Pericles Kondos

Assessing the risk of “Kirkendall voiding” in Cu3Sn

Abstract A common location of sporadic, usually unpredictable, premature failures of both SnPb and Pb-free solder joints is within the intermetallic bond to one of the contact pads. Cu pads tend to be less prone to such defects than Ni/Au coated pads, but one phenomenon is a cause of growing concern for high reliability applications. Sporadic intermetallic problems are notoriously difficult to screen for, but at least a number of them are detectable right after assembly. However, the occasional formation of a network of voids within the Cu3Sn layer forming when soldering to Cu pads is rarely noticeable until much later. To make matters worse, efforts to establish practical engineering tests for purposes of identification and screening have usually not been successful. The root cause of the so-called ‘Kirkendall voiding’ has been shown to be associated with the quality of the electroplated Cu, apparently as determined by the incorporation of minute amounts of specific organic impurities. The present work describes and discusses a series of potential tests that allow the sorting of electroplated Cu in terms of its propensity for Cu3Sn voiding during soldering and subsequent aging. The times and efforts involved in these tests differ greatly, as do their effectiveness and accuracy. Individual tests may therefore be useful at various stages from supplier qualification to monitoring of batch to batch variations.

Improving Copper Electrodeposition in the Microelectronics Industry

Sporadic voiding within the interfacial Cu3Sn intermetallic compound (IMC) layer-sometimes referred to as ¿Kirkendall voiding¿-has been found to lead to degradation of solder joint reliability in board level, mechanical shock testing. It has been suggested that the voiding phenomenon is a result of the incorporation of organic impurities in the copper (Cu) deposit during electroplating. In the present study, Cu samples were electroplated galvanostatically from a generic solution, containing Cl- ions, as well as a suppressor [polyethylene glycol (PEG)], and a brightener (bis(3-sulfopropyl) disulfide, SPS) as additives. Overpotential transients were measured during electroplating with a range of current densities (0.5-40 mA cm-2) in baths with various compositions. Effects of the bath chemistry on the Cu surface morphology, as well as on the propensity for voiding after soldering, were also investigated. Elemental analysis of selected samples was performed by SIMS. Plating at 10-20 mA·cm-2 with an optimized bath composition led to Cu with a fine-grain structure and smooth appearance. Solder joints formed from these deposits remained void free after soldering and thermal aging. Lower current densities, ran in the same plating bath, led to a significant propensity for voiding, apparently because of incorporation of, principally, SPS and its breakdown products into the growing layer. Continuous plating at 10 mA cm-2 for up to 18 hours without replenishment revealed a strong dependence on bath aging, with Cu changing from ¿void-proof¿ to clearly ¿void-prone.¿ These trends were attributed to the different rates of consumption for PEG and SPS and changes in the contaminants being incorporated in the deposits. In general, differences in the voiding behavior of the plated Cu could be predicted by monitoring a set of characteristic overpotential transient signatures.

Controlling Cu electroplating to prevent sporadic voiding in Cu 3 Sn

One reliability concern, when soldering to a Cu surface finish, is the sporadic mechanical degradation of solder joints due to the formation and growth of voids within the interfacial Cu3Sn intermetallic compound (IMC) layer and at its interface with the Cu pad structure. Excess organic impurity incorporation during Cu electroplating has been shown to cause this problem. The level of impurity incorporation is found to depend greatly on interactions between the plating additive chemistry and the plating process parameters. A general picture has been developed, based upon the parabolic adsorption behavior of organic molecules on metal electrodes in aqueous plating solutions as a function of the applied potential. Thus the propensity for voiding, in soldering and subsequent thermal aging, can be manipulated by varying a range of plating parameters. The mechanistic understanding required to devise practical control is outlined. Any remaining research required for the formulation of step-by-step process guidelines is identified.

On the evolution of the properties and microstructure of backward compatible solder joints during cycling and aging

A number of groups have published accelerated test results comparing backward compatible (mixed) solder joints to pure SnPb and lead free ones, but the question remains as to the actual reliability under realistic long term service conditions. We are addressing this in a comprehensive fundamental study of the evolution of the microstructure and resulting joint properties and performance for mixed solder joints. The present work reports results of reflowing 30 mil SAC305 balls onto Cu coated BGA pads with different amounts of SnPb paste, aging and/or cycling the joints and inspecting the microstructure by cross polarizer microscopy and SEM. The addition of small amounts of Pb was found to affect the solidification during cool-down from reflow, and thus the initial microstructure, in a profound fashion. The shapes and distributions of secondary precipitates were, as expected, different from those observed in ‘pure’ SAC305 joints and a large number of very small Pb inclusions were distributed within the Sn dendrites. As a result the mixed joints started out harder and did not soften as rapidly during thermal cycling and aging. In fact, an initial break up of elongated Ag3Sn precipitates first led to a slight hardening and strengthening. In addition thermal cycling induced recrystallization of the Sn was delayed and acceleration factors were lower. However, the strongly reduced sensitivity to aging suggests that mixed joints may compare more favorably to their ‘pure’ lead free counterparts in long term service than reflected by accelerated test results.

Passive Optoelectronics Component Manufacturing: A Case Study

The infrastructure required for fiber optic communication systems to become truly affordable includes a supply of individual components at a price that can only be achieved through full automation of the packaging processes. We illustrate the manufacturing of typical passive photonics components through the case of a polarization dependent optical isolator. Identification of the available options here requires an understanding of the underlying optical principles and functionality, as well as of the influences of process variations and part tolerances on performance. Issues relating to cost, ease of manufacturing and automation are discussed. These include component design and materials selection, as well as questions of intellectual property.Copyright

Packaging of Single Mode Laser Diodes

From a manufacturing perspective optoelectronics packaging is often quite immature and significant optimization still possible. This may, however, require some understanding of the optics as well as of the mechanical and thermal design, materials, and process issues. This is certainly the case for single mode laser diode packaging, where optical alignment and coupling plays a dominant role. Referring to a couple of current generic packages for illustration the present work offers a discussion of some of the manufacturing issues involved and potentials for improvements. This includes alternatives in terms of package design and contents, optical alignment and coupling schemes, and the use of laser welding, soldering, and adhesives.Copyright