Takashi AOKI Shinichi OKAMOTO Osamu INA Akio SUGIURA

Transcription

1 Takashi AOKI Shinichi OKAMOTO Osamu INA Akio SUGIURA Ryoichi NARITA Up to now, while automobile electric packages have demanded the high density for small and light products, electric leaks, which have occurred due to condensation, have been a major problem. To prevent any electric leaks, a conformal coating on the electric parts (ex. ECU : Electronic Control Unit ) has been needed, but in general the design rules of the conformal coating (ex. Application area) is vague. Therefore, DENSO demands to clarify the design rules of conformal coating electric package for higher reliability. To meet the demand, DENSO has developed a condensation simulation method using CAE that can show the occurrence condition of condensation fast and accurately. In the result, DENSO has been able to get the design rules where to need the conformal coating in the electric package for automobiles. Key words : Condensation simulation, Automobile electric package, Conformal coating, Electric leak In recent years, car electronic products are characterized by smaller gaps between terminal pins and high-density packaging using CSP, as shown in. a drop of condensation forms between terminals, current leakage will result. If the drop grows, as shown in, there will be a short-circuit. There are generally two methods of preventing this, as depicted in. [QFP] mm gap CSP [52P].4mm gap QFP [256P] 1.mm gap BGA.5mm gap QFP [28-256P] 1.2mm gap PLCC [68P] [CSP].65-.8mm gap QFP [12-16P] Fig. 1 Trend in electronic packaging.4mm gap CSP QFP: Quad Flat Package CSP: Chip Scale Package Condensed (specific residence: 1MΩcm) (Positive) Current leakage Terminal Fig. 2 Leakage by condensation e <Moisture-proof coating> Moisture-proof material (Negative) Condensation These trends lead to concerns that the increased strain on soldered points may result in shortened service life due to thermal fatigue, and that moisture resistance may deteriorate, as is often observed in leakage due to condensation. Conventional acrylic coating protection <Completely sealed structure> Condensation Water vapor Casing against moisture only increases the thermal strain on soldered points; furthermore, the use of acrylic resin itself Fig. 3 Moisture-proof structure is being scrutinized under the stringency of environmental regulations. The objective of moisture-proof treatment is to prevent current leakage and migration between terminals in the event of condensation on a printed circuit board. When (1) Moisture-proof coating (insulation) on the circuit board (2) Complete sealing of the circuit board to prevent the ingress of moisture (without moisture-proof material)

2 Thus far, however, both methods have shortcomings. In method (1), it is still unclear which portion of the board is likely to experience condensation, and therefore, where the coating must be applied. As for method (2), guaranteeing completeness of the seal may require higher manufacturing cost. To maintain product competitiveness, it is necessary to : minimize amount of moisture-proof material by applying it only to necessary points, and minimize cost of casing by defining conditions for preventing condensation (e.g. maximum gap allowed) to guarantee insulation reliability equal to that of a completely sealed structure even when the casing is not completely sealed. Given and above, it is essential to do the following to improve product competitiveness. 1) Quantitative analysis of condensation process 2) Establish structural design technique optimized to practical environments in which products are used In the meantime, the solution cannot be found merely by evaluating actual products, due to the complexity of the condensation process and the difficulty of measurement in minute areas. With the aim of improving problem-solving efficiency, the development of a condensation simulator was initiated by applying thermohydrodynamic analysis methods to quantitative analysis of the condensation process under transient temperature and humidity conditions in electronic products. Condensation occurs on a circuit board when, by convection or diffusion, warm air or moisture in the ambient air enters via an opening in the casing around a circuit board and liquefies on the board surface, as shown in. The parameters involved in the process are temperature, relative humidity, and the latent heat transfer that takes place in phase change. In general, the condensation in car electronic products is evaluated by testing under different combinations of temperature and humidity specified in. Under the transient condition in which the temperature rises, a temperature differential arises between circuit board and air Water vapor rises (due to difference in specific gravity with air) differential Water vapor Air Condensed Board surface Phase change Liquefaction and vaporization Latent heat transfer Board wettability Fig. 4 Condensation process model due to difference in the speed of the temperature rise, as shown in. When the dew point is within the range between air temperature and board temperature, condensation occurs. When the temperature stabilizes after a certain time, the condensation disappears. Under the transient condition in which the temperature falls, the circuit board temperature will be relatively higher than that of the air; therefore, condensation occurs on the inner walls of the casing. Although condensation is a familiar phenomenon, it can occur transiently within a minute space with very little temperature difference. Considering limitations in the forms and accuracy of measuring instruments, it is difficult to solve the problem by relying (T) (T) (ºC) P=1atm (%RH) (t) (ºC) 4 Fig. 5 Condensation evaluation pattern -3 Dew point Board surface temperature Fig. 6 Condensation range 75 5 Air temperature within casing Condensation range on the circuit board (t) Relative humidity (RH)

