(not available)

(not available)

(not available)

This paper had as an aim to correlate the degree of voiding with reliability. Here the a purpose designed test vehicle was
used,in which seven different solder pastes from three manufacturers were evaluated through a range of reflow profiles. In
spite of strenuous efforts we had the surprising result of only producing limited voiding. Hence,end–users can have a high
degree of confidence that voiding levels in lead-free solder pastes can be kept below the IPC specified maximum of 25% by
area. No paste exhibited voids in any PBGA joint greater than 15% despite attempts to produce higher voiding levels.
Three different voiding levels were produced and samples subsequently subjected to 2000 thermal cycles (-55 to +125oC).
Despite these samples having higher voiding levels than those stipulated for class 3 of the IPC’s BGA Assembly and
Inspection Guidelines,no adverse effect on reliability was seen. The shear strength deterioration during thermal cycling of
chip resistors also with these levels of voiding,was also unaffected.

With the transition to high tin solders,and 100% tin finishes,some concerns have been raised about the possible occurrence
of “Tin Pest”. This paper reviews the historical and recent data collected on this phenomenon. No attempt has been made to
collect and explain thermodynamic data. Various myths are either explained or discounted,including one about “Napoleon’s
buttons”. All linear rate data that could be found is included. A correlation with atomic radii of elemental additives is also
expounded. A summary of analytical techniques which can be used for examining the results of this transformation is also
included.

The implementation of both RoHS and WEEE Directives by the European Union (EU) mandate that electrical and electronic
products put on the market within the EU shall contain restrictive amounts of lead. Bright tin-lead plating has been used for
decades for electronic components and suitable alternatives have been investigated. One candidate that appears to meet most
of the required criteria,such as corrosion resistance,solderability and low cost,is pure tin,however,the use of pure tin
inevitably raises the specter of tin whiskers. The mechanism of tin whisker growth,despite the very significant amount of
research effort devoted to investigating this phenomenon,remains incomplete. It is understood that compressive stress,
introduced into the tin deposit and sometimes inherent within it,is a significant cause of whiskering. Likewise,methods of
whisker growth mitigation such as the use of a nickel pre-plate are also well documented. On reviewing the literature it
quickly becomes clear that there is a strong bias towards the use of matte tin plating processes. This is at least partially
attributable to some basic characteristics of matte and bright processes. Matte tin electrolytes are generally less chemically
“complex” than bright ones,and the resultant deposits normally contain less organic materials. Our recent research has
characterized the whisker growth propensity of multiple matte and bright plating formulations utilizing recently accepted
whisker test methods. We have found that the choice of organic additives used in both matte and bright tin electrolytes can
have a profound effect on their respective tendency to initiate whisker growth. We will outline our whisker results in detail
and examine key process and coating characteristics which may explain this preferred whisker performance.

A comparison study was carried out with Sn3.5Ag and Sn3.8Ag0.7Cu solder balls on Ball Grid Array (BGA) components
with Cu/Ni/Au pad finishing. This study shows that Sn3.5Ag C5 solder system performs better than Sn3.8Ag0.7Cu in terms
of joint strength and brittle mode failure. Experimental works were carried out to observe the melting properties of the solder
alloys by Differential Scanning Calorimetry (DSC). Solder ball shear and cold pull strength after ball attach,high
temperature storage (HTS) and multiple reflow were measured by Dage to gauge the solder joint strength and intermettalic
compound (IMC) thicknesses were measured after cross-sectioning,. Drop Tests were done per ASE & Freescale methods to
study the solder joint performance against vibration and impact shock. Liquid-liquid thermal shock was done to assess Board
Level Reliability. A comprehensive study was done using SEM and EDX to study the effect of microstructure and interface
intermetallics of both solder system at ambient,HTS at 150ºC for 168 hours and 6x multiple reflow towards the joint
integrity. Microstructure studies on SnAg solder reveals that formation of rod shape Ag3Sn IMC distributed across the solder
surface helps to act as dispersion hardening that increases the mechanical strength for the Sn3.5Ag solder. EDX analysis
confirmed that in SnAgCu solder/Ni interface,Cu-rich IMC formed on top of the Ni-rich IMC. For SnAg system,only Nirich
IMC is found. Therefore,it is highly suspected that the presence of Cu-rich IMC posed a detrimental effect on the joint
strength and tends to cause brittle joint failure. Both of the effect is then showed in ball pull result that after HTS,SnAgCu
solder has 99.5% brittle mode failure,where SnAg solder has 0%. This result correlates with missing ball responses after
packing drop tests as well as liquid-liquid thermal shock result. Thus,despite having 4ºC higher melting temperature than
SnAgCu,improvement on SnAg was obtained using the SnAgCu reflow profile. Thus,SnAg eutectic solder is a potential
candidate for lead-free solder joint improvement for overall lead-free package robustness.

