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.

This paper will discuss the technical challenges associated with the selection of chemical additive for the printed circuit board assembly (PCBA) and stencil cleaning processes. The removal of residues from lead free and tin lead fluxes are increasingly causing problems for printed circuit board (PCB) assemblers. The heat requirements of lead free chemicals have made removing the residues difficult,and all residues can cause problems with components that have decreased in size and those with increased electrical sensitivity. The problems associated with residues exist with both lead free and tin lead organic and rosin based chemicals. Residues from organic chemicals can cause corrosion or lead to electrochemical migration. Residues from rosin based no clean chemicals can cause problems with the application of conformal coatings,test connections,and
high frequency components. The practice of using of high purity deionized water has been unsuccessful in removing many types of residues and can actually contribute additional problems.
For these situations,additional chemical additives,such as saponifiers and surfactants,can be used to assist in removing hard
to clean residues. These additives are used in all style of cleaners such as in-line,batch style,and stencil cleaners. Because not all additives have the same formula and can be used with all cleaning processes,manufacturing locations must decide which additive can be used to clean the residues in question and meet local environmental regulations. The cost of the additive must also be considered. Since the use of chemical additives is a large investment for manufacturing facilities,the correct type and concentration must be determined to achieve the appropriate cleanliness level for the least amount of investment.

Highly dense circuit assemblies increase the cleaning challenge. Batch cleaning equipment designs provide a small footprint,low cleaning fluid consumption,and low cost of ownership. Batch cleaning machines use flow,time,temperature,impingement,and advanced cleaning fluids as critical drivers for delivering a clean part. Increased density,low standoff components,and Pb-free flux residues place increased importance on the cleaning fluid design. There is a need for improved cleaning fluids to remove Pb-free flux residues from populated circuit assemblies in batch cleaning machines. The purpose of this designed experiment is test populated circuit cards using innovative new cleaning fluid designs on a range of popular Pb-free flux residues in batch cleaning equipment. Validation will be reported using visual images of the test assemblies processed within the designed experiment.

One of the most popular pieces of equipment in the PCB fabrication industry is the inline processor. These machines are used for a variety of tasks including resist removal,etching and cleaning. In most cases,the machines are multi chambered,with a counter flowing chemical solution. The solution is normally pumped from the chamber to spray nozzles which deliver the solution to the work piece; a PCB panel. After impinging upon the panel the solution drains back into the chamber along with the dissolved superfluous material. Anecdotally,the effectiveness of these processors is related to the rate of product throughput,rate of turnover of the chemistry in chamber,the rate of introduction of fresh chemistry and volume of the chambers. The intent of this investigation is to mathematically model the performance of these processors and thereby characterize the behavior. The most important result of the analysis is identifying an initial transient in these processors which occurs at startup and may require a half an hour or more to reach a steady state condition. During this period,the so called breakpoint will fluctuate (that is the position in the processor where visually the superfluous material on the panel appears to have been removed down to the base material) Most PCB processing engineers have empirically accounted for this behavior by processing “dummy” panels at the outset of the process,hopefully exceeding the time of the transient. During this transient the performance parameters will vary in both time and position which will render machine characterizations (such as DOEs) futile. For purposes of this model it is assumed that the principal mechanism for removal of the superfluous material is dissolution. It is also assumed that the dissolution rate is proportional to the saturation level of the working solution and that when saturation is reached; the solution becomes inert for purposes of this process.

In recent years,various studies have been issued on cleaning under low standoff components; most however,with incomplete
information. It is essential to revisit and describe the latest challenges in the market,identifying obvious gaps in available
information. Such information is crucial for potential and existing users to fully address the cleanliness levels under their respective components. With the emergence of lead-free soldering and even smaller components,new challenges have arisen including cleaning in gaps of less than 1-mil.
This study was initially designed to investigate the impact of mechanical vs. chemical energy contributions during the removal of contamination under 1-2 mil standoff components. To validate the results obtained,extensive studies were conducted,specifically prepared test-assemblies,iterative experimentation,as well as new mechanical innovations that might help users in the future. The latter include,but are not limited to,various flow pattern designs and industry-leading cleaning agents. As a result,the authors will also include experimental data to address fluid flow mechanics,temperature and solvent concentration-related effects.
Initial results obtained indicate that cleanability of residues under low standoff components has become a non-trivial issue. Not only are residues becoming harder to remove,the penetration of the cleaning agent seems to be in direct relationship with the geometry and height of the components in question.

The flux management system for a reflow oven is highly critical to the quality,cost,and yield of a reflow process. Flux accumulation and dripping inside the oven not only requires frequent maintenance,it can also result in poor quality,low throughput,and safety issues. Understanding that high volume electronics manufacturers do not like downtime for maintenance,this is the driver for continued development of advanced flux management systems that incorporate self-clean features and do not require interruption of production for maintenance activities.
This paper will review some basic past and present flux chemistries that affect flux collection methodology. It will also review some of the most common flux collection methods,self-cleaning techniques,and maintenance goals. And,finally,data will be presented from high volume production testing of an advanced flux management system.

Mixed leaded and lead-free processing requires that reflow ovens change temperature. Cooling a modern oven can take a long time,even longer than an older oven because modern ovens are more insulated. We recently acquired new ovens and needed a method to cool them quickly. We analyzed two methods,a method of letting the oven cool without intervention,and a method of running aluminum heat sinks through the oven. Other methods,such as opening the oven or injecting liquid nitrogen,are also considered,but no analysis was performed on these methods.