IR imaging applied to the quality control of miniaturized electronicsIn the portable electronics product business there is a strong trend towards miniaturization. This trend means that the packaging density is increasing significantly. Flexible printed circuit boards are more and more used in high-density applications like in smart cards, hearing aids and low current display (LCD) modules. A small pitch and the use of flip chip technology are common to all of these applications. Classifying flip-chips and contacts of an ACA join by IR camera Anisotropically conductive adhesives (ACA) have emerged as an important joining technology in a number of significant application areas such as flat panel assembly and smart cards. These materials rely upon the trapping of conductive particles between the conductive pad the other part being locked by residual stresses to ensure retention of sufficient contact. In the adhesive no metallurgical interactions occur between pad and component. Therefore, the reliability of joining is difficult to ensure by conventional tests. Satakunta Polytechnic has qualified contacts together with its research partners using IR thermal measurement instead of x-ray or laser. The research is a part of the project “Innovative network of automation technology”, financed by the State Provincial Office of Western Finland. In this article the research results are described. Tests were made in high density flip-chip applications (pitch 200, 80 µm) connected to the flexible substrate. The quality of contact resistances was measured with four-probe-method using daisy-chain circuits. In tests, circuits were loaded with different currents and generated temperatures were measured using IR image processing. The aim was to evaluate the capability of computer aided IR camera system of classifying the flip-chip and contacts of an ACA join. Contact resistance in the ACA interconnection The reliability and electrical performance of the ACA interconnection is a result of several factors, such as process parameters, adhesive, contact area and substrate material. A good and reliable electronic contact in the ACA joining can be achieved by using large contact areas and a high conductive particle density. In small pitch applications the particle density must be higher in order to achieve a low contact resistance. The particle resistance is a part of the total resistance in the ACA mechanical contact. The resistance depends on whether a hard or a soft metal-coated polymer particle in ACA is being used. The total contact resistance, R, for an adhesive with “n” particles in contact between materials 1 and 2 can be described by the formula: (1). R = 1/n x (Rparticle + Rc interface 1 + Rc interface 2 + Rf interface 1 + Rf interface 2), where, Rc is the constriction resistance of the interface and Rf is a film or a tunneling resistance (formed oxide layer on the surface or some other high resistance layer). Results of IR-Measurements of ACA Contacts
In the measurements thermal camera of type Flir PM 595 and Flir Thermacam SC3000 and lens were used. The resolution of the first named system is >100 µm and its thermal sensitivity is 0.14 degrees and the other >26 µm and 0.02 degrees. Daisy chain flip chips were tested with 80µm pitch using 0,5 mA current and taking photos from the substrate side. Using Thermacam SC 3000 the difference of the accuracy is clearly observed in Picture 1.
The contacts in FC are not on a straight line. In Picture 1, we can see that only a small amount of current can raise the temperature in a flip-chip by some degrees and expose fault contacts. If more current is added, bad contacts can be clearly seen. See the white areas in Picture 2.
 IR- thermography - the effect of the surface emissivity The application of the IR - thermography enables us to measure processes related to the energy, for instance changes in the flow outside the pipe. The method does not require transparent substrates to ”see” inside. The heat transfer is very difficult to analyze quantitatively because the mathematical models are very complicated and it is almost impossible to verify the results. Many heat transfer questions exist in miniaturized electronics especially in the quality control of components or devices. The thermal camera uses infrared radiation wavelength of 2-13 µm. It is not physically in contact with the measuring sample. It measures the radiation temperature intensity of the surface of the image and calculates according to this information the temperature of the measuring point. When calculating the temperature we have to know the emissivity of the radiation area. The emissivity varies from 1 (a black body) to 0 (cleaned metal surfaces). The emissivity depends on frequency, temperature and angle used in the measuring. When the wavelength is fixed, a constant value of the emissivity can be used. The effects of temperature and angle are normally very small. Metal surfaces are very complicated to analyze because they act like mirrors in which the IR-radiation reflects. Therefore, a uniform emissivity of the measuring area without any reflect is needed. Very often some kind of painted surface is used to equalize the emissivity, but it is not easy to strip away.
In the tests at Satakunta Polytehnic talcum was used, because it is very easy to clean off, its resistance is very high and it is not transparent. The emissivity value of talcum used in the measurement was 0.96. In Picture 3, we can see the thermal behavior of the golden coil on the DIL-case measuring with and without talcum. The difference of the measured temperatures was over 60 % higher than the average with talcum.
 In the thermal computational techniques “Agema Report and Thermal CAM Researcher of Flir Systems” thermal analysis software programs were used in evaluating thermal models and in determining the effects of the different contacts between the bumping component and substrate. Conclusions In the tests it was shown that by using the computer aided IR-camera it is possible to define the differences even in ACA flip chip contacts. This way it is much easier to qualify contacts than with the four probe method. When measuring the temperature with the IR-camera it is suitable to equalize the emissivity of the area by using talcum instead of painting. The research in this field is continuing at Satakunta Polytechnic in a project financed by National Technology Agency of Finland. Satakunta Polytechnic
Satakunta Polytechnic is one of the largest polytechnics in Finland. The Institute of Automation and Information Technologies at Satakunta Polytechnic focuses on the development of applications for automation, electronics and information industries. Its expertise in Thermal imaging covers time-critical in-line inspection and monitoring of fluid flows, paper webs and electronics. Satakunta Polytechnic is an important player in the R&D of machine vision. It acted as a principal contractor and R&D partner in the European Union’s successful EUTIST Integrated Machine Vision project (2000 – 2003). Satakunta Polytechnic’s expertise in machine vision includes 3D surface inspection, 3D measurement and robot guidance. On October 2004, Satakunta Polytechnic will demonstrate its new research areas and results at the Vision Boulevard exhibition area of Tekniikka 2004, in Jyväskylä, Finland.
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