1 Fraunhofer ISIT, Itzehoe Critical Use Conditions and their Effect on the Reliability of Soldered Interconnects in Under the Hood Application Dr. T. Ahrens and Mr. F. W. Wulff, CEM GmbH, Mr. S. Wiese, Dresden University of Technology, Germany and Dr. L. Nitopi, Magneti Marelli, Italy 1 ABSTRACT The life time of Under the Hood automotive electronics is mainly limited by thermal fatigue of solder joints. In order to assess the reliability of the assembly, Magneti Marelli initiated a study including: climate analysis of european locations measurement of temperature profiles near hot spots on an injection and ignition unit during actual and simulated test ride ageing of a new test pattern with high, low and simulated life temperature cycles thermomechanic strain measurements on components and assemblies during thermal aging. The damage is induced mainly above room temperature. Microstructure analysis reveals fatigue crack initiation in solder joints on large SMD resistors and on power diode PTH (pin through hole). 2 Introduction ECU (Electronic control units) are the key systems for the proper operation of sophisticated gasoline engines for automotive products. Most of engine functions, such as fuel injectors, spark ignition, air intake, or exhaust gas composition are monitored and/or controlled by electronic units. The standard packaging technology for ECU s is printed circuits on FR4 laminate, double side mounted in mixed surface mount and pin through hole technology (SMT and PTH), assembled in a metallic case. From this kind of unit a very high reliability and product robustness is requested. While standard reliability tests are specified by omologation requirements, there are conditions during real life use with worse effects than those found in standard reliability testing. There are three factors that determine the temperature changes on the ECU solder joints: Ambient temperature, engine produced heat, and heat dissipation of
2 Fraunhofer ISIT, Itzehoe electronic components during operation. These local heating and short period thermal cycles are cases not included in standard omologation tests. Most of the current literature about solder joint ageing covers computer/telecom environment /1/. The current investigation was performed to elucidate the solder joint behaviour of a representative injection/ignition control unit. This ECU is operating in a 'hostile' automotive environment, e. g. in the tight engine chamber of a small city car, allowing only little air flow. The ECU consists of a typical electronic PCB (printed circuit board) assembly with components like 68 J-lead Microprocessor, power diodes, power transistors, large chip-capacitors and chip-resistors. An important reason for choosing this ECU is its high application number - more than 1 million units have been produced in Pavia by now. 3 Characterisation of actual loading profile 3.1 Ambient Temperature: Meteorological Search A search was made to evaluate the possible 'use conditions' around the year in the possible different places were the product is mostly sold. The following histogram shows the minimum temperature frequencies in one year for Palermo in Italy and Helsinki in Finland. The two cities were chosen as European extremes for high and low average temperatures. FREQUENCY [DAYS] 12 1 8 6 4 2 Palermo Helsinki -2-15 -1-5 5 1 15 2 25 3 Figure 1: Yearly minimum temperature frequency in a southern (Palermo) and a northern (Helsinki) european city TEMPERATURE [ C]
3 Fraunhofer ISIT, Itzehoe Palermo has a high medium temperature combined with a small temperature range, whereas Helsinki has a low medium temperature with a high range. Other European locations have climate situations between the two cities named above. The continental climate of Russia was not considered because of very little sales numbers in that area. 3.2 Heat distribution due to electronics operation In order to determine the hot spots during operation of the electronics, a Thermography image of the unit was taken, while the unit was driven by a simulator. Figure 2 shows the temperature distribution during a high touring condition. Figure 2: 1: 7. 3: 46.4 4: 32. 5: 39. 6: 65. 1 5 6. C Thermography image of an open ECU board under operation on an engine simulator 6 35. 1: Zener protection diode 3: voltage divider 3 4 1. 4: 68 J-lead microprocessor 5: protection Diode 6: protection resistors Point 4 next to the 68 J-lead microprocessor is no hot spot. The temperatures in the marked points were reconfirmed using thermocouple measurements. 3.3 Operational heating during actual drive test Seven thermocouples were mounted on the ECU in a test car. Thermocouples No. 1 and 3-6 were placed on the circuit board on the same sites as in the thermography image (figure 2). Thermocouple No. 2 was placed on the bottom side of the board opposite point 1. Thermocouple No. 7 was mounted outside the ECU on the aluminium case in order to determine the effect of engine heating on the unit.