3 only on the evaluation of actual products. Therefore, our aim was to solve the problem rationally and effectively by combining parameters measured in theoretic product evaluation using CAE. Studying simulation method/ procedure Verification (Conditions / Amount of condition) 1 st step Condensation is a phenomenon in which the vapors in moist air liquefy upon reaching the state of supersaturation. Condensation on a circuit board is governed by three factors, shown in : the product structure, atmosphere and mounting conditions. Thus, the objective was to find the optimum structure efficiently by standardizing each parameter. Atmosphere ( / Humidity) Condensed Electronic device Mounting condition (Gravitational force) Product structure Circuit Board Opening Number of openings Self-heating Opening structure Thermal capacity Casing Position Area Inner volume Condensation Humidity Profile Position Direction Atmosphere Mounting condition Fig. 7 Cause and effect diagram of condensation in car electronic products describes the research procedure adopted. This paper focuses on the first step from the study of simulation procedure to defining elements of structural optimization. Air Defining elements of structural optimization Calculating optimum structure Fig. 8 Research procedure humidity, and their transitions at respective positions. From this observation, the modeling involves, at least a combination of structure and fluid (internal air). The ambient air may be included in the modeling, or represented by setting up a boundary that transfers heat at the outer surface of the casing. Judging from these, a multiphase thermo hydrodynamic analysis is generally considered the appropriate simulation method that covers all governing formula involved in the condensation process. However, it is also extremely complex and, as shown in, is probably inapplicable in terms of calculation time and convergence. In addition, this method is of the structural mesh type and poses difficulty in modeling the subject faithfully. Table 1 Comparison of simulation methods Simulation model Convergence Calculation time Calculation precision Cost ratio Ease of use Single-phase thermohydrodynamics + sub-program Greater freedom (Non-structural mesh) Yes ca.1hours (Parallel operation) Good Multi-phase thermohydrodynamics Some limitations (Structural mesh) No over 1hours (Convergence program) NG 1 over 1 Good (Interactive) NG Evaluation Good NG In electronic products to be simulated, condensation occurs when the moisture in the internal air, or ambient air that has entered the interior, liquefies on various elements of circuit boards depending on the ambient temperature and Therefore, it was decided to adopt a method in which moist air is regarded as a single-phase fluid whose three elements (dry air, vapor, and condensed ) are defined as scalars. The fraction of the masses of three elements is determined by a sub-program that calculates the phase change from vapor to condensed by

