Applied Soft Computing 48 (2016) 124–136 Contents lists available at ScienceDirect Applied Soft Computing journal homepage: www.elsevier.com/locate/asoc Robust parameter design for the micro-BGA stencil printing process using a fuzzy logic-based Taguchi method Tsung-Nan Tsai a,∗ , Mika Liukkonen b a Department of Distribution Management, Shu-Te University, 59, Hengshan Rd. Yenchao, Kaohsiung, Taiwan b University of Eastern Finland, Department of Environmental Science, P.O. Box 1627, FIN-70211 Kuopio, Finland a r t i c l e i n f o Article history: Received 12 January 2016 Received in revised form 29 May 2016 Accepted 16 June 2016 Available online 30 June 2016 Keywords: Fuzzy logic Surface mount technology Stencil printing Ball grid array Printed circuit board Neural network Genetic algorithm a b s t r a c t Solder paste is the main soldering material used to form strong solder joints between printed circuit boards (PCB) and surface mount devices in the surface mount assembly (SMA). On average 60% of end- of-line soldering defects can be attributed to inadequate performance of solder paste stencil printing. Recently, lead-free solder paste has been adopted by electronics manufacturers in compliance with the RoHS directive. However, soldering defects in the ball grid array (BGA) packages used in lead-free SMA have become more prevalent and are difficult to detect. In this study, a fuzzy logic-based Taguchi method is proposed to optimize the fine-pitch stencil printing process with multiple quality characteristics for the micro ball grid array (micro-BGA) packages using a lead-free solder paste. A structured data set is first collected from an L 18 (2 1 × 3 7 ) fractional factorial design experiment, followed by multi-response optimizations and analysis of variance (ANOVA) for identifying significant factors. The optimization per- formance gained by the proposed fuzzy logic-based Taguchi method is compared with the results of other two hybrid methods including a combination of neural networks and genetic algorithms, and the inte- gration of the response surface methodology with a desirability function. The confirmation experiments show that the proposed fuzzy logic-based Taguchi method outperforms the other two methods in terms of the signal-to-noise ratios and process capability index. © 2016 Elsevier B.V. All rights reserved. 1. Introduction Surface mount technology (SMT) is the primary method used for the fabrication of many modern electronic products. The surface mount assembly (SMA) process consists of three sub-processes, including solder paste stencil printing application, component placement, and solder reflow, to form strong metal solder joints between the printed circuit board (PCB) and surface mount devices (SMDs). Changing consumer preferences have driven product designers to continue the move towards miniaturization that makes SMA process more complicated to control. Especially, the solder paste stencil printing is recognized as the most difficult pro- cess for quality assurance in electronic assembly, and averagely 60% of end-of-line soldering defects can be attributed to poor stencil printing performance [34,46]. The purpose of the stencil printing process is to consistently deposit the desired amount of solder paste in the appropriate form ∗ Corresponding author. E-mail addresses: tntsai@stu.edu.tw (T.-N. Tsai), mika.liukkonen@uef.fi (M. Liukkonen). at the correct position for every printing cycle [6]. Taking the BGA package as an example of stencil printing, in the first step, air pres- sure is applied to force the solder paste to roll to the front of the squeegee blade and fill in the stencil apertures, as displayed in Fig. 1(a). The squeegee travels across the surface of the stencil to transfer and deposit solder paste onto the corresponding PCB pads for the BGA package after the stencil is released, as depicted in Fig. 1(b). After that, the BGA package is placed on the corresponding printed pads using a placement machine in the component place- ment stage, as shown in Fig. 1(c). Finally, the solder paste is heated and solder joints are formed through a reflow oven. The stencil printing performance is influenced by many vari- ables, including stencil aperture design, solder paste properties, tool selection, and printing method [40]. The stencil printing factors are frequently considered by researchers and process practitioners, as summarized in Table 1. For examples, Coleman [10] and Smith [43] revealed that the amount of solder paste deposited is directly restricted by the stencil aperture area ratio and stencil thickness. The squeegee pressure and speed, as well as the speed used to sep- arate the stencil from the PCB also have consolidated effects on the uniform formation of paste bricks [34,45]. Too low a squeegee pressure can leave a thin film of solder paste on the stencil surface, http://dx.doi.org/10.1016/j.asoc.2016.06.020 1568-4946/© 2016 Elsevier B.V. All rights reserved.