Bonding of SMD components with light-curing adhesives by radiation transmission. Part 1

Transcription

1 Gummi Fasern Kunststoffe, No. 11, 2001, pp. 728 Bonding of SMD components with light-curing adhesives by radiation transmission. Part 1 G.W. Ehrenstein Chair of Plastics Technology, Erlangen-Nuremberg University Selected from International Polymer Science and Technology, 29, No. 1, 2002, reference GK 01/11/728; transl. serial no Translation submitted by C. Hinchliffe INTRODUCTION In electronics production, bonding by the application of light-curing epoxy resin based adhesives is a rapid and variable alternative to conventional bonding techniques [1, 2]. It may, for example, be necessary to bond SMD components to a plastic substrate to prevent them from sliding off inclined surfaces when producing threedimensional circuit substrates using 3D-MID technology [3]. In the field of flexible circuit substrates, this method may also be used to fit SMD components in a non-positive way to avoid damage to the soldered joint in the event of deformation of the circuit board foil. Until now, conventional epoxy-based resins have been used in flipchip technology or for bonding housings or coatings (eg for smart cards). In particular, with SMD bonds, it is possible to apply procedures such as radiation transmission or preactivation to reduce the time of the adhesive process and for the simultaneous bonding of SMD components with the application of light-curing adhesives. For both procedures, we have identified the basic relationships between the curing process or degree of cure of the adhesive applied and the process parameters relevant to metering and assembly. Here, the objective was to achieve reproducible high-quality adhesive strengths with process times similar to those used in actual production processes. The following article deals with radiation transmission technology, a subsequent article (Adhesion in Electronics Part 2) will deal with the preactivation process. 1 BONDING SMD COMPONENTS BY RADIATION TRANSMISSION Combining the processes of assembly and bonding reduces the time required for the firm bonding of SMD electronic components. The substrate to which an SMD component is to be fixed may be exposed to radiation to cure the droplets of adhesive applied to the substrate (Figure 1). This requires the substrate to be sufficiently permeable to the radiation used. The majority of plastics used in electronics production are not very transparent in the UV radiation range. Therefore, in addition to UV curing adhesives, increasing use is being made of resin systems with photoinitiators which absorb blue light. Absorption is inhibited by structured conductors applied to the circuit substrate before assembly or fillers in the substrate. Figure 1 Schematic diagram of the radiation transmission process International Polymer Science and Technology, Vol. 29, No. 5, 2002 T/5

