Reflow Soldering Processes Development Using Infrared Thermography Petko Mashkov, Tamara Pencheva, and Berkant Gyoch Physics Department, University of Rousse, Bulgaria tgp@ru.acad.bg Abstract: Investigation deals with application of new kind of soldering equipment and development of reflow soldering processes for electronic devices using this equipment. It is based on low inert heaters for the middle IR spectral region. Due to low inertia of the heaters temperature changes in the soldering camera may be controlled precisely and the soldering cycle may be realized at the same place without conveyor belt. The printed circuit board (PCB) of the device is relatively big with different electronic components density in different zones of its surface. Used electronic components demand precise soldering cycle parameters realization. Temperature monitoring of the whole PCB s surface necessary for technological operation development practically is impossible by conventional measurement techniques. Good results are obtained using IR thermography with IR camera for the middle IR spectral region. Results of this investigation allow adjusting proper regime of operation of the soldering equipment and successful realization of soldering processes. 1. INTRODUCTION The purpose of this investigation is connected with development of soldering technological processes for an electronic device. The PCB for this device is relatively big with different electronic components density in different zones of its surface. Electronic components are with quite different properties and soldering of some of them (for example BGA packages) is quite delicate technological operation. Soldering equipment designed and produced in our laboratory is without conveyor belt [1-3]. Due to low inertia of heaters the temperature onto PCB may be controlled in situ during soldering cycle and a lot of operating parameters can be adjusted in dependence of size and type of the PCB, electronic components density, etc. Temperature usually is controlled in one point on PCB during soldering processes. In practice it is impossible to put a lot of thermocouples on PCB. Because of soldering cycle s duration is about 4 minutes, usage of more than one thermocouple would disturb operation and service of the soldering camera and may be a reason for different problems. At the same time it is desirable to have opportunity to control electronic elements temperature regimes, especially when electronic components are quite different, PCBs are big, etc. Temperature monitoring on the whole surface of PCB and different kind electronic components necessary for technological operation development practically is impossible by conventional measurement techniques. Our previous experiments [1] showed that this problem may be solved partially using infrared thermometers, but as it is shown [1], temperature can be measured at the moment in one point only. It is impossible to determine temperature distribution onto the whole PCB, surface temperature of different kind electronic components during soldering cycles which is necessary for optimization of soldering regimes technological parameters. More over, temperature measurements by IR thermometers can t be applied when the surface of the object is metal due to its high reflectivity [1]. This is a disadvantage because it is impossible to obtain reliable results concerning surface temperature of electronic components with metal packages during soldering. Proper solution of this problem is possible by application of IR thermography equipment for the middle IR spectral region (8-13 m). The method allows realizing precise measurement and control of temperature distribution on PCB during soldering processes, possibility of optimization of temperature profiles for different printed boards in dependence of their size and electronic components density [1, 4, 5].
2. EXPERIMENTAL EQUIPMENT Cross-section view of experimental soldering camera without conveyor is shown in Figure 1. In this equipment a lot of operating parameters may be controlled: direction and hot gas flow rate; cooling rate by entrance of outer air, etc. Operation regimes of upper and lower parts of the heaters can be controlled independently. Some small constructive changes in the equipment, Figure 1, allow investigation of thermal processes on PCB at different stages of the soldering cycle by IR camera. Experiments are carried out using ThermaCam E300 FLIR Systems. 5 6 1b 7 5 3 4 2 1a 8 Fig. 1. Cross-section view of experimental soldering camera without conveyor: 1a and 1b heaters; 2 metal grid for PCBs mounting; 3 heating chamber; 4 PCB; 5 reflecting shields; 6 metal grid, 7 aluminum gate, 8 fan, 9 ThermaCam E300 camera. Some of parameters of the camera are: Thermal Sensitivity: <0.08 C at 25 C. Detector Type: Focal plane array (FPA) uncooled Vanadium Oxide microbolometer, 320 x 240 pixels. Spectral range: 7.5 to 13 µm. Measurement Temperature Ranges: -20 to + 250 C (-4 F to + 482 F) and 0 C to +500 C (+32 F to 932 F), Up to 1200 C (2192 F), optional. Accuracy (% of reading) ± 2 C (3.6 F) or ± 2% of absolute temperature in C. Application of IR camera at these conditions is a delicate process and obtaining correct results from temperature measurements demands keeping some very important rules. The radiation measured by the camera does not depend on the temperature of the object only but is also a function of its emissivity. Radiation also originates from the surroundings and is 9 reflected in the object. The influence of other parameters - atmosphere s temperature and relative humidity is weak but in the heating camera infrared radiation is quite intensive and it is very important to determine correctly its influence on obtained results. 