Flexible printed circuits are a growing technology both in terms of numbers and in technology advancement. Applied as
cabling harness technology,IC packaging and as replacement for rigid board technologies,more designs are based on flexible
substrates than ever and the numbers are rapidly growing. As density and layer count increases,so do the concerns about
reliability and yield.
Proper care must be taken to ensure that the many risk factors in Flex manufacturing are kept under control. Layer stackup,
adhesives,dielectrics,stiffeners,cover layers,bend and flex,width transitions,trace curves,embedded components are all
well known factors in Flex design and manufacturing where each and every one requires special considerations.
Obviously,these factors have to be considered early in the design cycle as in all engineering: issues discovered late in the
process become extremely costly.
In this paper,we will look at all of these areas and how new CAD technology can assist in keeping these effects under
control.
Bend region stress,reliability and restrictions,adhesion concerns,layer stackup management together with design efficiency
issues such as effective trace routing in complex curved regions will be covered in detail.
We will show that although Flex technology is becoming more and more advanced,given the proper precautions,yield,
reliability and design efficiency can still be kept at a convincing high.

The kinetics of activator disappearance were investigated for a wave-soldering flux containing two activators,succinic and
glutaric acids. Three loadings of flux were applied to bare FR4 panels and copper-clad laminate coated with an organic
soldering protector (OSP),then baked at 110°C,95°C,80°C and 65°C for different times. The flux activator acids remaining
after baking were analyzed by ion chromatography. The experimental results were analyzed assuming 1st order kinetics of
acid disappearance and the Arrhenius equation. These results suggest:
• Glutaric acid disappears about 8 times faster than succinic acid on bare FR4 and about 2.5 times faster on OSP
panels.
• From the accelerated predictions,rates of glutaric and succinic acid disappearance on FR4 and OSP were
extrapolated for 40°C,and expected loss of these acids have been estimated as a function of time. While the glutaric
acid activator on both surfaces is 99% deactivated in 3 weeks at 40°C,the succinic acid requires 4.9 years for 99%
of the original amount to disappear at 40°C on bare FR4,but only 10 months at 40ºC on OSP.
• The measured actual rates of glutaric acid disappearance at 40ºC on bare FR4 and OSP were about ½ of the
predicted rates. The actual rate of disappearance of succinic acid at 40ºC on FR4 was about 6 times faster than the
extrapolated value. The 99% deactivation at 40°C for both acids on both surfaces have been calculated,and are 1.5
months for glutaric acid on both surfaces,one year for succinic acid on FR4 and 3.5 months for succinic acid on
OSP.

A fluxless bonding process between silicon and copper using Sn-rich Ag-Sn-Au multilayer composite has been developed.
The copper substrate is plated with a silver layer as stress buffer. We bond 5mm x 5mm silicon chips onto Ag/copper
substrates using electroplated lead-free Ag-Sn-Au solder. Electroplating method becomes an attractive deposition technique
in that thicker films can be fabricated. It also has an economical advantage over vacuum deposition technique. To achieve
high quality joint with few voids,a fluxless bonding process is developed in vacuum environment (50 militorrs) to suppress
tin oxidation. Comparing to bonding in air,the oxygen content is reduced by a factor of 15,200. Nearly void-free solder joints
are made. Initial joints comprising of three distinct layers of Sn-rich layer,Ag3Sn intermetallic compound,and Ag layer have
been achieved. Microstructure and composition of the joint are studied using optical microscope and Scanning Electron
Microscope (SEM) with energy dispersive X-ray spectroscopy (EDX). This technique becomes an innovative success for
overcoming the very large mismatch in thermal expansion between silicon of 3x10-6ppm/°C and copper of 17x10-6ppm/°C.
To evaluate the reliability of the solder joint and the bonded structure,samples will go through thermal cycling test and
failure modes will be evaluated in the future. Microstructural changes of the solder joints during thermal cycling test will be
investigated and assessed. Based on the results obtained,design recommendation can be made for producing joints with high
re-melting temperature.

The American Competitiveness Institute (ACI) performed a series of Environmental Stress Tests for the Joint Group of
Pollution Prevention / Joint Council of Aging Aircraft (JG-PP / JCAA) Lead Free Soldering Program. The objective was to
determine if Lead Free soldered hardware was equivalent to or better than its Tin Lead (SnPb) counterpart. The program’s
test vehicle was manufactured by BAE Systems in Irvine,Texas. The JG-PP / JCAA test vehicle was soldered with Tin Lead
(SnPb) as a baseline,Tin Silver Copper (95.5Sn3.9Ag0.6Cu or SAC),Tin Silver Copper Bismuth ((92.3Sn3.4Ag1.0Cu3.3Bi
or SACB),and stabilized Tin Copper (99.3Sn0.7Cu0.05Ni).
The Salt Atmosphere test was performed in accordance to ASTM B117 Test Method for 48 hours. The Temperature
Humidity test followed the procedure MIL-STD 810F; Test Method 507.4. In both tests,no failures were found that could be
attributed to the solder joints. Therefore,the Tin Lead and Lead Free soldered hardware can be considered equivalent.
Two types of Mechanical Shock tests were performed. The first Mechanical Shock test was performed using the test
procedure MIL-STD 810F; Method 516.5; Procedure 1. The test was performed on all 3 axes. 2 components soldered with
SACB failed the tests. The balance of the SnPb and Lead Free soldered components passed this mechanical shock test with
no failures.
The second Mechanical Shock test was performed to a modified version of MIL-STD 810F; Method 516.5; Procedure 1,
where the test vehicle was tested in the Z-Direction,at increasing levels to failure. The mechanical shock levels reached in
this test were above those in the first mechanical shock test. Across all test levels and component types,the SnPb soldered
hardware performed comparable or better than the SAC and the SACB soldered hardware.

