BGA Solder Balls Formation by Induction Heating

Transcription

1 International Journal of Scientific Research in Knowledge, 2(1), pp , 2014 Available online at ISSN: ; 2014 IJSRPUB Full Length Research Paper Vladimir L. Lanin*, Ivan I. Sergachou, Vitaly T. Khotskin State University of Informatics and Radioelectronics of Belarus, P. Brovki 6, Minsk, Belarus *Corresponding Author: Received 01 November 2013; Accepted 10 December 2013 Abstract. Induction devices with open magnetic circuit are efficient for forming solder balls from solder paste on contact pads PCB. Local distribution of eddy currents in solder balls it is possible to provide by optimize frequency and amplitude of the excitation current in inductor. Realization of the high-speed induction heating solder balls in frequencies range from 500 to 1000 khz are suitable in the magnetic circuit gap of inductor device. Keywords: Induction Heating, Solder Balls, BGA 1. INTRODUCTION BGA package are widely used in a modern radio electronic apparatus, such, as mobile telephones, smartphones, computers and other, integral microcircuits, that allows considerably saving a place on PCB and promoting the fast-acting of electronic circuits. For soldering of BGA package on the PCB it is necessary to form the matrix of solder ball pins (Li, 2006). Induction heating by the high-frequency electromagnetic field has long been successfully used in the industry, as it allows implementing a highperformance non-contact and local heating due to eddy currents induced in conducting materials. The problems of increase of power indexes and efficiency of application are actual for all induction heating devices. The multiple-turn solenoid type inductors are characterized by considerable dispersion of magnetic stream, low efficiency, necessity of electric and thermal isolation from the heated details and aquatic cooling in work. Application of induction devices on magnetic circuit allows to promote locality of heating, bring down a watts-in, to refuse from the aquatic cooling and thermal isolation from the heated details (Li, 2008). The local melt phenomenon in solder balls induced by induction heating can be used to control the shape of solder joins and obtain hourglass-shaped solder joints (Xu, 2009). At presence of the magnetic circuit induction devices has a much smaller magnetic stream in surrounding space and, accordingly, less losses substantially, and also ecologically safe for staff as 22 compared to inductors without the magnetic system. At the same time magnetic circuit plays the role of magnetic stream concentrator and allows localizing heating in solder balls. Induction device with open magnetic circuit used for soldering of coaxial cable to connector at power of heating 250 W and time of soldering 2,5 3,0 s and wires to the electronic module at power 190 W. At the simultaneous soldering of three wires to PCB a heat focuses so that an isolation on wires is not melted (Lanin, 2012). 2. MODELING THE DISTRIBUTION OF THE ELECTROMAGNETIC FIELDS For effective work of the induction system with magnetic circuit it is necessary to optimize frequency and amplitude of drive current in a winding, and also to get eddy currents distribution in heated solder balls. For this purpose modern modeling which based on methodology of eventual elements is used. A method is based on approximation of continuous function (in physical interpretation: temperature, pressure, moving etc.) by discrete model, that is built on the great number of piece-continuous functions, certain on the eventual elements. The investigated geometrical area is broken up on finite elements so that on each of them an unknown function is approximated by a polynomial. The modeling system ANSOFT MAXWELL allows the calculation of the harmonic electromagnetic and electric fields, and also transients. The presence of calculation module of eddy currents

2 Lanin et al. distribution allows to modeling the work of induction heating devices. Methodology of modeling of electromagnetic fields distribution includes: creation of geometrical model, task of material properties, source of excitation, border terms, tuning of options of calculation and net, decision of task of distribution and analysis of results. In ANSOFT MAXWELL the geometrical model of induction device (Figure 1, a) included next component parts: 1. Magnetic circuit is ferrite F- 86, the properties of which were selected from the ANSOFT MAXWELL library. 2. Excitation coils of N=25, winded on the magnetic circuit and realized in models as two hollow cylinders with the thickness of walls, to the equal height of winding. Applied to them current excitations is given in the section of the cylinder indicating the amp / turns and directions. 3. Border terms: the field of Н is continuously at crossing of solder ball borders; the Neumann condition on the boundary of modeling field H does not cross the boundary of the simulation. When the parameters of construction in SOLID WORKS are set and breaking up of model is produced on finite elements (Figure1,b). a b Fig. 1: Induction device (a) and finite element model (b): 1 magnetic circuit, 2 PCB with ball grid array, 3 excitation coils ANSOF MAXWELL is based on fundamental Maxwell equations of electromagnetic field. Skin effect is significantly affects at the efficiency of the induction heating. Skin depth is determined by the distance, at which value of eddy currents reduce to 1/e from original value: (1) where ρ specific electrical resistance, µ magnetic permeability, µ r relative magnetic permeability, ω angular frequency, f frequency. The surface density of the active power, released in the metal for the period T is given by (Nemkov, 1998): ( ) ( ) ( ) (2) where E, H intensity of electric and magnetic fields, respectively, I eddy current, W 0 winding density. Convectional heat exchange in melted balls wasn t taken into account in model because it is not essential. At the same time the latent heat of solder phase transition and isotropy of materials was taken into account. Equation for calculating the temperature field is given by: ( ) (3) where ρ material density, С р specific heat, λ thermal conductivity, q energy density which is generated by eddy currents. Boundary condition for the heated surfaces: ( ), (4) where β density of thermal flow, Т and Т 0 surface temperature and environment temperature. As a result of modeling should be received density distribution of the eddy currents for induction heating different materials and thickness of details in wide frequency range 500 khz -1 MHz. For the induction heating of Sn - Pb solder balls by a diameter 0,8 mm on PCB from the fiberglass FR4 in frequencies range of 500 khz 1 MHz and 2 A drive current was achieved the distribution of magnetic-field and eddy currents in solder balls. Results evidently show the role of magnetic circuit as a concentrator of electromagnetic energy, as the basic part of energy of the electromagnetic field is concentrated in area of gap (Figure 2). The small losses of electromagnetic energy are observed in the bends of magnetic circuit, therefore in the construction of magnetic circuit it is necessary to envisage rounding of direct corners for losses minimization. 23

