Chip scale packages (CSPs) are widely used in portable electronic products where there is also a growing trend to lead free
assembly. Many CSP designs will meet the thermal cycle or thermal shock requirements for these applications. However,
mechanical shock and bending requirements often necessitate the use of underfills to increase the mechanical strength of the
CSP-to-board connection. Three underfill options compatible with lead free assembly have been evaluated: capillary
underfill,fluxing underfill and corner bond underfill. CSPs with eutectic Sn/Pb solder were used for control samples. Without
underfill,lead free and Sn/Pb eutectic drop test results were comparable.
Capillary flow underfills,dispensed and cured after reflow,are commonly used in CSP assembly with eutectic Sn/Pb solder.
With capillary flow underfill,the drop test results were significantly better with lead free solder assembly than with Sn/Pb
eutectic.
Fluxing underfill is dispensed at the CSP site prior to CSP placement. No solder paste is printed at the site. The CSP is placed
and reflowed in a standard reflow cycle. A new fluxing underfill developed for compatibility with the higher lead free solder
reflow profiles was investigated. The fluxing underfill with lead free solder yielded the best drop test results.
Corner bond underfill is dispensed as four dots corresponding to the four corners of the CSP after solder paste print,but
before CSP placement. The corner bond material cures during the reflow cycle. It is a simpler process compared to capillary
or fluxing underfill. The drop test results with corner bond were intermediate between no underfill and capillary underfill and
similar for both lead free and Sn/Pb eutectic solder assembly.
The effect of aging on the drop test results with lead free solder and either no underfill or corner bond underfill was studied.
This test was to simulate drop performance after the product has been placed in service for some period of time. There was
degradation in the drop test results in both cases after 100 and 250 hours of storage at 125oC prior to the drop test.
The assembly processes,drop test results and failure analysis are presented.

The ever-increasing use of high frequency in high density interconnect (HDI) assemblies,combined with the worldwide
move toward lead-free manufacturing,has initiated a closer scrutiny towards effective flux removal processes. Since
adequate climatic operating conditions cannot always be assumed,system signal integrity maybe vulnerable to failure
through induced capacitive effects of hygroscopic activator residues. Furthermore,such contamination,particularly with the
new lead-free solder paste formulations,is no longer-detectable by ion-equivalent measurement alone.
Most failures of electronic components in humid environments are caused by electrochemical migration and corrosion
induced leakage currents. In this paper,the origins and effects of such failure mechanisms are examined. In addition,the
influence of alloy types,with particular reference to lead free formulations,is also discussed. The critical importance of
contamination free surfaces in high frequency circuits is outlined. Finally,different methods to determine climatic reliability
are discussed,in which a new and innovative test method is described.

Lead-free alloys under consideration have physical properties,which may directly impact industry standard electronic assembly
cleaning processes. The purpose of this study is to evaluate how the use of nitrogen versus non-nitrogen reflow atmospheres affect the
cleanability of flux residue from RMA,synthetic and water-soluble surface mount solder paste residues.
The designed experiment evaluated commercially available lead-free solder paste products and industry standard cleaning materials.
The cleaning evaluations were conducted in a controlled application lab while using thermally profiled reflow conditions and
cleaning equipment. The response variables used will be qualitative visual inspection of white residue and solder bump appearance.

Conductive Anodic Filamentation is a subsurface failure mode for woven glass reinforced laminates (FR4) materials,where a
copper salt filament allows bridging between via walls or other copper conductors. In this study FR4 laminates,in the form of
high via density multi-layer test circuits,are exposed to different manufacturing conditions and assessed for resistance to
Conductive Anodic Filamentation (CAF). CAF performance was assessed using high temperature and humidity conditions to
promote failures,with a voltage applied across adjacent vias. By application of a range of voltages and via geometries a
performance map for laminates can be obtained to compare materials for performance. The changes due to exposure of
laminate to lead-free temperatures and other processing steps are then examined using the technique.

Various test methods are used to characterize the PCB plated-thru-hole reliability. One such method is the Interconnect Stress
Test (IST). The results from this test are often used to qualify PCB materials and/or fabricators. This paper will discuss how
certain statistical analysis techniques may be used to decipher the results,and predict capabilities of PCB materials and/or
processes.

