The electronic industry is under extreme pressure to remove tin-lead solders from electronic components. Pure tin is
one of the alternatives and may be the simplest system as a “drop-in” replacement for SnPb surface finishes.
However,fear of “whiskers” has been a major concern inhibiting its imple mentation. This is because that tin
whiskers,with a length from a few micrometers to several millimeters,may grow from electroplated Sn and cause
electric shorts in electronic devices,particularly in fine-pitch high I/O (input/output) components.
In APEX2002,we described our systematic and in-depth investigation of various factors affecting whisker growth,
and the driving force for whisker formation. In this paper,we will discuss various methods for minimizing or
eliminating whisker growth,which are based on our current understanding of the whisker growth mechanisms.

Tin alloys such as Sn 0.7% Cu and Sn 2%Bi were identified as a viable alternative to tin-lead finish. Electroplating
of these alloys is challenging because of great difference in reduction potentials of the individual elements and
usually associated with strong immersion deposition and poor alloy control. To overcome these problems we
utilized metal specific non-complexing organic additives,which can significantly slow down or completely inhibit
deposition of the alloying element in certain range of potentials. In this paper,we present the recent developments
for Sn-Bi and Sn-Cu plating chemistries. We also discuss materials properties of the deposits and different plating
applications.

This paper provides an assembly process comparison of Pb-free alloys with a Sn/Pb alloy using a test vehicle
designed to resemble real world manufacturing applications. Four different Pb-free alloy systems,including two
compositions in the Sn/Ag/Cu alloy system,are investigated,and their board level assembly processing
characteristics are compared to the eutectic Sn/Pb alloy for a variety of surface mount components and packages,
and through-hole components. Presumably,Pb-free technology will adopt the same family of components,with
alterations only in their package materials. The key advantage of this approach is that the focus is on evaluating the
Pb-free manufacturing process and verifying its capability,independent of a specific component. As a result,a
matrix of different families of components,such as chip components,peripherally leaded components,ball grid
array packages,and chip scale packages with varied pitch sizes has been incorporated. The components were
assembled on printed circuit boards having five different Pb-free surface finishes. The sequential approach has been
adopted that involves the design of the test vehicle and component selection,comparison of printing characteristics
for Pb-free pastes,and comparison of processing defects. This paper is a follow-up to one that appears in the
proceedings of the 2003 Pan Pacific Microelectronics Conference. The earlier paper discussed in detail the design of
the test vehicle,stencil design and comparison of printing characteristics for Pb-free pastes. This paper discusses the
comparison of processing defects. The results of the assembly process defects are based on visual and x-ray
inspection of the assembled boards. Finally,a performance index is calculated to provide a relative comparison of
different combinations of Pb-free alloys and alternative surface finishes.

Shelf life is one of the most important characteristics of solder paste. Many pastes exhibit good print characteristics
when fresh but then suffer rapid degradation. Although rosin based pastes are most susceptible to this phenomena
water-soluble solder pastes can also see degradation in rheological properties.
In order to determine what parameters are most important for the stability of water-soluble paste an extensive study
of the change in rheology with time was undertaken for a series of water-soluble pastes. The parameters investigated
included alloy type,carrier base composition,surfactant properties,and solvent carrier composition.
Results indicate that the decay of the rheological properties is strongly influenced by alloy type and can be,to some
extent,retarded by proper choice of the activator package.

With the growth of the concern on the environment,more and more efforts have been done to protect the earth,to
minimize the pollution on the environment. The use of the lead free solder is replacing that of the conventional
solder-Sn-37Pb in many electronic industry fields.
Sn42Bi57Ag1 is a potential alternative for Sn63Pb37 solder paste in the application of consumer products. Due to
its lower melting temperature of 138°C,it requires a relatively lower assembly temperature compared with other Pbfree
alloy (Sn95.8Ag3.5Cu0.7) and Sn63Pb37 alloy solder paste. This study focused on the preliminary
investigation of the solderability of this solder paste on the different substrates. The influence of the reflow profiles
on paste’s solderability is also discussed in this paper. One character of Sn42Bi57Ag1 solder paste is the tendency of
forming some black spots on the surface of the joint; more details about this issue are given in this paper. The test
results of void,viscosity,solder balls,printing definition and stencil life indicate that with a proper paste flux
vehicle,no-clean low-temp lead-free paste may achieve a good performance.

The world is stating to focus on tin-silver copper as the lead-free alloy of choice. This paper discusses the
commercially available tin-silver-copper alloys and their processing and reliability as well as the world-wide level
of acceptance,forwards and backwards compatibility,contamination issues,environmental and economic impact.
In the processing section we will present practical data on surface finish compatibility and voiding.

