ring, corresponding to phenyl groups
1737, 1740	C = O stretching vibration associated with carbonyl groups of epoxy resin

2.5 Chemical Characterization (SEM/EDX)

Scanning Electron Microscopy (SEM) was used to visually inspect the TMAZ of the joints produced with FR4-II Cu. This was necessary because the RMS spectra of these joints exhibited interference due to fluorescence effects from the double layers of copper present in the FR4-II Cu substrate. The measurements were carried out using SEM (Quanta FEG 650, USA) coupled with an energy-dispersive X-ray spectroscopy (EDX) detector (TEAM™ EDS, EDAX, Germany). The analyses were performed at a voltage of 20 kV and working distance of 10 mm on the FR4-II Cu joint anchoring surfaces, which resulted in a process temperature of 480°C. To provide an electrically conductive thin film on the PCB for optimal imaging, samples were sputtered with gold (Q150R ES, Quorum Technologies Ltd., UK) at room temperature for 60 s at a current of 65 mA.

Cross-sections of friction-riveted FR4-I Cu joints produced at different process temperatures (250°C and 360°C) revealed distinct thermal effects, Fig. 2 (a)-(d). At 250°C, the rivet penetrated the FR4 laminate, but the temperature was insufficient to cause significant deformation or anchoring, Fig. 2(a). However, at 360°C, increased penetration and deformation of the rivet were achieved, indicating more pronounced thermal effects, Fig. 2(c). Despite these variations, both conditions exhibited overheating beneath the rivet head, resulting in the formation of black spots, which is potentially indicative of epoxy degradation, Fig. 2(b), (d).

Similarly, the FR4-II Cu joint produced at the highest process temperature of 480°C exhibited severe charring of the FR4 laminate [19]. This extensive thermal degradation, more severe than that observed in the FR4-I Cu joints, significantly impaired the anchoring efficiency of the joint, as insufficient resin remained around the rivet to provide adequate mechanical interlocking. This observation aligns with previous findings, in which high process temperatures were associated with reduced joint integrity.

To understand the origin of the black spots and charring observed in the friction-riveted joints, DSC analysis was conducted on both FR4-I Cu and FR4-II Cu base materials. The resulting DSC curves, Fig. 3, exhibit exothermic transition peaks between 300°C and 430°C. For FR4-I Cu, a peak was observed at 327°C, whereas for FR4-II Cu, the peak occurred at 329°C. These peaks indicate the onset of resin decomposition in FR4, where heat is released due to the breaking of chemical bonds in the epoxy resin structure, leading to char formation. Additionally, small peaks at approximately 410°C are attributed to the combustion processes and vaporization of low-molecular-weight products from the epoxy resin. This decomposition temperature range is consistent with the observations of Wen et al. [23] of thermal properties of epoxy resins in SF6/N2 mixtures.

TGA was employed to investigate the thermal decomposition behavior of FR4-I Cu and FR4-II Cu base materials. As shown in Fig. 4, the onset temperatures of decomposition, where significant mass loss begins, were similar for both FR4 laminates, indicating that the initial degradation process is not significantly affected by the presence of an additional copper layer.

However, the TGA curves, particularly the DTG curves, reveal distinct differences in the total mass loss and rate of mass loss between the two materials, Table 3. FR4-I Cu exhibits a greater mass loss than FR4-II Cu, suggesting that FR4-II Cu has enhanced thermal stability due to an additional copper layer. For instance, at a heating rate of 20°C min^− 1, the total mass loss for FR4-I Cu is 29.8%, while it is only 23.5% for FR4-II Cu. This additional layer is likely to facilitate heat dissipation, which may limit the heating rate within the material. A slower heating rate can delay the onset of certain decomposition reactions and reduce the overall mass loss.

The enhanced thermal stability of FR4-II Cu observed under controlled TGA conditions can be attributed to the presence of copper layers on both sides of the laminate. Copper, which is a good thermal conductor, facilitates heat dissipation, which may limit the rate of temperature increase within the material. Additionally, the copper layers may act as a barrier, hindering the escape of volatile decomposition products, and thereby influencing char yields.