The selection and use of solder joint modification configurations for printed wiring assemblies has traditionally been a design
specific activity. The implementation and use of a standardized set of solder joint modification configurations on an industrywide
basis would be cost effective and promote industry consistent modification practices. A set of 14 commonly used IPC
Class 3 modification configurations were selected for investigation. Thermal cycle testing was chosen to evaluate the solder
joint thermal cycle fatigue reliability of the configuration set. A total of 1,972 thermal cycles using a -55ºC to +125ºC recipe
in accordance with the IPC-9701 guidelines were completed. Failure analysis and photo-documentation were conducted
characterizing modification configuration solder joint geometries and wetting angles. The investigation
results/recommendations were disseminated to the IPC-A-610 and IPC-7711/7722 specification committees for potential use.

A variety of mechanical and thermal stress related problems related to Flip Chip Ball Grid Array Packages (FCBGAs) are
often solved by the Finite Element Method. Often,the question for the stress analyst is how to simplify the problem at hand
and come up with a sufficiently accurate model in a reasonable amount of time. The aim of this paper is to examine several
different modeling strategies and compare the performance of various models so as to provide a guideline for selection of a
modeling strategy. For the comparison of various models,the problem of thermal cycling is addressed here. The comparison
of various models is based on strains in the critical solder ball. A common geometric layout is assumed. Properties of the
laminate are varied so as to cover the range of properties of various laminates available. Pros and cons of various
methodologies are pointed out and are expected to provide useful guidelines for the stress analysts in the electronic packaging
area. The paper shows that making meaningful comparisons and predictions using modeling requires in-depth knowledge
experience in the field of modeling as well as the physical phenomena being modeled.

A new,green epoxy mold compound has been developed to encapsulate fine pitch PBGA and 3D-stacked die CSP packages.
When evaluated on these packages,the compound provided very good wire sweep performance. It also has the ability for
lead free solder re-flow at 260oC after JEDEC Level 3 pre-conditioning. In order to minimize wire sweep,a new resinhardener-
catalyst system was developed. The optimized combination of resin system provides extremely low viscosity and
long gel time. X-ray results showed that wire sweep less than 2% can be consistently achieved on PBGA and stacked die
packages using 0.8mil wire. The low moisture absorption of the new epoxy mold compound minimized delamination
between mold compound and die at lead free soldering parameters.

As the need for higher performance,higher I/O count packaging solutions at lower costs continues to grow,opportunities
exist to support these applications with higher performance wire bonded packages,as an alternative to some of the more
expensive FlipChip solutions.
This paper details the unique design layout and engineering development methodologies that were used to produce a family
of organic wire bonded BGA packages. This family of packages utilizes a ‘Stripline’ cross-sectional structure in contrast to
the more commonly used ‘Microstrip’ structure for electrical enhancement. This enhancement is combined with the package
designed to support both ‘Single ended’ and ‘Differential’ I/O’s as well as providing support for multiple I/O voltages. In
supporting the need for increased power dissipation,built in heat spreaders as well as optimized thermal via and solderball
layouts have been designed into the package.
The packages are able to support a high density of I/O’s per unit area of the die,by combining an optimal layout of multiple
rows of bonding pads on the die with a package layout that promotes the electrical enhancement features. Our modeling and
characterization of this family of packages conclude it meets the high performance requirements needed of the next
generation electronic packages.

The world-wide movement to phase out lead from electronic products presents many challenges for companies throughout
the electronics supply chain. The University of Massachusetts Lowell has brought together nine Massachusetts firms to
collaborate on the manufacture and testing of lead-free printed wiring boards (PWBs). The results of the first set of
experiments,published in 2001,showed that zero-defect soldering is achievable with lead-free materials. Following thermal
cycling,the PWBs were visually inspected and the leads were pull tested for reliability analysis. They compared favorably to
a baseline of lead soldered PWBs
A follow-on design of experiments was created in 2002 and a second set of test PWBs was made and tested in 2003. Several
lead free solder pastes (3) based on Sn/Ag/Cu were used with a variety of surface finishes (5),comp onent types (4)
component finishes (2) and reflowed using either air or nitrogen. Visual inspection and pull testing has been completed and
published in APEX,SMTI and IEEE conferences. This paper summarizes the effort and conclusions to date and discusses the
methodology of the pull-testing phase after thermal cycling.