4 Fraunhofer ISIT, Itzehoe Temperature measurements have been performed under the following driving conditions: start up, city traffic, motorway, interurban road, and along a narrow mountain road. The highest temperature increase during the start up phase (figure 3a) was found at a voltage divider dissipating 122 mw (point 3a in figure 3). The voltage divider was realised with a pair of two 47Ω SMD resistors of size 21. Diodes and resistors, which are protecting the circuit from high ignition-coil voltage, showed a temperature rise of 15 C in the first 5 minutes, then the temperature increased more slowly with a rate of ca..25 C/min. Other power components showed a linear temperature rise of approx..3 C/min. The metal case of the unit was warming up with a linear rate of ca..1 C/min. The highest temperature of the metal case was measured during city traffic, when the cooling is low due to low speed (figure 3b). The temperature during city traffic was 35 C above ambient temperature (ca. 5 C during the test drive). Motorway driving yields high cooling due to high speed (figure 3c). Here the case temperature was lowest with 1 C above ambient temperature. The temperature of the components on the PCB is very specific: The largest thermal amplitudes are found on the Zener Diode and the resistors which are protecting the circuit from high ignition coil voltage. These components perform temperature cycles of 15 C during motorway driving and 5 C in city traffic. The period of those cycles varies from 5 to 1 minutes. In contrast to that, very stable temperatures were found at the voltage divider and the large 68-J-lead microprocessor. The voltage divider showed the highest temperature on the PCB with 3 C above the case temperature, while the microprocessor showed the lowest temperature on the board with 5 C above the metal case temperature. After engine stop, the temperatures at all thermocouple locations were merging at 3 C above ambient temperature. 15min later they started to decrease to ambient temperature, which was reached after another 3 minutes.
5 Fraunhofer ISIT, Itzehoe TEMPERATURE [ C] 1 6 2 START UP PHASE 3 4 5 4 6 7 2 5 1 15 2 25 TIME [MIN.] 3a) Temperatures after engine start Figure 3: Temperature variations from thermocouple measurements on different ECU sites; the ambient temperature was ca. 5 C during the test: 1 - Zener protection Diodes 2 - back of the board opposite point 1 3 - voltage divider 4-68 J-lead microprocessor 5 - protection diode 6 - protection resistors 7 - outside of the ECU metal case 6 6 MOTORWAY TEMPERATURE [ C] 4 2 CITY TRAFFIC 1 2 3 4 5 6 7 TEMPERATURE [ C] 4 2 1 2 3 4 5 6 7 5 1 15 2 TIME [MIN.] 3b) Temperatures during city traffic driving 5 1 15 2 TIME [MIN.] 3c) Temperatures during motorway driving 4 Simulated ageing conditions on test pattern circuit board 4.1 Worst case climatic conditions Three worst case environments have been chosen with regard to the meteorological search : A) Seasonal simulation: slow cycles -1 C and +4 C, 5 days for each cycle B) Steady state cold at-1 C C) Steady state hot at +4 C
6 Fraunhofer ISIT, Itzehoe Conditions B) and C) were chosen to evaluate if the damaging part of the cycle is the hot or the cold one. 4.2 Test pattern project and assembly A test pattern PCB was laid out with a high number of the most critical devices identified during the simulator thermography inspection and test drives : 1. Large number of Voltage divider copies 2. PTH power resistors to heat the board locally Figure 4: Testboard for fatigue experiments 3. different chip capacitors, chip resistors, and SO components, heated by the power resistors Both voltage divider and power resistors are connected to a power supply. The resistor values and currents in the circuits are chosen to yield a thermal loading similar to that observed in the real control box during the test drive. The solder joints on the PTH resistors and on the voltage dividers served as weak points to receive active fatigue" loading. The SMD components arranged in between the heat sources served as test samples for passive fatigue". The test board was designed to fit inside a standard ECU aluminium case. A number of test boards were mounted with the standard reflow soldering process. A 2 µm stencil was used with the standard solder paste used in production. No wave soldering was performed. The PTH power resistors were hand-soldered. 4.3 Simulated drive cycles To simulate real life, a cyclic electrical loading ( microcycles") was superimposed to the worst case climatic conditions as described in section 4.1. From the analysis of
7 Fraunhofer ISIT, Itzehoe the field test drive charts it was found that a fast cycle of 4 by 4 s (power on - power off) is the most frequent, and probably damaging, situation. Similar temperature increase found in real life was reproduced on and around the several components under test. Temperature 1 9 8 7 6 5 4 3 2 1-1 -2 Thermal Response to Electrical Microcycles (calibration board, box open) 1 2 3 4 5 6 4 8 12 16 2 Time [sec] Figure 5: Active thermal cycles (microcycles) measured on the test board at - 1 C environmental temperature. Measurement sites were 1: edge solder joint on first PTH resistor 2: inner solder joint on center PTH resistor 3: solder joint on center SMD resistor 4: solder joint on edge SMD resistor 5: top surface of PTH resistor 6: outside of the metal case These ageing experiments have been performed for the three different environmental conditions running up to 3 months, or 12, electric microcycles. This simulation time is about equal to 1 years of operation of the car. Samples were taken for damage assessment every 4 microcycles. 5 Microstructure fatigue damage 5.1 Standard fatigue tests An ECU was subjected to 5 thermal shock cycles between -4 C and +125 C with 1 h dwell time at upper and lower temperature. After the cycling it was still functioning. A visual inspection did not reveal obvious fatigue cracks at the outside of the solder joints. The microsections on the solder joints, however, show fatigue cracks in the solder joints at the power diode, at the IC component, and at both investigated large (size 2212) SMD resistors. No fatigue traces were detected on the J-leads of the PLCC component.
