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Batch Microwave Plasma Cleaning for Robustification of Automotive Devices
Keywords: Batch, Microwave, Plasma
Emission and fuel standards continually set challenges for the automotive makers, compelling them to turn to smarter electronics. Smartening vehicles require automation of engine and transmission controls, as well as advancement of fuel pumps and body-related functions. Considering these functions, a more robust electronic system is required to ensure both safety and user satisfaction. The most common defect encountered in the IC packaging industry is interfacial delamination – caused either by material mismatch during thermal expansion or by surface contamination. The latter can be easily addressed with plasma cleaning techniques. This study attempts to explore the use of batch microwave (B- MW) plasma cleaning to address delamination on NiPdAu lead frames in the IC manufacturing industry. Factors affecting the efficiency of this plasma technique are studied and optimized. Batch-type vs. Strip-type Cleaning. Conventional plasma cleaning machines have a strip-type set-up i.e. a direct application of plasma onto lead frame strips. This configuration allows maximum concentration of reactive ions to contact the surface to be cleaned; and has good uniformity results. However, the direct flow of plasma has been observed to damage die and wire parts. To address this, several manufacturers install an optional fixture to induce indirect plasma flow. Although this technique has minimized consequential damage to IC parts, strip-type set-up can only clean an average of five strips per plasma shot. In terms of high-volume cleaning, batch-type set-up is preferred. Instead of loading one strip after another, strips are loaded onto slotted magazines which are then loaded onto the plasma machine chamber. One plasma shot in batch-type takes longer cleaning time but allows up to a hundred strips per shot. It also does not require frequent conversion of jigs every time a different lead frame size is to be loaded. However, uniformity is not as good as in strip-type. Microwave (MW) vs. Radiofrequency (RF) Plasma. Plasma cleaning mechanisms involve a combination of physical and chemical reactions; although, parameters can be adjusted to make either type more dominant [1]. Physical reactions are typically more dominant in RF while chemical reactions are more dominant in MW. Considering that MW plasma is formed at a higher frequency (i.e. 2.45 GHz) than RF (i.e. 13.56 MHz), it is theoretically expected to produce ions with higher energy & velocity (via calculations based on one of the two De Broglie equations). Based on a solid state equation for electron density, MW plasma is also expected to produce a higher concentration of reactive ions which explains why it favors chemical cleaning [2][3]. Batch Microwave Plasma Cleaning. The use of microwave plasma discharge offsets the long cleaning time requirement in batch-type cleaning. Six factors were initially identified to directly affect the efficiency of B-MW plasma cleaning: power, cleaning time, flow type, gas mix, ratio of reactive-to-inert gas and staging time. Among these six factors, the flow type has the most profound effect on the cleaning efficiency – even affecting the role of the other five factors. Flow type is categorized into two: (1) pulsed and (2) constant flow. In pulsed flow, the reactive and inert gases are introduced alternately into the chamber; the former at very high flow rates within a short time (e.g. 1-2 sec) and the latter at low flow rates for longer periods (e.g. 10 sec). The introduction of a reactive gas at high flow rate is expected to “brush off” contaminants from the lead frame surface; but only at a short period to avoid either too much oxidation or etching. In constant flow, the reactive and inert gases are introduced simultaneously into the chamber at fixed flow rates. This technique introduces a higher concentration of the reactive gas into the chamber and gives it longer time to react with surface contaminants before being pumped out. When using pulsed flow, the cleaning time and the type of gas mix used do not have significant effects on cleaning. Interestingly, these two factors become significant when using constant flow. The average contact angle is also reduced by 50% and uniformity improves. Three types of gas mix were evaluated in this study: (a) Ar/H2, (b) Ar/H2 with O2 and (c) Ar with O2. Gas mix b was found to be the most effective due to the combination of O2 for removing organic contamination and H2 for oxide removal. Better cleaning efficiency was observed with a ratio of reactive-to-inert gas that is at least 1:1. Based on our study on uniformity, there is a high probability that the position of magnets on the ECR tray affects the path of reactive ions thus affecting the cleaning uniformity [4]. An electromagnetic simulation & optimization of the magnet arrangement is pending; although, the authors have observed a significant improvement in uniformity with longer cleaning time. B-MW Plasma Cleaning vs. Strip-RF Plasma Cleaning. After optimization of the abovementioned parameters for a 77 x 240 mm NiPdAu lead frame, the cleaning performance of batch-type MW plasma is compared with that of a strip-type RF plasma. ANOVA confirms at 95% confidence level that both plasma techniques have a significant effect on improving the wetting of NiPdAu surfaces. However, B-MW plasma is 1.6x more effective in cleaning and can improve manufacturing productivity up to 2.5x. B-MW plasma application is intended prior molding of IC packages. Sample units have been sent for reliability testing, specifically C-mode Scanning Acoustic Microscopy after pre- conditioning at various moisture sensitivity levels. This is to validate the contact angle measurements and ensure that the plasma technique has indeed addressed mold-lead frame delamination.
Ghizelle Jane E. Abarro, Package Development Engineer
ON Semiconductor Philippines Inc.
Carmona, Cavite
Philippines


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