Characteristics of New Electroless Au/Pd/Au Process for Fine-Line Applications


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Abstract

The characteristics of electroless gold/palladium/gold (IGEPIG) deposit, which has been newly developed for fine-line application as electroless nickel-phosphorus (EN) free, has been compared with that of conventional electroless Ni-P/Pd/Au (ENEPIG) deposit and electroless Pd/Au (EPIG) deposit.

The IGEPIG deposit had excellent wire bonding reliability (WBR) compared with other deposits even if the gold thickness of IGEPIG at the top surface was less than that of the EPIG and ENEPIG deposits. From the results of AES analysis after heat treatment for 16 hours at 175°C, Pd and Cu was hardly detected at the top surface of Au, and it indicated that IGEPIG deposit prevented Cu and Pd diffusion. Also, it was revealed by EBSD (electron back scatter diffraction pattern) analysis that Pd grain size on Au was about ten times bigger than those on a Pd-activated copper surface. It was assumed that the prevention of Pd and Cu diffusion was relative with bigger Pd grain size of IGEPIG deposit.

Solder joint reliability (SJR) of IGEPIG deposit with SAC305 (Sn-3.0Ag-0.5Cu) solder ball is better than that of EPIG deposit, and almost the same with that of ENEPIG deposit. IGEPIG deposit had excellent pattern ability, because of no EN deposit and Pd activator process compared with ENEPIG and EPIG. 

Introduction

Recently, the pattern of copper lines and spaces on PCBs has become narrower with the miniaturization of the integrated circuit. If using a conventional ENEPIG process with a nickel thickness of 5–6 µm, it will be easy to cause short circuits between each pattern line because of over-plating. Electroless nickel plating grows both vertically and horizontally, thus increasing the chance of bridging traces when the distance between traces is small. Conversely, 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 reducing the EN deposit because less electronic signal loss is necessary for RF (radio frequency) 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 restrictions for extremely fine patterns because a Pd activator process is used. In some cases, residual Pd activator (catalyst) could remain between traces, even with thorough rinsing, and help to serve as sites for extraneous metal plating between traces. And the WBR of EPIG after heat treatment (HT) was inadequate, especially 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 surface 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 introduce the performance of the IGEPIG deposit.

All three finishes discussed are wet plating processes, depositing metals from aqueous solutions. ENEPIG utilizes both immersion reactions (palladium catalyst and immersion gold) and electroless reactions (electroless nickel and electroless palladium). EPIG utilizes immersion reactions (palladium catalyst and immersion gold) and one electroless palladium step. IGEPIG utilizes two immersion reactions (an initial immersion gold and final immersion gold) and one electroless palladium step. All three processes utilize the same aqueous chemical pre-treatment of cleaning, copper micro-etching, and acid dipping.

To read the full version of this article which originally appeared in the June 2018 issue of PCB007 Magazine, click here.

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