Many existing and some new laminate material test methods that are being proven useful for determining a laminate material's suitability for use in higher temperature lead-free assembly processing. Most notable of these is the new "Laminate Material Temperature of Decomposition" test method (IPC TM-650,2.4.24.6) which will be listed as a parameter for the more thermally robust material classifications listed in the new IPC-4101B,Specification for Base Materials for Rigid and Multilayer Printed Boards. However many OEMs are finding that larger,thicker multilayer boards (MLBs) are more prone to delaminate during higher temperature assembly processing and rework.

Several large OEMs (IBM,HP,CISCO,etc.) have been working together as a group to develop a new test method for evaluating the thermal robustness of multilayer boards. This test would to be primarily used for initial qualification and occasional on-going process/material monitoring. After developing a product history and a well populated database,this could lead to the specification of a minimum requirement for acceptance. This new multilayer board (MLB) delamination and laminate integrity test method should detect improper press lamination (cycle time too short or peak temperature not reached in center of stack-up/press opening),dry weave or excessive voids between fibers within a glass reinforcement bundle,delamination,and/or poor quality prepreg in prepreg layers. This "MLB Laminate Integrity" test should also detect "tail cracks" and "eyebrow" type localized delamination
(example: cracks in annular pad area of blind vias). Similar to T260 testing,labs should use the same "bumps" and starting points for determining test failure. A standard test board / test coupon containing the worst case design features should be used for using this new "MLB Laminate Integrity" test method to evaluate laminate materials.

Participants in this effort are needed to provide "good" multilayer boards and "bad" multilayer boards for a given preheat profile and peak solder reflow temperature. These will be used to evaluate the sensitivity of the new test method using a range of test parameters. Both the IPC and iNEMI are considering forming subcommittees that may take on this project of developing an industry standard MLB Delamination Test Method.

The debate on the effect of voiding on BGA reliability has continued for years. Many PWB assemblers strive to minimize
voiding,particularly with the advent of lead-free processing and in fine feature area array devices. Although solder pastes
have been designed to minimize voiding,and processing guidelines exist to mitigate void formation during reflow processing,
the presence of a microvia in a PWB pad can contribute significantly to void formation. It is believed that the depression in
the pad caused by the microvia traps air during the stencil printing process,and the air cannot fully escape during reflow.
A process of filling the vias with copper at the board fabrication phase,thereby eliminating the depression that contributes to
voids,was tested for its effectiveness in void mitigation during assembly. The test compares the voiding results of filled vias
with those of unfilled vias and flat pads with no vias at all. The test vehicle and methods,as well as the results of the tests are
presented and discussed in detail.

Nitrogen inerting has been widely reported to reduce defects in lead-free reflow soldering. However,many solder pastes
available claim that they either do not need nitrogen or work equally well in air. While some of these pastes can produce an
acceptable joint quality,they are very susceptible to any narrowing of the process window and some cannot produce high
quality joints even under the most advantageous conditions.
At a leading consumer electronics manufacturer in Asia,three pastes from major producers were compared. Commercial
boards were reflowed in air,and in nitrogen at two purity levels. The boards were then visually inspected for joint quality
using the manufacturer’s standards. The results showed that even the joint quality produced by the best paste could be
improved using nitrogen and the highest nitrogen purity tested could bring the worst paste up to the standard of the best.