4 incrementing time; the results are integrated into an ordinary loop of flow field calculation. shows the algorithm used in the calculation and output of each process. This method creates the added task of developing a sub-program for simulating the process of phase change from vapor (gas) to condensed (liquid). Calculations of enthalpy, radiation and form of condensed wetting are excluded here. Single-phase Thermohydrodynamics simulation increment Applicable fomula Outputs Applicable fomula Outputs Mixed gas Flow Calculation of mixed gas physical property Equation of state (Mixed gas) Density Thermal conductivity Specific heat Flow Calculation Navier-Stokes equation Equation of continuity Equation of energy (Mixed gas) Flow Pressure END START Fig. 9 Calculation algorithm Sub-program (Condensation simulation) Gas-liquid phase change simulation Applicable fomula Outputs increment Excluded All enthalpies Radiation Board wetting form Water vapor Condensed Equation of mass transfer (Mixed gas) Fraction of each element Dry air Flow Next, shows the formulas used to simulate the gas-liquid phase change for analyzing the condensation process. Normally, condensation occurrence is represented by the dew point, but here it is determined by defining the vapor saturation pressure (Ps) by an approximated high degree function of temperature. Ps is defined by formula (1). Ps=1 3 (6E-7X 4 +1E-5X E-3X E-2X+.6119) fomula (1) X=T-273(T: (k)) Using this method, it is possible to determine condensation occurrence by combining default outputs of the solver; this reduces the computational load and enhances calculation efficiency. Relative humidity >1% Dry air Sub-program Creation Ingress by Water Water Ingress by convection vapor convection Passage of time Diffusion Passage Diffusion of time <Equation of condensation> -Saturation pressure of vapor -Absolute humidity -Relative humidity -Water formation (Condensation) -Disappearance of vapor P s = P s (T) m H2O - ( w w f ) P [ ] 1 sm 3 = = Fig. 1 Formulas for gas-liquid phase change simulation (condensation process simulation) m m w = vapor m air w P f = ( w)p s vapor = shows the general concept of the test model. Inputs are histories of temperature and relative humidity under the transient condition in which the temperature rises (transient boundary conditions of ambient temperature and humidity), with transient analysis of 1,2 steps at 1 second intervals. In addition, these inputs are defined as the boundary conditions for the simulation model while temperature, relative humidity and pressure (1 atm, constant) are assigned for openings (end sections of internal air) and temperature and thermal conductivity (33 w/m 2 k) are assigned for the boundaries with thermal transfer at the respective outer walls of the casing and connectors. The transient behaviors of these parameters are defined by the sub-program. Here, the relative humidity of the atmosphere is defined by the fractions of the masses of three elements (dry air, vapor and condensed ) of moist air. To verify the precision of simulation using the basic model, condensation occurrences were compared with measurements taken on actual products under transient m H2O (T: temperature, t: time, P: density) t kg [ ] 3 kg sm

5 < ECU (Basic model) > Circuit board Casing Comb-shaped gage Section for simulation model Boundaries at openings (Boundaries with pressure,temperature, humidity and mixed gas defined) determined by transient changes in electric current leakage as measured using a comb-shaped gauge (with 3 µm gap) that conforms to JIS Z 3197, placed on the surface of a circuit board in an open casing as shown in. The current leakage was measured by the method depicted in the circuit diagram, while temperature and humidity of atmosphere, internal air and circuit board were monitored using thermometers and relative humidity gauges. Measurement points are also shown in the figure. Internal air Comb-shaped gage Circuit board - Determining condensation on actual products Constant temperature, humidity tank Casing walls (Boundaries with thermal transfer defined) Condensation leakage (Comb-shaped gauge) measurement ( sensor on board / in air) - Conditions for simulation Item Inputs Outputs Calculation method Governing formulas Pre/Post Solver CPU - Input boundary conditions Boundaries at openings Outer wall Properties Weight Specific heat Thermal conductivity Transient analysis Conditions Histories of temperature and humidity (Fig. 5) Histories of temperature, humidity, and amount of condensation Equation of continuity Navier-Stokes equation Equation of energy Equation of state Equation of mass transfer FEMAP + PROSTAR STAR/CD (v31a) 4-node parallel operation) - Physical properties of mixed gas Density Viscosity Item Standard pressure boundary (Pressure) Mixed gas definition (Scalar) Boundaries with thermal transfer (Wall) kg/kmol kg/m 3 J/(kg. K) Pas W/(m. K) Dry air Conditions Pressure : P=1atm (Constant) : T (Fig.5 equivalent) Relative humidity: RH (Fig. 5 equivalent) Water vapor Condensed Dry air : T (Fig.5 equivalent) Thermal conductivity : 33 W/(m 2. K) Conditions Water vapor Condensed E E-5 1. E Fig. 11 Conditions for simulation using basic model conditions in which the temperature and humidity change. First, condensation on actual products was observed and Condensed - Circuit diagram for condensational leakage measurement 2V Relative humidity measurement (Humidity sensor) R G Comb-shaped gauge 1kV V JIS-Z-3197 Condensation leakage : I a = V/1 Gage resistance : R G = K R (2/I L ) Fig. 12 Determining condensation on actual products shows an example of actual measurement and a simulation result. The two contors represent the temperature distribution at respective points and the distribution of condensed fractions around the comb-shaped gauge after 1, seconds. It was observed on actual products that, as the ambient temperature and humidity rose, those of the internal air and components also rose, due to heat transfer from the outer walls of the casing and convection. After 75 seconds, condensation commenced on the surface of the comb-shaped gauge, due to the difference in response time constants between circuit board and internal air, and peaked

