The continued functional densification and integration in networking products is driving the need to study large form factor
printed circuit boards that use high I/O packages (either ceramics column grid arrays,CCGA,or plastic ball grid array,
PBGA). As of today,there has been limited work on understanding the impact of lead-free soldering on these large and
complex assemblies. This paper will look at larger packages (up to 52.5mm square with 2577 I/O) in combination with
lead-free soldering. Assembly processes such as solder paste printing and reflow soldering will be studied and the results
presented. The rework of these component types will be evaluated and the key issues for developing a successful rework
process will be discussed.

Movements to lead-free assembly are being influenced by legislative and market requirements. Specifically Europe has
passed legislation requiring the removal of lead from electronics assembly by 2006. Also,the perceived marketing advantage
of a “green product” is beginning to be accepted. Though development work for lead-free components is increasing,work on
lead-free components with higher I/0 has been fairly limited at this time.
The present work describes the evaluation of a 780 I/O lead-free Flip chip BGA component in terms of assembly below
260°C peak reflow temperature. FCBGA components were assembled at peak reflow temperatures of 225°C,235°C,245°C,
reflow environments of air and nitrogen,and rework with peak reflow temperature 230 to 235°C with a novel rework nozzle.
Solder joint formations from the different reflow processes were inspected using visual analysis,XRAY analysis,and
physical cross sections. Each inspection showed good solder joints of all components mounted at the different peak reflow
temperatures and environments.
Reliability of the mounted components was then tested by temperature cycling from 0-100°C. All components mounted in the
different peak reflow temperatures and environments were stressed with the bulk of the stressed devices from reflow peak
temperature of 235°C in nitrogen environment. All mounted components survived greater than 3500 cycles.

For SnAgCu solders,the voiding rate at microvia was studied with the use of simulated microvia,and was the lowest with
95.5Sn3.8Ag0.7Cu and 95.5Sn3.5Ag1Cu. The voiding rate increased with decreasing Ag content from 3.5Ag,mainly due to
an increasing surface tension. Voiding at microvia was governed by via filling and exclusion of fluxe s. The voiding rate
decreased with decreasing surface tension and increasing wetting force which in turn was dictated by the solder wetting or
spreading. Both low surface tension and great solder wetting prevented the flux from being entrapped within microvia. A fast
wetting speed might also facilitate reducing voiding. However,this factor is considered not as important as the final solder
coverage area.

This paper presents a quantitative analysis of solder joint reliability data for lead-free Sn-Ag-Cu (SAC) and mixed assembly
(SnPb + SAC) circuit boards based on an extensive,but non-exhaustive,collection of thermal cycling test results. The
assembled database covers life test results under multiple test conditions and for a variety of components: conventional SMT
(LCCCs,resistors),Ball Grid Arrays,Chip Scale Packages (CSPs),wafer-level CSPs,and flip-chip assemblies with and
without underfill. First-order life correlations are developed for SAC assemblies under thermal cycling conditions. The
results of this analysis are put in perspective with the correlation of life test results for SnPb control assemblies. Fatigue life
correlations show different slopes for SAC versus SnPb assemblies,suggesting opposite reliability trends under low or high
stress conditions. The paper also presents an analysis of the effect of Pb contamination and board finish on lead-free solder
joint reliability. Last,test data are presented to compare the life of mixed solder assemblies to that of standard SnPb
assemblies for a wide variety of area-array components. The trend analysis compares the life of area-array assemblies with:
1) SAC balls and SAC or SnPb paste; 2) SnPb balls assembled with SAC or SnPb paste.

As we inch towards the somewhat shifting deadlines towards lead (Pb) restriction in Japan and Europe,there is an increase
seen in the amount of studies performed for electronics assemblies soldered with Pb-free alloys. This paper presents results of
an ongoing formal Pb-free activity (planned in 2000 and begun in 2001) with which we have been involved. Most Pb-free
studies involve test vehicle designs that are not typically representative of real-world printed circuit board assemblies. With
surface mount and through-hole components,a large number of soldering defect opportunities,and a large-sized substrate
(12”x10”,four-layers,62mil thick,and a four-up design),our test vehicle is one that is more challenging to assemble. It is
suggested that if this board can be assembled with Pb-free materials,then it goes a long way towards ensuring the successful
implementation of a more environmentally friendly product.

The commercial use of lead-free solder has been making significant gains worldwide in recent years. To identify the effects
of thermo-mechanical stress on Sn-Ag-Cu and Sn-Zn-Bi solder with different lead finishes (Sn-10Pb,Ni/Pd/Au plating),we
performed the following reliability tests: high temperature tests,thermal cycle tests,and combined thermal-vibration tests.
Following the tests,we investigated the causes of degradation by checking solder joint strength and observing solder joint
cross-sections.
Our investigations indicate that the same level of reliability can be obtained with Sn-Ag-Cu solder as with conventional Sn-
Pb eutectic solder. On the other hand,in response to thermo-mechanical stress,Sn-Zn-Bi solder forms voids and intermetallic
compounds at the joint interface between the solder and the printed circuit board (PCB),resulting in a loss of joint strength.
We then used Sn-Ag-Cu solder in mass production prototype PCBs. We subjected these PCBs to a variety of reliability tests
and carried out three years of field reliability testing. These PCBs with Sn-Ag-Cu solder held up successfully under a
minimum of 3,000 cycles in thermal cycle tests and a minimum of 20,000 hours in field reliability testing.