2 Due to the hardening of the external layer irradiated first, the higher absorption of UV-curing adhesives results in the formation of a skin on the surface, while with blue light, which is absorbed to a lesser degree, curing is uniformly distributed throughout the droplets [3]. Within this EP resin class of adhesive, in addition to a general suitability for electronics (good insulating properties, good heat resistance), the light-curing epoxy resins in cationic polymerisation are characterised by living polymerisation, ie they are also suitable for bonding shadow zones, as once started, polymerisation continues until the adhesive is completely cured, even without direct exposure to light. thick) and ABS (Novodur P2MC made by Bayer AG), as an amorphous thermoplastic, were also included in the investigations for purposes of comparison. 2.3 SMD components The SMD electronic components for bonding onto substrates were SO16 dummies based on PA66 (made by Topline, Wolfratshausen), with a bonding surface of 10 x 7 mm, weighing 0.42 g. Before bonding, the themoplastic substrates and the SMD components were cleaned with isopropanol and then rinsed with ultrapure water and dried. 2 MATERIALS AND TEST PIECE PREPARATION 3 INVESTIGATIVE METHODS 2.1 Light-curing epoxy resin The adhesive used was a cationic curing epoxy resin from the series Katiobond 4557 (made by Delo, Landsberg). The maximum tensile strength when fully cured was 9.9 N/mm 2, the modulus of elasticity was 120 N/mm 2, the elongation at break was 18%. The photoinitiator in the adhesive, which absorbed in the 400 nm-550 nm range (violet, blue, green to yellow-green) was matched to the lamp installed on the bonding machine, a Delo Lux 05 lowpressure mercury halide radiator. The maximum intensity of the radiator was 120 mw/cm Choice of substrate and preparation For the preparation of the substrates for the bonding, we produced flat thermoplastic plates (dimensions 125 x 35 x 1-3 mm) from PC (natural and 0.3% blue), PBT, PA6 and PA6/6T-GF by means of injection moulding. PC (Makrolon 2800 made by Bayer AG, Leverkusen) was used as an extremely transparent comparative material, which was also modified with the addition 0.3% blue pigment to determine of the influence of the pigment concentration on transmission. PBT (Ultradur B4500 made by BASF AG, Ludwigshafen) is increasingly used in the electronics industry, PA6 (Ultramid B3SK, BASF AG) is a conventional housing material, while the partially aromatic, 25% glass-fibre reinforced PA6/6T=GF25 (Ultramid TKR 4355, BASF AG) is used specially for MID applications. We also investigated an extruded 100 µm thick polyether etherketone film (Victrex PEEK TM 380 G, made by Victrex, Hofheim) and 3 mm thick plates made of COC/TOPAS 8007/X10 (cylcoolefin copolymer, made by Ticona, Frankfurt am Main), a material with good electrical insulating properties and high heat resistance. Finally, the traditional printed-circuit board material, FR4, (glass-fibre reinforced epoxy resin, 1.5 mm 3.1 Measuring transmission The investigations into the transmission of the plastic substrates were measured on a Lambda 18 UV/VIS spectrometer (made by Perkin-Elmer, Langen). A monochromator varied the light emitted from a radiation source successively in largely monochromatic radiation with wavelengths of from 200 nm to 900 nm. This light was guided through the test piece to a detector which determined the intensity of the light passing through to provide a spectrum of the transmission in relation to the wavelength. In order to include the radiation which passed through the test piece but was scattered, the spectrometer was also fitted with an Ulbricht sphere, which focussed the scattered light onto the detector as well. 3.2 Measuring temperature evolution during curing In order to measure the influence of radiation-induced, or to a lesser degree reaction-induced, temperature evolution on adhesive curing, a parallel series of investigations were performed in which the temperature changes at the bond area were measured using an embedded thermocouple during the radiation transmission (Figure 2). 3.3 Determination of adhesive curing The degree of curing of the adhesive was determined thermoanalytically from the residual enthalpy by means of DSC. A Photo-DSC 7 device (Perkin-Elmer, Xenon lamp spectrum) was first used to determine the overall enthalpy. For this, an isothermal Photo-DSC measurement was performed at 80 C. As the adhesive was not necessarily completely cured after this, it was then heated up again. The total of the enthalpies obtained from the two T/6 International Polymer Science and Technology, Vol. 29, No. 5, 2002