Vibration testing was conducted by Boeing Phantom Works (Seattle) for the Joint Council on Aging Aircraft/Joint Group on
Pollution Prevention (JCAA/JG-PP) No-Lead Solder Project. The JCAA/JG-PP Consortium is the first group to test the
reliability of lead-free solder joints against the requirements of the aerospace/military community.
A complete modal analysis was conducted on one test vehicle (the “Pathfinder” PWA) using a laser vibrometer system. This
system measured velocities,accelerations and displacements of the PWA during the vibration test. The laser vibrometer data
was used to determine the resonant frequencies of the “Pathfinder” PWA and the actual deflection shapes of the PWA during
test. In addition,the strains generated during a 1 G sine dwell were calculated for 1189 points on the “Pathfinder” PWA.
After completion of the modal analysis,thirty test vehicles (in two batches of 15 test vehicles each) were subjected to the
vibration test conditions. The input power spectral density was increased during the test at 60 minute intervals in an effort to
fail as many components as possible within the time allotted for the test.
The solder joints on the components were electrically monitored using event detectors and any solder joint failures were
recorded on a Labview-based data collection system. The time to failure of a given component attached with SnPb solder
was then compared to the time to failure of the same component attached with lead-free solders.
After completion of the testing,all of the test vehicles were visually inspected. Broken component leads and other unwanted
failure modes were documented.

Bare printed wiring board materials require changes from today’s typical standard dicy cured FR4 materials in order to
support lead-free assembly. The High Density Packaging User’s Group has completed a study of via reliability through airto-
air thermal cycling after various SnPb and lead-free reflow profiles. Six different materials,with different numbers of
reflow profiles were studied in this test through 6000 accelerated thermal cycles. Data from this testing will be presented that
clearly shows that all materials claimed to be lead-free compatible are not created equal. A review of relevant material
properties vs. the results will also be attempted.

The Pb-free solder interconnect reliability performance of a wide variety of common SMT component types was measured in
an IPC-9701 TC1 thermal cycle (0-100°C / 40 min cycle). Seventeen different Pb-free SMT components were evaluated,
including PBGAs,CBGAs,CCGAs,power modules,QFPs,PLCCs,CSPs,chip capacitors and resistors. All were attached to a
six layer PCB using SAC387 solder paste according to the HDPug GPLF defined assembly process. This study specifically
compares the reliability behavior of these various component types when subjected to expected Pb-free assembly process
extremes such as minimum peak reflow temperature and extended reflow soak times. Microstructural features of as-assembled
SAC solder connections are first explored. Failure data is reported for the various component types and assembly conditions.
All SAC solder thermal cycling data was collected along with eutectic SnPb solder controls.

Elevated SMT reflow temperatures for Pb-free soldering are placing excessive thermal demands on certain families of
electronic components. The High Density Package Users Group,HDPUG,Consortium conducted an extensive study on
optimizing the time / temperature profile for reflow to reduce temperature variations from high to low thermal mass
components and to identify the minimum peak temperatures that will produce acceptable solder joints. This paper details the
preliminary findings of profile characterization work for a range of PCB design types,assessing the impact of adjusting both
time and temperatures. Development of thermal profiles for rework of SMT components is also included. Results of
metallurgical analysis are given to support provisional recommendations on minimum peak temperature requirements. These
were subsequently used for the build of the GPLF (General Purpose Lead Free) test vehicle to assess reliability of Sn/Ag/Cu
solder joints produced at the limits of the process window.

Many words have been written about the impending lead free transition,during this period of frantic discovery lots has
been communicated about the reflow and alloy concerns; But the print process,which lets face it is the 1st process that
adds value,is often over looked but this is where a process can be made or broke. Much has been documented about the
differences of Lead free materials versus Lead rich materials; Mainly the discussion goes along the lines of questioning
the self-centring capability and the wetting characteristics. This leads onto questions such as,“What should I do with my
aperture design”,“Will I see more assembly defects?” This paper will look at these questions plus others such as what
happens if the process window is intentionally violated.

A brand name Pb-free solder paste made with three different sizes of solder paste spheres was studied. The soldering ability
of Types III,IV and V solder paste in terms of opens,shorts,solder spread,solder balls,solder beads and tombstoning was
examined using two different types of test boards.