Developing lead free connector products involves at least two distinct steps: removing the lead from the product and ensuring
the product has sufficient thermal stability. Lead is most commonly found in terminal finishes and has been removed from
most thermoplastic materials used in connectors. Ensuring sufficient thermal stability requires knowledge of the thermal
excursions involved in soldering and how these excursions translate into product performance metrics.
For reflow soldering,we know the maximum soldering temperatures will increase by 20 to 30 °C. The magnitude of this
change is not large,however,the temperature value,260 °C,exceeds the melt point of many engineering thermoplastics.
Since the cost of these plastics typically scales with melt temperature,an increase in thermal requirements can mean a
significant cost increase.
In this paper we strive to understand the fundamental response of the plastics to the transient thermal excursions involved in
wave soldering. FEM simulations demonstrate the thermal gradients that exist during these processes. These results can be
used to understand the heat transfer and then to engineer the products to ensure reliability. Wave solder process simulation
shows that the pin to plastic interface resides at a temperature very near to that of the solder. Connector terminals,made from
copper based alloys,often have very high thermal diffusivities,increasing heat flow from the solder pot into the plastic. FEM
results are compared to experimental results from lab and production manufactured testing of solderable interconnects. A test
method for evaluating plastic performance in wave solder applications is proposed.

An issue that has emerged from the increasing use by the electronics industry of lead-free solders in mass production wave
soldering is the erosion of the copper of printed circuit board patterns and component terminations and the stainless steel of
the wave solder bath. In the study reported here the wetting and erosion of copper and Type 304 stainless steel by two widelyused
lead-free solders,Sn-3.0Ag-0.5Cu and Ni-stabilized Sn-0.7Cu,was compared with that of Sn-37Pb and Sn-0.7Cu with
and without the addition of phosphorus antioxidant. The rate of dissolution of copper by the Ni-stabilized Sn-Cu alloy was
found to be lower than that of pure Sn-37Pb alloy while that of the Sn-3.0Ag-0.5Cu and Sn-0.7Cu alloys was higher. The
addition of phosphorus increased the copper erosion rate of both lead-free alloys well beyond that of Sn-Pb. The rate of
erosion of stainless steel by lead-free solder was confirmed as faster than that of Sn-Pb eutectic solder and phosphorus was
found to promote the wetting that precedes erosion. The rate of erosion of stainless steel by the Sn-0.7Cu solder was
significantly slowed by the addition of nickel.

Major industrial nations,around the world,are rapidly moving to eliminate lead from the electronic manufacturing processes.
While some companies are taking advantage of the situation and are using “lead-free” as a major marketing initiative in the
consumer market,others are delaying the inevitable,because of the world wide lead-free legislation.
Lead-Free Legislation
Europe
• OECD: Lower the lead content limit in underground water from 0.05mg/L to 0.025mg/L in 2000.
• Total abolition of lead,cadmium,hexa-chromium,and non-flammable agent halogen starting 2005/6,according to the
EU directive (WEEE & Rosh).
USA
• 1990: Introduced a bill prohibiting use of solder containing over 0.1% lead. (However,this excludes the electronics
industry.)
• 1999: Industrial organization NEMI,formed by the USA electronic parts manufacturing industry,government
organizations and universities,started research and development targeting the total abolition of lead products by 2004.
• 2002: Proposition 65 California.
• End Of Life legislation pending in 20 plus states.
Japan
• 1991: The Waste Disposal Law requires disposal within the facility when the detected lead amount is over 0.3mg/L by
eluting test of industrial waste.
• 1994: The Water Pollution Prevention Law lowers the lead content of rivers from 0.1mg/L to 0.01mg/L.
• 2001-4: The Consumer Electronics Recycle Law requires manufacturers to recover harmful materials.
The move to lead-free solder has an impact on all phases of PCB assembly,including test and inspection. Let’s take a look at
some of the technical issues involved and the impact of lead-free solder on the major test and inspection technologies:
automated optical inspection (AOI),automated X-ray inspection (AXI),in-circuit test (ICT) and functional test.

The hypothesis that the “whisker growth phenomenon” in electrodeposited tin is a re-crystallization process driven by stress
has gained popularity among leading research institutes and industrial laboratories. However,there exist varying opinions as
to the type of stress responsible for this phenomenon. Recently,various studies have demonstrated that compressive stress,
whether intrinsic or externally applied,is most likely the cause of whisker growth.
There are three main sources of compressive stress that a component finish experiences after plating. They are: stress
generated by intermetallic compound formation between the tin finish and the copper alloy substrate; mechanical stress such
as trim-and-form introduced in current manufacturing practice; and thermal stress generated by temperature cycling and
propagated into tin layer due to CTE mismatch among the constituent materials of a component.
It is well recognized that both IMC formation and CTE mismatch are largely affected by the substrate material and
underlayer/barrier between Sn and substrates,as well as aging conditions. In this paper,we attempted to understand the
relative contribution of stress generated from IMC and from CTE mismatch on various leadframe and connector substrates at
both isothermal and temperature cycling conditions. Ultimately,we hope to delineate the whisker accelerating factors to
provide input for the industry to derive a set of standard yet realistic whisker test methods.