Solder joints tend to crack after extended thermal cycling,if the component and the circuit board are CTE mis-matched. Predicting t-cycle lifetime is a crucial first step in optimizing product design and/or in-service conditions. Predictor models embody cyclic fatigue physics and math,and require inputs of the materials and geometry of the hardware as well as the thermal conditions of the environment. The output is the predicted number of t-cycles to fail (i.e. to develop electrical-open cracks thru the solder-fillet). Several predictor models are in use within the industry. This paper describes a comparison among several predictor models,rating them for ease of use,and for accuracy against known actual test results and against each other. The study uses a round-robin approach; wherein each participant was given the same input data for ten different components,but the actuals were withheld until the respective predictor results were in. Also,this paper describes a related study on the ability of each model to perform parametric analyses: i.e. to define the effect of variations in hardware and environmental conditions on t-cycle life. The results offer guidance on t-cycle life prediction,as well as on improving t-cycle life

A series of advanced screen-printing processes have been developed to build functional high density flexible electronic circuits. Not only do the single layer circuits have fine conductor traces,but double and multi-layer circuits are connected by micro via holes with embedded passives. Utilizing the whole process,an entire electronic circuit system can be built on a flexible substrate.

The peel test will be reviewed,with special attention given to deposited adhesiveless copperclads. Details of the specification are reviewed The relevance of the title subject will be addressed from the perspective that a vendor and a customer of a flexible material can head off disaster if they spend time communicating on what the requirements of that substrate material are.
We will consider the mechanics of the test,IPC specifications,will be reviewed with specific case histories in which the choice of peel method was critical to problem resolution.
We will discuss the many influences on peel strength values by presenting data on variables such as conductor thickness,conductor width,and copper treatment,as well as more subtle things such as surface finish and even simple choice of test method. Details such as the effect of surface finish will receive comment. Data concerning ENIG-plating will serve as the backdrop for this segment.
Audience members will be encouraged to participate by asking the “expert” to answer questions such as,“which test is most important,” and “how much peel strength is enough?” This will be used as a teaching opportunity to exemplify the value of close communication between customer and supplier.
Failure modes will be described for deposited clad flexible substrates. The value of investigation of this characteristic will be stressed,together with other investigational techniques for the engineer who may be new to the industry. Among these nuggets will be the exhortation to learn to write good English,which will permit a partial answer to the question posed in the title.
Choice of substrate material with respect to the tiecoat will be clarified by discussion of processing and product characteristics.
A brief mention will be made of alternative methods of adhesion measurement,including shear testing and tensile testing. Again,with the author?s own data on adhesiveless FCCLs,a picture will be provided of the relative strengths and weaknesses of each method.

Flexible electronics is the future trend of the worldwide electronic industry. The RFID tag is one of the main applications in flexible electronics currently. This paper presents a flexible HF RFID tag which is manufactured by a low cost screen printing technique. The inductive coupling antenna is constructed by printing conductive silver paste on PET substrate to achieve good flexibility. The inductance and quality factor value of the antenna were designed using an EM simulator tool. Several design issues such as metal thickness,line width,and spacing between conductive coils related to the performance of inductive coupled antenna have also been analyzed carefully. The optimized antenna structure is properly chosen based on EM simulation results and measurements of several prototypes. A Philips I-CODE label IC was mounted on the inlay after being thinned. (The details of the manufacturing process flow will be examined in this paper later.) Finally,a half-wave rectifier circuit is proposed in this paper to demonstrate the potential of designing flexible circuitry using a screen printing process. In this design,one special High-DK material (with DK=20) which was developed by ITRI MCL was adopted to manufacture the capacitors in rectifier circuit. The experimental results shows that the rectifier which is powered by general RFID reader can be used to light up one typical SMD type LED successfully. The whole circuit size is about 16 cm2.