6 at around 9 seconds. Subsequently, the ambient temperature and humidity became constant, forcing component and internal air temperatures to converge at the ambient temperature, reducing the amount of condensation. On the other hand, the simulation shows condensation starting after about 7 seconds, peaking at around 9 seconds, and diminishing thereafter. The results are consistent with each other, with little discrepancy in starting time. Condensation occurrences and disappearances were thus successfully visualized and qualitative level precision was achieved. As application examples, the effects of changing the area and direction of the opening were simulated. The focal points in these simulations were vent hole shape and direction. Two standards were set for each factor: an open casing without a lid and a proof casing with a 6mm diameter vent hole for the former, and vent holes on top and bottom for the latter. The models of these simulation examples and the vent holes (location of boundaries with atmosphere) are shown in. Note that boundary conditions were set following the example of the basic model. Transient temperature distribution (1s) 39.1ºC 34.1ºC Distribution of condensed around comb-shaped gauge (1s) - Actual measurement (ºC) humidity (RH) Simulation result (ºC) humidity Relative humidity Humidity setting (µa) 3 2 Humidity setting Leakage current 1-2 Board -4 setting (RH) Relative humidity Board setting Fig. 13 Simulation results and precision verification 2 Current leakage at gauge (%) 1 Condensation amount 5 Fraction of condensed Opening casing Waterproof casing Top Top Bottom Bottom Simulation models Casing Output point Fig. 14 Simulation models Next, the condensation evaluation results are shown in. Evaluated values were maximum leakage at the gauge for actual measurement and maximum fractions of condensed for the simulation. Both measurement and simulation show that the open casing has condensation more than 1 4 times that of the proof casing. From this it can be concluded that the area of openings is the major contributing factor to condensation. As for correlation between vent hole direction and condensation, more condensed was observed with the bottom vent hole for both types of casing. This is because with the vent hole on top, vapor in the moist air does not usually enter the casing due to its lightness compared to dry air and because the moisture in the internal air tends to liquefy on the ceiling. Therefore, it is more advantageous to have a