2.1. Determination the reflected apparent temperature: The parameter reflected apparent temperature is used to compensate the radiation reflected in the object, especially when the object s emissivity is low and its temperature is relatively far from that of the reflected apparent temperature. That is why it is very important to be able to measure this temperature during soldering in the heating camera. Two methods for determining of reflected apparent temperature and avoiding the influence of radiation from surroundings reflected in the object are recommended [2]. The first (direct method) is impracticable it is impossible to put IR camera into the reflow oven during soldering cycle. The second described method (reflector method) [2] demands to place an aluminum shield over the PCB for determining the reflected apparent temperature Fig. 2. Measuring the reflected apparent temperature using aluminum shield. (Figure 2). Because the duration of this process is small (about 1 2 seconds) it can be made without disturbing a lot the soldering process. Small constructive changes in the heating camera are made and a moving aluminum shield is mounted in it. At a choused moment it can be placed over the PCB and
replaced when the value of reflected apparent temperature is read. 2.2. Determination emissivity of different electronic components on PCB: Fig. 3. Determination emissivity of IC s surface. Temperature measured by thermocouple is 120 0 C. Good correspondence with temperature shown in thermo photograph is achieved when emissivity is set to be 0.95. A lot of experiments for verification of the method s possibilities and accuracy of obtained results from thermal pictures are made. Some thermocouples (4-5) on different electronic components (with different emissivity and physical properties) upon the PCB were mounted. Temperatures in these points at chosen moments during typical soldering cycle were measured. The obtained results are compared to those from IR pictures for the same points and moments. IR photographs are processed by ThermaCAM Quick Report (Version 1.1) FLIR and results for reflected apparent temperature are used. The correspondence between temperature values obtained from these two methods is very good practically results are the same. Experiments show that this method for compensation the influence of reflected radiation from the object is applicable (despite of its high intensity in the reflow oven) and obtained temperature measurements results are correct. Fig. 4. Determination emissivity of metal surface of quartz resonator. Temperature measured by thermocouple is 120 0 C. Good correspondence with temperature shown in thermo photograph is achieved when emissivity is set to be 0.12. [2] allows determining object s emissivity easily and quickly but sometimes may be unpractical. When it is necessary to measure emissivity of electronic components mounted on PCB, usage of electrical tape with known high emissivity is not a good decision. The most interesting temperature interval for our investigations is between 100 0 C and 300 0 C. At these temperatures it is impossible to use plastic tape. Experiments show that it is possible to obtain correct results for emissivity values by a little different measuring method. A thermocouple is mounted on the surface of the electronic component which emissivity has to be measured. The object is heated by hot air flow which temperature is controlled to be constant. After reaching stationary conditions the temperature (measured by thermocouple) is read and thermo photo is made. During processing the IR picture by FLIR Quick Report application the emissivity setting has to be changed until temperature the same as measured by thermocouple to be read. Pictures shown in Figures 3
and 4 illustrate upper described measuring procedure. Experiments show that results obtained by the method recommended in [2] and by the method with thermocouple practically are the same. But the method proposed here has important advantages. It allows quickly determination of emissivity of the objects with very different optical properties directly on PCB in the whole temperature range between 100 0 C and 300 0 C. 3. RESULTS Investigations by IR thermograph measurements give a lot of advantages during development of characteristics power and operation parameters of the heaters (Fig.1), gas flow rate and its direction in the heating camera, parameters of soldering cycle and others. A lot of experiments are made to establish optimal operating parameters during soldering cycle for large size PCB (250/ 310 mm) with very different electronic components density on its surface. A considerable disadvantage of investigation method described above is determination of reflecting apparent temperature (RAT). This procedure delays measurement process, disturbs soldering and causes difficulties for operators. More over, the essential problems for this measuring method are connected with fast changes of radiation intensity in the heating camera during soldering process. That is why Fig. 