3 contact angle measuring system made by Dataphysics, Filterstadt) and using the following equation of state in accordance with Neumann et al converted to surface tension σ B [6]: Figure 2 Temperature measurements in adhesive during curing measurements corresponds to the overall enthalpy. In order to determine the degree of curing in adhesives from bonding experiments which had already been irradiated, these were only subject to one heating cycle and the residual enthalpy determined in this way compared to the overall enthalpy. 3.4 Surface preparation The surface preparation of SMD components to improve the adhesive strength was performed in a Plasma Prozessor 440 low-pressure plasma chamber (made by Technics- Plasma, Kirchheim). The process gas used to generate the plasma was oxygen (2.5 mbar); the plasma energy was 500 W in all cases. The objective of the preparation was to increase the adhesive strength of the bonded joint by improving the wettability of the SMD components [5]. 3.5 Adhesive energy The adhesion on bonding was determined from the surface tensions of the SMD components and the adhesive and the interfacial surface tensions between the two. Thermodynamically, the adhesion of a bonded joint is characterised by the adhesive energy W A which needs to be expended to separate the two joined parts. This is described by the Dupre equation: W A = σ B + σ Kl - γ BKl (1) According to this, the adhesion is good if the surface tensions of the bonded SMD component σ B and adhesive σ Kl have high values. At the same time, the interfacial surface tension γ BKL between the two should be low. It disappears when the surface tensions of the substrate and adhesive are equal, ie σ B /σ Kl = 1. To determine σ B, the SMD components to be bonded were wetted with drops of two different test liquids (formamide with σ Fo = 58.4 mn/m and glycerine with σ Gl = 62.5 mn/m). When the test liquid was added continuously, the contact angle dynamically formed on the horizontal drop θ was determined with the assistance of a computer (ADSA σ 2 B ( ( σb σtestfl ) ) cosθ = 1+ 2 e (2) σ Testfl. Testfl = Test liquid The surface tension of the liquid adhesive σ Kl was determined on the same apparatus by means of computerassisted contour analysis on a meniscus (lamella), which forms when a metal hemisphere immersed in the adhesive is withdrawn. The interfacial surface tension γ BKl may also be determined directly from σ B and σ Kl using a numerical procedure developed by Neumann et al (3) [6]. γ = σ + σ 2 σ σ 2 ( ( σb σkl) BKl Kl B B Kl e ) (3) σ B, σ Kl and γ BKl were used to estimate the adhesion energy (W A in equation 1) as a measure for adhesion and this was compared to the adhesive strength determined in a compressive shear test. 3.6 Microscopic characterisation of bonded joints The bonded SMD components were investigated following a compressive shear test to determine their surface topography and the fracture chromatograph both under the microscope and in a scanning electron microscope. 3.7 Measuring adhesive strength To evaluate the adhesive strength on the plastic substrates for SMD components, we recorded the force necessary to induce the failure of the bonded joint. For this, the substrates were firmly clamped, vertically due to the design of the apparatus, in a universal tensile testing machine (UPM 1464 made by Zwick). A sliding metal ram was used to exert a lateral force and the shear strength of the SMD component determined along the plane of the substrate (shear rate: 1 mm/s). The adhesive strength (unit: N/mm 2 ) between the SMD component and the plastic substrate was determined from the maximum force which generally occurred directly before the failure of the bonded joint and the bonded surface of the SMD component. 4 ASSEMBLING AND BONDING MACHINE To convert the bonding of SMD components by radiation transmission into a practical procedure, an existing metering system (KD x-y-z metering unit made by Panacol-Elosol) was expanded by an assembly system. The metering and assembly devices were mounted on a International Polymer Science and Technology, Vol. 29, No. 5, 2002 T/7

4 sliding unit which could be moved over the substrate in directions x, y and z. To apply the substrate, the feed head was moved over the substrate and then the adhesive metered pneumatically (Figure 3 (1)). In the next stage, the SMD component to be fitted, which was located next to the substrate on a base, was picked up pneumatically by the component mounting device and precisely positioned on the bonding point (Figure 3 (2)). As the mounting head is fitted with a spring mechanism, deposition could be performed under a slight pressure in order to ensure firm bonding. The light exposure used during assembly to cure the adhesive was passed through the substrate from below (Figure 3 (3)). To facilitate this, the substrate was placed on a plate, in which a channel covered with a glass plate had been milled, through which the light reached the substrate through a mirror system. The process times shown in Figure 3 are due to the low travel times and the accompanying analysis of the process and materials. Industrial PCB assembly machines can assemble from 2-30 items a second. 5 RESULTS OF THE INVESTIGATIONS 5.1 Substrate transmission The wavelength-dependent transmission rates determined by UV/VIS spectrometry on the substrates are shown in Figure 4. As a general rule, the spectra of the thermoplastic substrates reveal a clear increase in transmission from short to long wavelengths. From 300 nm to 350 nm, highly transmitting substrates have high transmission rates. At the wavelength of blue light (approximately 400 nm to 500 nm), the conditions are relatively good for transmission. Non-coloured polycarbonate is apparently highly transmitting. Coloration with blue pigment (PC 0.3%) reduced absorption not only in the complementary yellow range, but also probably due to internal scatter effects in the rest of the spectrum. Due to their molecularly inducted absorption, poorly transmitting substrates include aromatic thermoplastics such as PBT and in particular (1) Start of process (2) After 19 s (3) Process end after 49 s Figure 3 Process of the radiation transmission procedure with examples of process times Figure 4 UV/VIS transmission spectra of relevant substrate materials. Left: highly transmitting substrate materials; right: poorly transmitting substrate materials T/8 International Polymer Science and Technology, Vol. 29, No. 5, 2002