8 Fraunhofer ISIT, Itzehoe From visual inspection, no indications of fatigue appear on the solder joints of the voltage divider resistors (Figure 6, insert). The microsection shows on both ends small, but apparent indications of fatigue damage (Figure 6). Figure 6: Small fatigue cracks at the solder joint underneath the voltage divider resistor large image: microsection insert: inspection microscopy. 5.2 Cold simulated life Very little damage occurs after twelve thousand microcycles in the steady state cold. On the SMT-resistor, initial fatigue cracks of 3µm length are found at the top meniscus of the solder joint. 5.3 Hot simulated life The steady state hot (+4 C) condition with electric microcycles yields severe damage. The SMT solder joints are cracked already after 8 cycles to a length of ca. 3 µm, starting from the top meniscus at the front end of the component. After 12 cycles, this crack length is ca. 1µm. On one terminal, there is a 5µm long crack underneath the SMT-resistor (figure 7a). However, no indications of fatigue are found in the solder joints of the separate (passive) SM components on this board. A fatal crack through a PTH solder joint is shown in figure 7b: The microsection of the PTH solder joint on the resistor shows a fatigue crack length of more than 5µm. The crack extends from the meniscus along the pin surface, but within the solder joint into the hole. Although there is still electric contact, the joint has mechanically failed.
9 Fraunhofer ISIT, Itzehoe a) large crack underneath the SM voltage divider resistor b) fatal crack on PTH resistor pin Figure 7: Solder fatigue cracks after 12, microcycles at +4 C environmental temperature 5.4 Seasonal cycling simulated life After 3 months of testing there are 23 slow environmental cycles and 12, microcacles. The active SMD resistor shows ca. 1 µm crack length from the top meniscus into the solder joint. No cracks appear underneath the component. The PTH solder joints on the heater resistor show fatigue cracks shorter than 4µm, starting only at the solder side. Under this environment, the microcycles act partly under hot, partly under cold conditions. The crack appearance is somewhat closer to the cold than to the hot environment. 5.5 Thermomechanics of the test pattern The thermal strain of the soldered 21 resistor and the laminate was measured using strain gauges as described in /2/ in accord with ASTM standards /3, 4/. The measurements were performed on a test board designated as a calibration board during environmental temperature cycles. Figure 8 shows the strain difference between resistor and substrate, measured on the calibration board. The curve consists of several cycles. The slope of the curve is shown in table 1 for three different temperature regimes. The higher the slope, the more creep has occurred (irreversible time dependent solder deformation).
1 Fraunhofer ISIT, Itzehoe strain gauge measurement temperature range [ C] CTE (coefficient of thermal expansion) (ppm/ C) I -1 to 35 II 35 to 75 III 75 to 9 Table 1: free substrate 15 16 16 Thermal expansion and soldered resistor 4.8 4.4 4.4 strain difference between strain difference between resistor and substrate 5.1 8. 1.7 laminate and soldered component The eutectic solder has a melting point of 183 C, which means that 2 C are a homologous temperature of.64 for this alloy. Therefore, soft solder starts to creep notably above room temperature, i. e. there is time dependent yielding almost without an elastic limit /5/. The creep rate increases with temperature. Damage is accumulating due to creep fatigue in the solder joint, causing fatigue crack growth. Figure 8 also shows that no spontaneous yielding has occurred at low temperatures, as the line is smooth down to -1 C. Also shown in table 1 are the values for the free board and the soldered resistor expansion. the value for the resitor appears rather low. A possible reason can be bending of the board. 6 "Calibration Board" Strain difference between resistor and substrate (slow cycle) 5 4 3 2 S 1-1 t -2-2 -1 1 2 3 4 5 6 7 8 9 1 Temperature [ C] Figure 8: r Strain difference between SMT-resistor and board surface a
11 Fraunhofer ISIT, Itzehoe 6 Conclusions. The simulated life test creates loading conditions different from the standard test, as the fatigue crack takes a different path through the solder joint, both for the pinthrough-hole and for the surface mount technology From the study it has been found that even long term exposure to hostile environment is not fatally damaging to this kind of electronic control unit. The most damaging effect is seen from the hot simulated life condition, where electric microcycles are superimposed over a constant environmental temperature of +4 C. 7 Literature /1/ Lau, J. H. (Ed.): Solder Joint Reliability - Theory and Applications. Van Nostrand Reinhold, New York (1991). /2/ Ahrens, T.; Krumm, M.: Deformation Measurements at Components, Printed Wiring Boards and Microelectronic Assemblies to Ensure the Reliability of a System. Proc. 2. Intern. European Packaging Conf. EuPac 96, DVS-Berichte 173 (1996) 18-112. /3/ ASTM E 1237-88 Standard Guide for Installing Bonded Resistance Strain Gages. /4/ ASTM E251-92 Standard Test Methods for Performance Characteristics of Metallic Bonded Resistance Strain Gages. /5/ Hall, P.M.: Creep and Stress Relaxation in Solder Joints. In /1/, 36-332.