Over the years,electronic components have become
increasingly smaller and integrated. Spacing between
components and line to line spacing has continually
shrunk. This miniaturization has magnified the
importance of choosing appropriate components and
assembly materials.
As the assembly materials change to accommodate
the latest technology and industry desires,it
continues to be imperative to verify compatibility
between the materials and the individual company’s
assembly process. There is a wide variety of different
solder masks,solder pastes,fluxes,and solder from a
host of suppliers. The introduction of low
solids/residue fluxes years ago was in response to the
industry wanting no-clean processes. The current
impetus for the switch to lead-free solder is a current
example of how industry standards are continually
changing.
As more electrical components get packed into
smaller devices,the cost and complexity per device
increases,and the ability to fix or repair the device
becomes more difficult. The desire for increased
service life and reliability is easily justified,as it
translates to less warranty work,greater end user
satisfaction,and more profit to the manufacturer.

Conformal or protective coatings are widely used in
the electronics industry today and are available in a
large variety of different types and adjustments. The
"classic" or conventional conformal coatings are
mainly based on epoxy,polyurethane or acrylic
modified alkyd resins dissolved in organic solvents,
where often aromatic solvents are used for this
purpose. These systems are either physically and/or
oxidatively drying or curing.
These proven products which have been used for
many years still appear on many approval lists of end
users,even though conformal coatings are available
today that are based on different formulations,which
either avoid the use of organic solvents completely or
where organic solvents are used only in a very
limited amount. Moreover,some of today’s
protective coatings offer commercial and technical
advantages that would easily justify an even wider
use of these alternatives which are discussed in this
paper.

As line speeds increase and production looks to decrease size and energy requirements,radiation curing becomes a
more attractive technology option over the incumbent thermal and moisture curing technologies. This paper reviews
what the impact of UV curing silicone conformal coating formulation is on cure and performance attributes. The
functional requirements of radical and cationic UV cured silicone systems are compared and contrasted. The
physical properties of UV cured silicone conformal coatings are evaluated against the widely available thermal and
moisture curing systems,and the issue of “shadow” and “dark” cure in complex 3D structures is also discussed.

Boeing Commercial Electronics (BCE),a subsidiary of The Boeing Company,is a leading supplier of avionics and
cabin management systems for the Boeing family of commercial airplanes. Boeing specifications require conformal
coating on electronic assemblies to protect them from moisture and contamination. To meet this requirement,BCE
sprays a solvent-based acrylic (AR) conformal coating on their printed wiring assembles (PWAs). In anticipation of
further EPA spray coating restrictions,BCE launched an extensive test program to select a low VOC (volatile
organic compound) conformal coating. BCE customers require a coating that is easily repaired. The selection of the
low VOC coating is further constrained by a Federal Aviation Administration (FAA) flammability requirement for
materials used on commercial aircraft. In order to systematically select the best conformal coating,a phased
approach was adopted: Phase I - Industry Survey,Phase II -Evaluation of Factory Acceptance,Rework and
Flammability Testing,Phase III - Engineering Reliability Testing,Phase IV - Qualification,Implementation and
Optimization. The scope of this paper encompasses Phase I through Phase III.
In Phase I,a comprehensive industry survey of available conformal coatings was completed. Fifty-five potential
coatings were ranked based on the following attributes: (1) material type (2) percent solvent (3) Underwriters
Laboratory (UL) flammability recognition (4) ease of repair (5) 1 or 2 part coating system (6) viscosity (7) cure (8)
impact to production,customers and service centers (9) equipment compatibility and (10) cost. Based on these
attributes,the list of coatings was down selected to eleven coatings [one water-based acrylic (AR),three UV curable
acrylated urethanes (AR/UR),and seven silicones (SR)] to advance to Phase II.
The objectives of Phase II were accomplished by breaking the testing into three smaller parallel efforts. In Part A,
populated test boards were coated at the suppliers’ facilities and then used to determine the rework operator’s
acceptability of removal and rework. In Part B,test boards were coated on-site to provide BCE’s conformal coating
operators an opportunity to evaluate the coating for ease of use and human factors. In Part C,flammability tests were
completed. Based on the combined results from Parts A,B,and C,six coatings (1 water-based acrylic,2 UV curable
acrylated urethanes,and 3 silicones) were selected to advance to Phase III.
Phase III was broken in two parts. In Part A,coatings were tested for compatibility with materials utilized in
production of PWAs. In Part B,the following engineering reliability tests were conducted: (1) Fluorescence (2)
Appearance (3) Moisture and Insulation Resistance (4) Thermal Shock (5) Temperature & Humidity Aging
(Hydrolytic Stability) (6) Tape Adhesion (7) Surface Insulation Resistance (SIR).
Based on the combination of evaluation completed in Phase II and III,a 100% solids (0% VOCs) UV curable
acrylated urethane was down selected for Phase IV (qualification,implementation and optimization). Currently,
BCE is continuing the effort by examining implementation options. Several other low VOC conformal coatings
were qualified and added to the BCE specification for use on special applications.