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PCB-Jun2018

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60 PCB007 MAGAZINE I JUNE 2018 ly, the electroless palladium and immersion gold plated deposits are normally quite thin which reduces chance of metal bridging. In addition, there is the requirement for re- ducing the EN deposit because less electronic signal loss is necessary for RF (radio frequen- cy) module. As an EN-less or EN-free process, the electroless thin Ni/Pd/Au (thin ENEPIG) or EPIG process has been proposed. However, the pattern ability of thin ENEPIG has some re- strictions for extremely fine patterns because a Pd activator process is used. In some cases, re- sidual Pd activator (catalyst) could remain be- tween traces, even with thorough rinsing, and help to serve as sites for extraneous metal plat- ing between traces. And the WBR of EPIG after heat treatment (HT) was inadequate, especial- ly when the gold thickness is less than 0.1 µm. The main factor is that WBR becomes worse by HT is Pd and Cu diffusion to the top sur- face of Au. After much examination, we developed the IGEPIG process without Pd activator or any EN deposit as a new process which has excellent WBR and pattern ability. In this study, we in - troduce the performance of the IGEPIG deposit. All three finishes discussed are wet plating processes, depositing metals from aqueous so- lutions. ENEPIG utilizes both immersion reac- tions (palladium catalyst and immersion gold) and electroless reactions (electroless nickel and electroless palladium). EPIG utilizes im- mersion reactions (palladium catalyst and im- mersion gold) and one electroless palladium step. IGEPIG utilizes two immersion reactions (an initial immersion gold and final immer- sion gold) and one electroless palladium step. All three processes utilize the same aqueous chemical pre-treatment of cleaning, copper mi- cro-etching, and acid dipping. Experimental Methods The coupons used in this study consisted of a copper-clad laminated substrate which was copper plated to a thickness of 20 µm using an acid copper electroplating process. For SJR tests, the copper-plated substrate was coated with solder mask and imaged to form the sol- der ball pads of 0.25 mm diameter. Further- more, the substrate of the copper pattern with 20 µm space of the wiring line was used for evaluating of the pattern ability. Each substrate was plated with EPIG, ENEPIG and IGEPIG by using plating chemicals commercially avail- able from Uyemura. Table 1 shows the plat- ing process of each deposit. For Pd plating of ENEPIG deposit, pure Pd and Pd-P type were compared. And pure Pd type was used for Pd plating of EPIG and IGEPIG. As the solder ball for the evaluation of SJR, 0.3 mm diameter of Sn-3.0Ag-0.5Cu was used. The reflow profile with the top temperature of Table 1: Plating processes of ENEPIG, EPIG and IGEPIG.

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