5. Determination of reflected apparent temperature during soldering cycle using aluminum sheet and thermocouple under it. Temperature measured by thermocouple is 150 0 C. Good correspondence with temperature shown in thermo photograph is achieved when reflected apparent temperature is set to be 190 0 C. soldering processes. Thermograph photos allow receiving temperature distribution on the whole area of the PCB at each moment of soldering cycle independently on PCB s size and type of electronic components on it. Obtained data are very useful for optimization of the soldering equipment s Fig. 6. Determination of reflected apparent temperature during soldering cycle using aluminum sheet and thermocouple under it. Temperature measured by thermocouple is 130 0 C. Good correspondence with temperature shown in thermo photograph is achieved when reflected apparent temperature is set to be 147 0 C. different method to determine the reflecting apparent temperature much more fast and suitable is proposed. A thermocouple is mounted on the bottom side of a little piece thin aluminum sheet (15/15 mm). The surface of this aluminum sheet is good to have diffuse reflection. The emissivity of the sheet is determined
by methods described above. The sheet is fixed at a proper place on the PCB. When the thermo photo is made, at this moment the temperature measured by thermocouple is written down. Pictures processing includes: setting emissivity of the aluminum sheet (0.09); put the flying spot meter onto the sheet; change the reflected apparent temperature until reaching the temperature (shown by spot meter) equal and upper parts of the heaters. This is one of the main advantages of the method because it gives possibilities to achieve the whole view on thermal processes in the heating camera. IR PCB s photos illustrating different stages of soldering cycle are shown in Figures 7a and 7b. Fig. 7 a). IR photos of PCBs at different temperatures in heating camera during soldering cycles. to that measured by thermocouple; write down the value of reflected apparent temperature. After that this obtained value of RAT can be used during determination of temperature distribution onto the whole PCB. Some examples illustrating this procedure can be seen in Figures 5 and 6. Using upper mentioned methods thermal processes on PCB at different stages of soldering cycles are investigated. Processing and analysis of thermal photographs allow evaluation of uniformity of thermal distribution on different electronic components on PCB. It is very useful in order to adjust proper operation parameters of soldering equipment air flow rate, power and manner of operation of lower Fig. 7 b). IR photos of PCBs at different temperatures in heating camera during soldering cycles. Analyzing IR thermography pictures allow investigating real temperature conditions for different kinds of electronic components mounted on the PCB during soldering cycle. Obtained results of IR thermography measurements are used for determination of optimal operating parameters of the soldering equipment (temperature regimes for lower and upper parts of the heaters, gas flow rate and others). 4. CONCLUSIONS Investigation deals with development of reflow soldering processes for electronic devices using IR camera (ThermaCam E300 FLIR) for the middle IR
spectral region. Temperature monitoring of the whole PCB s surface is realized using IR thermography. Suitable method for fast determination of emissivity of different kind of electronic component on PCB in temperature range 100 0 C - 300 0 C is proposed. New approach for determination of reflected apparent temperature in the heating camera during soldering cycle is proposed. It allows convenient realization of thermograph investigations of thermal processes on PCB during soldering. It is shown that IR thermography can be used successfully for investigations of thermal processes in heating camera during soldering cycles. Results of these investigations allow optimization of operations regime for soldering equipment in dependence of concrete technological requirements. ACKNOWLEDGEMENT The National Science Fund, Ministry of Education and Science of Bulgaria, is gratefully acknowledged for the financial support of research project VU-EES- 301/2007. Part of the investigation was supported by Project 09-FЕЕА-01, National Science Fund, Ministry of Education, Bulgaria. REFERENCES [1] Mashkov P. H., T. G. Pencheva, A. V. Valchev and B. S. Gyoch. In situ non contact temperature measurements on PCB during soldering process, IEEE, Proc. of 31 th International Spring Seminar on Electronics Technology - ISSE 2008, Budapest, Hungary, September 25 th 27 th 2008, pp. 207 212. [2] User s manual ThermaCam 300 FLIR system www.flirthermography.com [3] Enachescu M., S. Belikov, L. Molnar. Infrared thermography for deffect detection and analysis. WO 03/062809 A1. [4] Mashkov P., T. Pencheva, D. Popov, B. Gyoch. Equipment and Method for Lead Free Soldering of SMDs. 15 th International Scientific and Applied Science Conference ELECTRONICS 2006, Sozopol, BULGARIA, 20 22 September 2006, book 2, pp. 77 82. [5] Mashkov P., T. Pencheva, D. Popov, B. Gyoch. Apparatus and Method for Soldering Electronic Components to Printed Circuit Boards Proceedings of IEEE, 28 th International Spring Seminar on Electronics Technology ISSE 2005, Wiener Neustadt, Austria, 18 22 May 2005, pp. 406-411.