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56 SMT Magazine • January 2017 used for surface finishing of package substrates. However, this technique cannot be applied to high-density package substrates, because it re- quires bus lines to each terminal and the nec- essary area for those lines. Therefore, electroless Ni/Au plating was adopted because it does not require bus lines. However, drop tests revealed that the reliability of ball joints prepared by this technique is insufficient. To solve this prob- lem, electroless Ni/Pd/Au plating was adopted for surface finishing of the terminals of package substrates 17-19 . Recently, electroless Ni/Pd/Au plating is be- ing offered as an alternative surface finishing process with high solder joint reliability and wire bondability. In the previous studies, the details regarding the influence of Pd thickness on solder joint reliability and IMC growth have not been provided. In this study, the influence of Pd film thickness on solder ball joint reliabil- ity was investigated to clearly identify the opti- mum thickness of the Pd film in electroless Ni/ Pd/Au plating. Experiments Sample Preparation A test pattern was formed on an epoxy res- in copper cladding laminate (MCL-E-679F; Hit- achi Chemical Co. Ltd) using the semi-additive method. The thicknesses of the board and cop- per pad were 0.6 mm and 25 µm, respective- ly. Then, solder resist was formed with a solder mask using a photo-definable type resist. The opening diameter of the ball pad was 0.45 mm. These test substrates were covered with electro- less Ni/Au (ENIG: electroless Ni/immersion Au) and electroless Ni/Pd/Au (ENEPIG: electroless Ni/electroless Pd/immersion Au) plating. Af- ter applying flux, solder balls (SAC305) were at- tached to the test substrates and passed through a nitrogen-reflow furnace. The thickness of each plating film and the evaluation conditions are listed in Table 1. Reliability Test on Solder Ball Joints (High- Speed Shear) As shown in Figure 1, the distance between the shear tool tip and the package substrate was 50 µm. The shear speeds were 20, 200, and 500 mm/s. A high-speed bond tester 4000HS (Dage Precision Industries, Ltd.) was used (Figure 2). The fracture surface was observed with an optical microscope, and the fracture modes were classi- fied into four failure modes. Typical images of these four failure modes are shown in Figure 3. Method of Evaluating the Covering Property of Pd Plating Film on Ni Plating Film To evaluate the covering property of the Pd plating film on the Ni plating film, ultrasonic Figure 1: Outline of solder-ball shear test. Figure 2: Externals of high-speed shear test equipment (DAGE 4000HS Bond Tester). SOLDER BALL JOINT RELIABILITY WITH ELECTROLESS NI/PD/AU PLATING