7 vent hole on top and the circuit board at the bottom of the casing. shows the fractions of condensed (simulation output) for both the casing with a vent hole on top under transient conditions in which the temperature rises. shows the contour outputs at the peak of condensation for each simulation standard. Given these examples, it can be said that quantitative analysis and visualization of the condensation/vaporization process, which is difficult to measure with actual products, have been made possible. These examples also suggest that the product development method and procedures established here will apply effectively to the design of optimum moisture-proof structures and the development of moistureproof treatment with theoretical support. In the future, efforts will be made to propose appropriate moisture-proof structures and conditions for coating, through planned research into defining elements of structural optimization. Opening casing Waterproof casing Condensed fraction Table 2 Condensation evaluation results (%) Gravitational force +1G -1G +1G -1G Actual measurement (leakage at gauge) 2 ~ 2µA.2µA Opening casting / top vent hole Waterproof casting / bottom vent hole Simulation outputs (condensed fraction) 9% 34%.5%.7% Fig. 15 History of condensed fraction Opening casing Waterproof casing Top Top Bottom Bottom 1% Simulation models 34%.1% 9%.5%.3%.5% Fig. 16 Contour outputs of condensed fraction.7% As indicated in earlier sections, it has become generally possible to simulate the extent of condensation. This section reviews the correlation between amount of condensation and insulation resistance. Condensation leakage current is determined by the resistance of the condensed that bridges two conductors on the circuit board, and the difference in electrical potential between the two conductors. Condensation typically begins with a hemispheric drop of condensation 1 µm diameter or less at the core, growing with expansion of the diameter and by joining with adjacent droplets. The process is very complex and depends on the temperature, humidity, surface conditions of the board etc. Assuming that condensed is a film of constant thickness and that the specific resistance of condensed is ρω-cm, the correlation between amount of condensed and leakage current is expressed by formula (2) in. is a chart that depicts this correlation, based on formula (2) (ρ=1 MΩ-cm), and the correspondence between simulated condensation amount using the basic model and leakage measurements taken on actual products. Through comparison of these two relations, it can be said that the latter relation, based on measurement, tends to indicate smaller leakage. This is presumably due to various

8 Condensed IR V t Terminal [Factor1] Wetting form : Increased resistance Condensed Terminal Circuit board L : Length between terminals t : thickness of condensed V : Voltage ρ : Specific resistance of condensed r : Length of terminal IR : Leakage current W : Condensation amount Assumption : condenses evenly on board surface IR = V / R = V L t r W1 = V -3 r L ρ L ρ Fig. 17 Formula for calculating leakage by condensation [Factor2] Board temperature increased due to latent heat from condensation : Less difference in temperature I R Heat radiation [Factor3] Effects of specific resistance of condensed Small Large ρ Large Small Condensed leakage I R (µa) Fig. 18 Correlation between condensation and leakage real-life factors, such as wetting forms of condensed, board temperature increase due to latent heat from condensation, and the specific resistance of condensed as shown in. To improve the accuracy of simulation, it will be necessary to extract these factors, quantify them and reflect them in the algorithm. Theoretical line (ρ = 1MΩcm) Measured condensation leakage V : 2V L :.318mm r : 16mm Condensation amount W (µg / mm 2 ) By combining the transient thermohydrodynamic calculation feature from a general purpose program written for single-phase fluids and an optional feature of gas-liquid phase change simulation (condensation simulation), a condensation simulator that is easy-to-use as a development Fig. 19 Major factors affecting leakage by condensation tool has been developed, while achieving qualitative precision. The simulator makes it possible to understand and visualize the condensation process that takes place in a minute area, which is impossible to measure in actual product evaluations. It will also be used in the future to solve problems in optimizing moisture-proof structures and developing new moisture-proof treatments for electronic products. 1) Nobuyuki Kato etc., The Society of Heating, Air- Conditioning and Sanitary Engineering of Japan No.74 (1999.7) 2) You Matsuo, The Society of Heating, Air-Conditioning and Sanitary Engineering of Japan C39 (1999.1) 3) Nobuyuki Kato etc., The Society of Heating, Air- Conditioning and Sanitary Engineering of Japan No.72-1 (1998) 4) Hitoshi Takeda, The Society of Heating, Air-Conditioning and Sanitary Engineering of Japan C4 (199.1) 5) Masanori Yadoya, Architectural Institute of Japan, (1987.1), p.921.

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