Rodrigues et al. [19] examined the influence of the FR4 material configuration on the Friction Riveting process temperature and found that the presence of copper layers in FR4-II Cu significantly increased the temperature during joint formation. Specifically, joints produced with FR4-II Cu, where the rivet directly contacted the copper layer, exhibited peak temperatures up to 26% higher than those produced with FR4-I Cu. This is attributed to the higher thermal conductivity and friction of the copper with the aluminum rivet, which facilitates heat generation and transfer within the epoxy matrix. These findings highlight the essential role of copper layers in FR4-II Cu, contributing to both enhanced thermal stability under controlled conditions and elevated process temperatures during joining.

To further investigate the influence of heating rate on FR4 degradation, TGA curves were analyzed at 5, 10, and 20°C/min, see Fig. 4. The results indicate that the onset of decomposition shifts to slightly higher temperatures with increasing heating rate, indicating that the degradation process is kinetically controlled. As shown in Table 3, the onset temperature for FR4-I Cu increases from 300°C at 5°C/min to 328.2°C at 20°C/min, while for FR4-II Cu, it increases from 299.2°C to 327.1°C. This observation implies that the rapid heating inherent in Friction Riveting may delay the onset of decomposition, potentially allowing for successful joint formation before significant degradation occurs. However, the maximum mass loss rate also increased with higher heating rates, suggesting that once degradation begins, it progresses more rapidly under faster heating conditions. As shown in Table 3, the maximum rate of mass loss for FR4-I Cu increases from 9.0%/min at 5°C/min to 30.9%/min at 20°C/min, while for FR4-II Cu, it increases from 6.9%/min to 25.3%/min. This highlights the importance of carefully controlling the heating rate during Friction Riveting to balance the need for efficient joining while minimizing thermal damage.

Table 3

– Values of mass loss, onset temperature, and maximum rate of mass loss obtained from TGA and DTG analyses of FR4-I Cu and FR4-II Cu.
FR4 material configuration	Heating rate [°C.min^− 1]	Mass loss [%]	Onset Temp [°C]	Max. rate of mass loss [%/min]
FR4-I Cu	5	30.4	300	9.0
FR4-I Cu	10	29.6	313.6	19.4
FR4-I Cu	20	29.8	328.2	30.9
FR4-II Cu	5	22.3	299.2	6.9
FR4-II Cu	10	23.3	312.2	14.3
FR4-II Cu	20	23.5	327.1	25.3

To assess the impact of Friction Riveting on the chemical structure of the FR4 laminate, RMS was employed to analyze cross-sections of FR4-I Cu joints produced at 250°C (C_{250 ºC}) and 360°C (C_{360 ºC}). These temperatures correspond to the conditions in which black spots, indicative of potential epoxy degradation, were observed in Fig. 2(b), (d). In Fig. 5(a), (b) the Raman spectra reveal similar peaks for both cross-sections of C250 and C360. However, variations in the intensities of peaks associated with vibrations and deformations of the epoxy ring or groups were observed. In particular, the intensities of the peaks at 917 cm⁻¹ and 1649 cm⁻¹, corresponding to stretching of the epoxy and phenyl groups, respectively, gradually weakened with increasing temperature. In addition, the characteristic peak at 600 cm⁻¹, assigned to the C-O-C bending vibration of the epoxy group, disappeared completely at 360°C. Considering that Raman peak intensity correlates with substance concentration [24, 25], the decrease in epoxy-related peak intensities, along with the disappearance of the C-O-C bending peak, suggests progressive degradation and charring of the epoxy resin between 250°C and 360°C. This observation is consistent with the significant mass loss observed in the TGA analysis, see Fig. 4.

To investigate the effect of temperature on friction-riveted FR4-II Cu joints, EDX analyses were performed on two different areas of the joint produced at 480°C. The first area, shown in Fig. 6(a), was not directly exposed to the peak temperature, whereas the second area, shown in Fig. 6(c), was exposed to 480°C and exhibited residual char morphology. The corresponding EDX spectra, Fig. 6(b),(d), showed significant compositional differences between the two regions. In the region not exposed to 480°C, the presence of bromine (Br) was detected, probably originating from the flame-retardant compounds used in FR4 laminates. However, no Br was detected in the area exposed to the peak temperature of 480°C. This suggests the potential volatilization of flame-retardant products at high temperatures, a phenomenon commonly observed in the thermal decomposition of epoxy polymers [26]. This loss of flame-retardants could compromise the fire safety of the joint, particularly under prolonged exposure to high temperatures.