The recent use of lead free alloys has made the wave solder process more challenging in terms of achieving acceptable solder joints for both SMT and PTH components. It has been found that the Design for Manufacturability (DFM) guidelines,which were established for tin lead processes,in many cases do not result in the same level of quality joints when soldering with lead free alloy. Therefore,in order to improve the process yields and reduce manufacturing costs when converting to lead free,it is essential to establish DFM guidelines specifically for lead free soldering. The effect of pin to hole ratio,quantity of large copper planes connected to a pin through hole barrel,connection types for PTH and land patterns for glue and wave chip components are some of the main features which require further investigation for design optimization.
As there are a variety of lead free alloys available on the market today,each with differing properties,it is also important to determine if a set of DFM guidelines result in similar results among these various alloys.
This paper will discuss the outcome of a project which studied several DFM features incorporated on an internally designed wave test vehicle,which was created to evaluate alternative lead free alloys. The DFM features included in this study were: land pattern design and varying component spacing for chip components,pin to hole ratio and its interaction with the quantity of large copper planes connected to a PTH,quantity of large copper planes connected to a PTH and its interaction with the type of connection either solid or four spokes. The test vehicle was assembled with four Pb-free alloys: Sn-Cu-Ni,Sn-Ag-Cu-Bi,Sn-Cu-X & SAC405 as the baseline. The quality level of each of the described DFM features will be discussed. In addition to this,a detailed barrel fill analysis for the PTH components will be shown.

SN100C (Sn99Cu0.7 with Ni- and Ge-micro-additions) is a lead-free solder alloy that is finding increased acceptance globally and holds promise as a mainstream solution in terms of long-term solder joint integrity when compared to SAC-alloys. This material has also been reported to offer superior characteristics to SAC-alloys in terms of reduced Cu ero-sion of assemblies,better fluidity and drainage in wave and selective soldering,and superior wettability.
However,like virtually all lead-free solders,the solder melts at a higher temperature than lead-based solders and there-fore drives the industry toward thermal profiles that are considerably more demanding to all of the materials comprising the circuit assembly,including the wave soldering flux.
As temperatures rise,the flux materials undergo changes in their physical and chemical properties such as the evapora-tion of volatile fractions,their surface activity,and their melt viscosity. The consequence for the solder flux is early displacement by the scrubbing action of the solder wave,and ultimately the thermal breakdown of the material. This results in loss of its functionality as a protective blanket,and the loss of an insulating film over the liquid solder when it wicks up the barrel of the via or through-hole. The latter result,in conjunction with the larger ?-T in Pb-free processes between the bottom and the top side of the printed circuit assembly passing through the solder wave,results in early solidification before the liquid solder is able to wick up the barrel and wet the top side of the pad. This defect is com-monly referred to as ‘inferior topside wicking.’
Whereas the use of a N2 blanket over the solder wave prevents oxidation and thereby assists the wetting and wicking,it does not impact the melt viscosity,and thus the displacement,of the organic materials in the solder wave.
Unlike ordinary rosins,modern fluxes may consist of multiple polymer species and property modifying additives. The additives affect the mobility of the system,solvent retention properties,long and short term dielectric properties,and thermal behavior. The key to maintaining all desired product attributes as well as maximizing topside fillet performance lies in a thorough understanding of the interactions between these polymers and certain properties of the modifying additives.
This paper describes the development and implementation of state-of-the art fluxes in the categories alcohol-based,low-VOC,and VOC-free technology,for use in Pb-free and N2-free,SN100C-based wave soldering processes. It explains the need for flux systems that incorporate organic materials of a more advanced molecular structure. Additionally,the chemical functionalities for enhancing the mobility of these materials,impacting topside fillet performance has been studied using methods including Thermogravimetric analysis and differential scanning calorimetry. Melt viscosity,sublima-tion energy,optimum activity range,weight loss,and phenomena such as surface energy of a number of organic materials suitable for use in Pb-free wave solder fluxes have been characterized.
With this information,it is possible to tailor the organic system for a specific thermal profile and dramatically accelerate the wetting of the metallic regions of the circuit assembly. Properly applied,these techniques may allow substantive increases in wicking performance mass while still maintaining all other desired product attributes.

This paper discusses micro-filled epoxy-based conducting adhesives modified with nanoparticles for z-axis interconnections,
especially as it relates to package level fabrication,integration,and reliability. A variety of conducting adhesives with particle sizes ranging from 80 nm to 15 µm were incorporated as interconnects in printed wiring board (PWB) or laminate chip carrier (LCC) substrates. SEM and optical microscopy were used to investigate the micro-structure,and conducting and sintering mechanisms. Volume resistivity of nanoparticle-modified adhesives is in the range of 10¯5 to 10¯6 ohm-cm. The present process allows fabrication of z-interconnect conductive joints having diameters in the range of 55-300 microns. There was no delamination of conductive joints after 3X IR-reflow (assembly precondition),pressure cooker test (PCT),and solder shock. The processes and materials used to achieve smaller feature dimensions,satisfy stringent registration requirements,and achieve robust electrical interconnections are discussed.