5 PEEK (note the 100 µm foil thickness compared to the 1 mm thick substrate). Amorphous ABS on the other hand has comparatively good transmission. By means of an example, we also included in the spectra the materials COC/TOPAS and FR4 commonly used in the electronics industry. The highly transparent TOPAS achieved similar values to those of polycarbonate, while FR4 only had transmission rates of between 8% and 10% for wavelengths between 400 nm and 450 nm. For more detailed investigations, we selected the MID relevant materials PC (0.3% blue), PA6 and PEEK as materials for the production of flexible circuit substrates. 5.2 Bonding in the radiation transmission procedure In order to evaluate the bonding process, the degree of curing of the adhesive in relation to the transmission period was determined (Figure 5 left). The high degrees of hardness obtained with when using radiation transmission on the comparatively poorly transmitting PEEK are striking. The good curing cannot therefore be solely due to the substrate s transmission properties. Figure 5 right shows the temperature profile in the adhesive; there is a clear rise during the irradiation period. With PEEK, in addition to the exothermic characteristics of the adhesive curing, the heat formed in the substrate mainly by the curing of the adhesive results in a temperature increase to approximately 125 C, thus achieving an extreme acceleration of curing and explaining the high degrees of curing. With the other substrates as well, the heat evolution is sufficient to achieve complete curing of the adhesive after irradiation periods of s. The investigations show that the choice of the substrate to subject to radiation transmission should not be based on transmission properties alone, but also on the radiationinduced heat evolution. 5.3 Adhesive strengths of bonded joints as used in practice Preliminary investigations in bonding S016 SMD components revealed that failure in compressive shear tests often occurs between the adhesive film and the SMD component. In order to increase the adhesive strength at this point, the SMD components were plasma-treated before bonding. As an example for radiation transmission bonding on PC substrates (0.3% blue), Figure 6 left shows the compressive shear strength of untreated and plasmatreated SMD components. We can see that the adhesive strength increases with longer transmission times and is much higher in the plasma-treated SMD components. To keep the process times low, a standardised transmission period of 15 s was chosen for further bonds. Figure 6 shows that adhesive strength may also be improved by prolonging the duration of the preliminary plasma treatment. The reason for the improved adhesive strength is the increase surface tension of the SMD surface as a result of the O 2 plasma treatment (500 W plasma energy for 5 minutes). Figure 7 left shows the surface tensions calculated from the contact angles with the test liquids (glycerine, formamide) for the SMD components cleaned with isopropanol and ultrapure water after different durations of plasma treatment. When the SMD components were not cleaned, no significant energy could be measured on the surface, as the contact angle underwent extreme fluctuations. Figure 7 right shows the correlation between the Figure 5 Degree of hardening and temperature evolution in the adhesive. Left: relationship between degree of hardening and transmission time. Right: temperature profiles at the bonding point during curing International Polymer Science and Technology, Vol. 29, No. 5, 2002 T/9

