Microelectronics Reliability
Volume 47, Issue 12,
December 2007
, Pages 2152-2160
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Abstract
This paper presents a study of the optimization of the gold plating thickness for the use of both wire bonding and soldered interconnects on a flexible printed circuit board sample module. Wire bondability is typically better, when the gold plating thickness is greater than 30μin.; however, the risk of problems with solder joint embrittlement becomes a concern with thick gold plating. In order to better understand the effect of the gold plating thickness on wire bondability and solder joint embrittlement, an evaluation was performed on samples with three ranges of gold plating thicknesses (10–20μin., 20–30μin., and 30–45μin.), on flexible printed circuit board (PCB), substrates. Mechanical shear testing and metallurgical analyses were conducted on chip component solder joints in this three thickness gold study. Thermal shock and drop testing were conducted to evaluate the reliability of the sample modules. Drop testing is especially critical for determining the reliability of the sample modules, which are used in portable consumer electronics products. Reliability testing and metallurgical analyses have been performed to characterize the effect of gold embrittlement on the mechanical integrity of the solder joints with a gold content ranging from 1 to 4wt.%.
Introduction
Gold has a long history of applications in the electronics industry and is commonly used as a mechanical contact finish, a wire bondable surface finish, and a solderable coating. The application of gold as a solderable coating has been a concern because of the impact of gold on solderability and the effects of gold on the mechanical and metallurgical properties of the solder joints. Much of the concern is based on a history of problems with gold embrittlement that were experienced in the past, when the thicknesses of the gold surface finishes were much greater than in more current applications. More recently, gold embrittlement has caused missing ball failures in ball grid array packages (BGA), and premature fatigue failure problems in newer technology BGA packages [1], [2], [3], [4], [2].
Gold plating is very versatile and its properties vary greatly depending on the plating process conditions. When gold is used as a mechanical contact surface finish, harder, fine-grained deposits are formed by the use of hardening elements, grain refining elements and brighteners, which produce smooth, shiny, and generally thinner coatings. Gold deposits with few or no additives produce rougher, duller, matte finishes, which are desirable for wire bonding and soldering because they are metallurgically soft and pure.
Solder forms intermetallic compounds with the various metals in the component and printed circuit board, (PCB) solder pads, which is a requirement for a good solder joint. The metallurgy of the solder and the surface finish of the solder pads play an important part in the reliability and the failure modes of the solder joints. In this paper, electrolytic gold–nickel PCB surface finish, tin–lead eutectic solder, and tin–silver–copper lead free solder are evaluated in solder joints to chip components and gull wing style connector leads. When solder is reflowed in contact with a gold–nickel surface finish, the gold quickly dissolves into the solder and forms a distribution of gold, nickel, and tin bearing intermetallic phases which are brittle structures in the solder. A loss of ductility of the solder joint, or so-called gold embrittlement can occur if the gold concentration in the solder joints is too high. Gold embrittlement can lead to a significant reduction in the fatigue life and the mechanical integrity of the solder joints.
It is perceived that gold embrittlement is a concern in solder joints with greater than 3% gold [5]. Zhong gives a range of gold content greater than 3–5% [2]. Although there are several reports that gold embrittlement can occur in at a much lower percentage in a ball grid array (BGA), packages [6], [7], [8]. A rule of thumb is that gold embrittlement is not a problem as long as the thickness is kept below 15μin. Fortunately, most current applications have very thin gold plating in the range 3–15μin. The emergence of improved plating processes that produce dense, low porosity plating along with the reduction in storage time between plating and assembly have allowed for adequate solderability with relatively thin gold surface finishes. The key to reducing the extent of gold embrittlement in solder joints is keeping the gold thickness to a minimum.
There is a trade-off between maintaining a low enough gold thickness to prevent gold embrittlement, and a high enough gold thickness for robust wire bondability, because wire bondability is typically better with thicker gold pads for either gold or aluminum bonding wire. Some electronics assembly manufacturers prefer to use thinner gold to minimize the risk of gold embrittlement problems, while others prefer thicker gold plating, which affords a longer shelf life and more robust wire bonding processes.
In this study with wire bonding to electrolytically plated flex circuits, the first bond is made by ball bonding gold wire to an aluminum pad on the die and then stitch bonding to the flex circuit substrate pad. The process is generally robust, yet if a problem occurs, it is usually related to either a problem with the plating metallurgy or surface contamination. Cross-contamination from a previous soldering process or a die attach process can cause problems with the wire bond process. Wire bonding requires clean and metallurgically soft wire bond pads. The use of thicker gold allows for the use of more aggressive wire bonding conditions for scrubbing through surface contaminants. Wire bond pull testing is commonly used to test wire bonds; however, the effectiveness of wire pull testing for detecting wire bonding problems in the manufacturing process and screening good and bad lots of printed circuit boards has been debated [9], [3].
Gold plating thickness requirements vary widely from customer to customer, yet many customers require a minimum of 30–40μin. There is little agreement on the optimum gold thickness for either wire bonding or for making solder joints in microelectronics packages. Holcomb concluded that 24μin. is a reasonable lower limit for a well-controlled electrolytic gold process on organic substrates in BGA packages [9]. Plasma cleaning removes adventitious carbon and inter diffused nickel oxides from the surface of the gold, and allows for good wire bondability with thinner gold plating. The success of a wire bonding process is very dependent on the quality of the plating and the manufacturing process conditions. For example, wire bonding to a flexible printed circuit board on our sample modules is much more difficult than to a die on a lead frame, because the lead frame can be heated to a much higher temperature during wire bonding.
Section snippets
Reliability testing
Air to air thermal shocks tests were performed from −40 to 85°C, with 30min dwell times at each temperature. The sample modules were functionally tested at intervals of 250 thermal shocks. Hot storage testing might have been a better indicator of problems with gold embrittlement than thermal shock; however, the optics in our sample modules become damaged at the temperatures typically used for hot storage tests, so hot storage was impractical [2], [3], [4], [2], [10]. Hot storage testing is also
Three thickness gold study with tin–lead solder
This study was conducted on sample modules with flexible printed circuit boards (PCB’s), with electrolytically plated gold–nickel pads and FR4 dielectric, Fig. 1.
The sample modules were assembled using standard assembly and wire bonding practices using a 63% tin–37% lead solder alloy. The gold thicknesses were 10–20μin., 20–30μin. and 30–45μin., and were measured by X-ray fluorescence. The thickness of the nickel barrier layer was approximately 80–120μin. Theoretical values for the gold content
Conclusions
In the study with tin–lead solder, the results of the theoretical percentage gold calculations, capacitor shear testing, metallurgical studies, and reliability testing did not allow for a conclusive determination of whether gold embrittlement was a reliability problem in sample modules with thick gold plating (30–45μin.). The modules passed thermal shock testing, and exhibited capacitor shear test failure modes that consisted of mostly PCB pad lifts and end-terminal metallization failures
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Infrared heater is commonly used as the heat source in reflow oven. A literature on the mechanical properties of solder joints in the component assembly on flexible printed circuit board (FPCB) was proposed by [1]. Reflow process, mechanical shear test and metallurgical analysis of solder joint were reported in their work.
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