Embedded resistors and capacitors provide high-density high-performance solutions,freeing up the surface for other
components and enabling tailored interconnect topology. This technology needs exploration and characterization before it can
become a routine resource in high-reliability,harsh-environment and aerospace applications. This paper discusses two critical
factors,a) uniformity as-received from the fabricator,and b) long-term stability under extreme environmental conditions.
PWB specimens spanned a range of embedded resistors and embedded (distributed and discrete) capacitors,several values of
each,from several suppliers,using a variety of materials and processes.
Data on as-received boards and coupons includes: Comparisons between target vs. measured values; comparisons between
"expected" vs. measured values; comparison of board–to-board uniformity of equivalent features,uniformity of nominally
identical features at various spots on the same PWB; comparison edge-to-edge of the same board; and comparisons between
coupon feature vs. equivalent PWB feature,to document how representative the coupon is.
Environmental stability. Performance data includes: resistance and capacitance values as a function of temperature; values
measured after long-term storage at elevated humidity and temperature; stability after vibration; stability after long-term
thermal-cycling; stability after water immersion,stability after molten-solder-dip thermal-shock; and stability after surface
over-heating.
Results are interesting. As-received uniformity data reveals substantial differences (up to 20-40%) between the target value
and the actual value,as well as among nominally equivalent features,and between coupon and board. Differences depend on
type of element. This is before any of the cherry-picking or screening by the supplier that could happen in typical jobs.
Environmental stability is reassuringly robust. Most exposures cause relatively little change. Stability depends on materials,
processes and geometries.
This information could help guide procurement documents,to provide realistic expectations regarding tolerances and yield,
as well as to develop QC sampling and acceptance protocols. It also should provide guidance to designers and users towards
qualification and performance expectations,under nominal and adverse in-service conditions.

Some Ball Grid Array suppliers are migrating their sphere alloys from SAC305 (3% Ag) or 405 (4% Ag) to alloys with lower silver contents. There are a numerous perceived benefits to this move in terms of cost and performance,but process compatibility and reliability concerns have yet to be addressed.
Process compatibility concerns stem from the fact that the low-silver SAC replacement alloys have higher melting temperatures than SAC305,approximately 227C as compared to 221C. Certain families of electronic assemblies,such as consumer portables,are often heat-sensitive and are reflowed in the low end of the established lead-free peak temperature range,typically 230-235C. The small temperature difference between the spheres’ melting temperature and the peak reflow temperature raises questions about the reliability of the solder joints that are formed under this tight thermal margin. These are similar to the concerns raised with the backward compatibility of SAC305/405 spheres with tin-lead solder processes. Some of the solutions identified in the lead-free ball/tin-lead paste scenario may apply to the low-silver ball/SAC305 paste combination,but they require review for their applicability with this new set of mixed metals.
A study has been undertaken to characterize the influence of alloy type and reflow parameters on low-silver SAC spheres assembled with backward compatible pastes and profiles. The DOE combines low-silver sphere materials with tin-lead and lead-free solders at different combinations of peak temperature and times above liquidus. Solder joint formation and reliability are assessed to provide a basis for developing practical reflow processing guidelines.

Addition of Al into SAC alloys reduces the number of hard Ag3Sn and Cu6Sn5 IMC particles,and forms larger,softer non-stoichiometric AlAg and AlCu particles. This results in a significant reduction in yield strength,and also causes some moderate increase in creep rate. For high Ag SAC alloys,adding Al 0.1-0.6% to SAC alloys is most effective in softening,and brings the yield strength down to the level of SAC105 and SAC1505,while the creep rate is still maintained at SAC305 level. Addition of Ni results in formation of large (Ni,Cu)3Sn4 IMC particles and loss of Cu6Sn5 particles. This also causes softening of SAC alloys,although to a less extent than that of Al addition. Addition of Al also drives the microstructure to shift from near-ternary SnAgCu eutectic toward combination of eutectic SnAg and eutectic SnCu. Addition of Ni drives shifting toward eutectic SnAg. For SAC+Al+Ni alloys,the pasty range and liquidus temperature are about 4?C less than that of SAC105 or SAC1505 if the addition quantity is less than about 0.6%. Addition of Al and Ni also results in a slight decrease in modulus and elongation at break,although the tensile strength is not affected.