Furthermore, the increased carbon (C) peak intensity in the high-temperature zone indicates significant carbonization of the epoxy resin [27, 28]. This carbonization process, which results from the breakdown of the molecular structure of epoxy, leads to embrittlement and weakening of the resin matrix. The loss of resin integrity not only reduces the interfacial adhesion between the rivet and FR4 substrate, but also compromises the overall mechanical strength of the joint.

This study provides a comprehensive analysis of the thermal degradation mechanisms of FR4 laminate PCB sheets during Friction Riveting using AA2024 rivets. The results demonstrated that the process temperature significantly influenced the extent of thermal degradation and the resulting joint integrity.

Thermal characterization using DSC and TGA revealed distinct differences in the thermal stability and decomposition behavior of FR4-I Cu and FR4-II Cu, with FR4-II Cu exhibiting enhanced thermal stability due to an additional copper layer. However, the higher thermal conductivity of FR4-II Cu also leads to increased peak temperatures during Friction Riveting, thereby accelerating its overall decomposition rate.

Raman spectroscopy analysis confirmed the progressive destruction of epoxy chains at elevated temperatures, contributing to the observed charring and mass loss. This degradation process is further evidenced by the disappearance of the characteristic C-O-C bending peak in the Raman spectra of FR4-I Cu joints produced at 360°C. Furthermore, EDX analysis of the FR4-II Cu joint produced at 480°C revealed the absence of bromine in the region exposed to the peak temperature, suggesting complete volatilization of flame-retardant compounds. Conversely, the presence of bromine in the region not directly exposed to 480°C indicated that flame-retardants remained intact in the areas subjected to lower temperatures. This finding highlights the complex interaction between the temperature and release of flame retardants during Friction Riveting.

Overall, this study provides insights into the thermal behavior of FR4 laminates during Friction Riveting, emphasizing the need to consider specific FR4 configurations and process parameters when designing and optimizing this joining technique. The findings presented here contribute to a deeper understanding of the degradation mechanisms of FR4, which can assist in the development of more robust and reliable friction-riveted joints for electronic applications.

Acknowledgments

We acknowledge the use of the Raman microspectroscopy SENTERRA equipment, (Bruker Optik GmbH, Germany) from PETRA III synchrotron facility at the Deutsches Elektronen-Synchrotron DESY in Hamburg, Germany, as well as their contributions to this study.

Funding

This work was supported by the Hereon Technology Transfer Fund in cooperation with Panasonic Industrial Devices Europe GmbH within the “FricBoard” project.

Competing interests

The authors have no relevant financial or non-financial interests to disclose.

Author contributions

All the authors contributed to the study's conception and design. Camila F. Rodrigues and Lucian Blaga performed material preparation, data collection, and analysis. Camila F. Rodrigues wrote the first draft of the manuscript, and all the authors commented on previous versions of the manuscript. All the authors read and approved the final manuscript.

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To be more precise, FR4-I Cu has only one copper layer on the top surface, where FR4-II Cu has a copper layer on top and bottom surfaces.
Samples for both DSC and TGA were small sections cut from the bulk FR4-I Cu and FR4-II laminates.

No competing interests reported.

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Thermal degradation and decomposition of FR4 laminate PCB substrates joined by Friction Riveting

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Abstract

Figures

1. INTRODUCTION

2. Material and methods

2.1 PCB substrates

2.3 Friction Riveting equipment and joining conditions

2.2 Thermal Characterization (DSC and TGA)

2.4 Chemical and microstructural characterization (Raman Spectroscopy)

2.5 Chemical Characterization (SEM/EDX)

3. Results and discussion

4. Conclusions

Declarations

References

Footnotes

Additional Declarations

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