3 International Journal of Scientific Research in Knowledge, 2(1), pp , 2014 Fig. 2: Distribution of magnetic-field intensity on frequency 800 khz. 3. EXPERIMENTAL RESULTS AND DISCUSSION Density distribution of the eddy currents for induction heating copper and steel plates was obtained with the following conditions: thickness 1mm, overlap factor is 4, excitation coils current 4 8 A, frequency range 500 khz 1 MHz. The density of eddy currents increases with the height of frequency, as a skin-effect affects stronger. The increase of frequency higher than 950 khz does not result in the substantial increase of efficiency of heating. It is possible to manage speed of heating, changing the size of current in excitation winding. The analysis of dependences (Figure 3, 4) shows that the highest power of heating is characteristic for metals with the highest conductivity, i.e. copper, because density of eddy currents in 3 times above. Fig. 3: Eddy currents density vs. frequency and current for a copper. Fig. 4: Eddy currents density vs. frequency and current for steel. With the increase of frequency there is expulsing of eddy currents to the surface of solder ball and increase of numeral value of density of current (Table 1). The results of eddy currents distribution in solder balls confirm the presence of skin-effect on high frequencies (Figure 5). 24

4 Lanin et al. Table 1: Density of skin-currents vs. frequency Frequency, khz Density of skin-currents, А/m , , , , , , Thus, frequencies in a range from 500 to 1000 khz are suitable for realization of the induction soldering in the gap of magnetic circuit. According to the figure 5 the depth of eddy currents penetration is 0,1 mm, that allows to use frequencies more than 800 khz to control the geometry of the soldered connection. a b Fig. 5: Distribution of eddy currents in solder balls on frequencies: a khz, b khz In this case, the superficial layer of solder ball will be melted and form the soldered connection, and a hard kernel in a center will support BGA package on necessary distance from the PCB. The original appearance of solder balls on the copper pad with a diameter 400 µm is shown on figure 6,a and array of solder balls on figure 6,b. Time of solder balls formation of the set sizes by induction heating from soldering paste equally s. a b Fig. 6: Appearance of solder ball on pad (a) and array of solder balls (b). 4. CONCLUSION Thus, for the efficient management of power and speed of induction heating is necessary to control the size of current power and speed of the induction heating in working winds, and it is possible to carry out the general or local superficial heating by the frequency change of the supply current. Electoral character of the induction heating of conducting 25 materials allows realizing the process of soldering of BGA packages on a PCB without overheating of packages. By means of the induction heating it is possible to carry out forming of solder balls array on PCB pads from soldering paste. By modeling in ANSOFT MAXWELL received density distribution of eddy currents for induction heating different materials and thickness in wide frequency range. Frequencies in a range from 500 to 1000 khz are

5 International Journal of Scientific Research in Knowledge, 2(1), pp , 2014 suitable for realization of the high-speed induction heating solder balls in the gap of magnetic circuit of inductor device. REFERENCES Lanin VL (2012). High-Frequency Heating for Soldering in Electronics. Circuits and Systems, 3: (2012). Lanin VL, Sergachev II (2012). Induction Devices for Assembly Soldering in Electronics. Surface Engineering and Applied Electrochemistry, 4: Li NC (2006). Reflow Soldering Processes and Troubleshooting: SMT, BGA, CSP and Flip- Chip Technology. Boston: Newnes (2006). Li M, Xu H, Lee SWR, Kim J, Kim D (2008). Eddy Current Induced Heating for the Soldering Reflow of Area Array Packages. IEEE Trans. on Advanced Packaging, 2: MAXWELL 3D (2010): Electromagnetic and Electromechanical Analysis: Maxwell_3D_v11_full_book.pdf. Nemkov VS, Demidovich VB (1998). Theory and Design of Induction Heating Devices. Leningrad: Energy-atom-print. Xu H, Li M, Fu Y, Wang L, Kim J (2009). Local Melt Process of Solder Bumping by Induction Heating Reflow. Surface Mount Technology, 4:

6 Lanin et al. Vladimir Lanin received the PhD in electronic engineering from Sankt Petersburg Technical Institute, Russia in He is currently Dr. Sci. Tech., professor with the Electronic Engineering and Technology Department of the State University of Informatics and Radioelectronics, Minsk, Belarus. He has more than 30 years of experience in Electronics Production Technology. Basic research area: technology of electronic devices. He is the author of 7 books, 25 patents and 250 scientific papers. vlanin@bsuir.by Ivan Serhachou received the Engineering Diplomas and, M.S. Degree from State University of Informatics and Radioelectronics, Minsk, Belarus in 2012 and 2013 respectively, all in electronics engineering. His research interests are induction heating for soldering electronic assemblies and modeling induction device in Ansoft Maxwell. He is the author of 7 scientific papers. Vitali Khotskin received the Engineering Diplomas and, M.S. degree from State University of Informatics and Radioelectronics, Minsk, Belarus in 2013 and 2014 respectively, all in electronics engineering. His research interests are induction heating, infrared heating and hot air heating for mounting electronic components. He is the author of 5 scientific papers. 27