6 Figure 6 Compressive shear strengths of bonded SMD components. Left: adhesive strengths of SMD components on PC 0.3% blue with and without plasma treatment. Right: adhesive strengths of SMD components on PC 0.3% blue after different lengths of plasma treatment Figure 7 Adhesion properties of plasma-treated SMD components. Left: surface tension of SMD components in relation to the duration of plasma treatment. Right: compression shear strength of SMD elements bonded to PC (0.3% blue) in correlation to adhesive energy (surface tension of the adhesive, measured in liquid state: 38.1 mn(m) numerically determined adhesion energy for the adhesive/ SMD component and the mechanically measured compressive shear strengths. In Figure 7 left, we can see that the surface tension or wettability of the SMD components may be significantly increased by the plasma treatment resulting in an improvement to the bond strength between the adhesive and SMD component. Supplementary investigations revealed that the influence of the plasma treatment only subsides slowly as a result of recombination of the plasma-activated surface with the surrounding atmosphere. Figure 8 shows the surface tension of the SMD components at different times. We can see that after 5-minutes plasma treatment and a subsequent residence of the SMD components in the plasma chamber (O 2 ambient pressure approximately 0.1 mbar), the surface tension falls slightly as the residence time increases. In addition to the measurements of adhesive strength and bond strength, macroscopic and raster electron microscope (REM) photographs on bonded SMD components revealed that the test pieces subjected to plasma treatment for 5 minutes underwent a somewhat different type of failure (Figure 9). Compared to the untreated test pieces, in the pictures on the right-hand side of the photograph showing plasma-treated surfaces of SMD components, we can clearly see the adhesion of remnants of adhesive. At a given adhesive tensile strength σ zb of 9.9 N/mm 2 T/10 International Polymer Science and Technology, Vol. 29, No. 5, 2002

7 6 CONCLUSION Figure 8 Relationship between surface tension and recombination time (log-scaling) after 5-minutes of plasma treatment (according to the manufacturer), in accordance with the deformation hypothesis (HMH), theoretically we can assume the maximum shear strength to be τ B = 0.68 σ zb = 6.7 N/mm 2 [7]. The maximum values shown in Figures 6 and 7 are close to this value, so that shear failure in the adhesive in the event of incomplete curing (82% degree of cure with a transmission time of 15 s, cf Figure 5) is understandable. The investigations into the bonding of SMD components using light-curing epoxy resins revealed that radiation transmission curing requires the materials chosen with regard to transmission to be within a wavelength range of between 400 nm and 550 nm to initiate the polymerisation of the adhesive. The absorption-induced heat evolution observed in certain substrate materials may be selectively used to accelerate curing. Therefore, substrates which let through sufficient light to initiate curing and forward the absorbed radiation as heat to the bond area appear suitable for this procedure. The strength of the bond depends on the transmission time, but may also be significantly increased by preliminary plasma treatment of the SMD component. ACKNOWLEDGEMENT This article is based on numerous individual investigations which were carefully checked and evaluated to reach an informative conclusion. I am particularly grateful for the preparatory work by Dipl-Phys. N. Hallschmid, Dipl-Ing X. Nie and Dr E. Bittmann. The German Research Association generously sponsored the work as part of research project SFB 356 Production Systems in Electrical Engineering. We are extremely grateful to them. Figure 9 Photographs of the surfaces of bonded S016 SMD components after shearing (arrows indicate remnants of adhesives). Top left: surface without plasma treatment (macroscope). Top right: surface with plasma treatment (macroscope). Bottom left: without plasma treatment (REM). Bottom right: with plasma treatment (REM) International Polymer Science and Technology, Vol. 29, No. 5, 2002 T/11

8 REFERENCES 1. Marx, P., Schutz and Trutz: The use of lighthardening adhesives in electronics. KEM 9 (1996), pp Messmer, R, Riege R.: Light-hardening adhesives with new possibilities. Adhäsion 37 (1993), pp Leopold D.: Bonding in electronics. Kleben & Dichten 39 (1995) pp Bittmann, E., Ehrenstein, G.W.: UV curing epoxy resins into microchip covering, Plaste und Kautschuk 41 (1994), pp Gleich, H: Improving wettability by plasma treatment. Kleben & Dichten 33 (1989), pp Neumann, A.W.: Spelt, J.K. Applied Surface Thermodynamics Marcel Dekker New York Ehrenstein, G.W.: Constructing with adhesives. 2 nd edition. Carl Hanser Verlag, Munich, 2001 (No date given) T/12 International Polymer Science and Technology, Vol. 29, No. 5, 2002