Papers about SMT
by Dr. Ning-Cheng Lee , Iris Artaki, James Slattery, John R. Sovinsky, Paul T. Vianco
Environmental and toxicity concerns related to the use of lead have initiated the search for acceptable, alternate joining materials for electronics assembly. This paper describes a novel
lead-free solder designed as a "drop in" replacement for common tin/lead eutectic solder. The physical and mechanical properties of this solder are discussed in detail with comparison to tin/lead eutectic solder. The performance of this solder when used for electronics assembly is discussed and compared to other common solders. Fatigue testing results are reported for thermal cycling electronics assemblies soldered with this lead-free composition. The paper concludes with a discussion on indium metal availability, supply and price.
pb-free, surface mount, SMT, solder paste, reflow, electronic, lead-free, soldering, solder
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Dr. Ning-Cheng Lee , Dr. Benlih Huang, William Manning, Dr. Yan Liu
Voiding is attributed to the flux outgassing within the solder joints when the solder is at molten state. The effect of reflow profile on voiding at microvia for lead-free soldering is strongly dependent on the flux chemistry. In general, wetting is more important than
melting outgasing behavior, and can be enhanced by employing a higher melting energy, including both higher peak temperature and longer dwell time. Use of a high soaking energy can help drying out volatiles hence reduce the melting outgasing and result in low
voiding, but may also increase oxidation for pastes with poor oxidation resistance and cause a high voiding. Testing oxidation resistance of solder paste beforehand will promise a more accurate selection of soaking energy.
pb-free, soldering, BGA, CSP, void, voiding, SMT, solder, lead-free, microvia, profile, reflow
[Permanent Link to this Paper ]
Posted on 2 Mar 2010
by Sunil Chhabra, Sergio Porcari, Steven Rocco Marongelli, Dr. Richard Ludwig, Dr. Ning-Cheng Lee
In order to achieve ultra-fine dot solder paste dispensing, both solder material and dispensing equipment have to be optimized. Dispensability of solder paste was evaluated in terms of “dispensing rate”, consistency of dispensing rate, and the stability of dispensing rate with time. Within the given conditions, threshold values for dispensability seem to exist for viscosity, powder size, and metal content. Small nozzle inner diameter is definitely needed to deliver a small dot size. Archimedes Metering Valve shows a greater flexibility in metering the volume than Positive Displacement Pump, primarily due to a greater sensitivity in dispensing volume to variation in pressure, and nozzle ID, besides being very sensitive to variation in encoder count. For success in high speed ultra-fine dot dispensing process, solder pastes with a low viscosity, small powder size, low metal content, and a high thixotropy are desired to deliver a high dispensing throughput. Controlwise, a high pressure and high encoder count may be promising. The consistency improves with increasing metal content, thixotropy, pressure, nozzle size, and encoder count. Viscosity, powder size, and delay time appear to have negligible effect on consistency. The stability increases with increasing flux activation temperature, and is expected to be poor for low thixotropy and low viscosity. Large powder size may cause immediate clogging, while small powder size may cold weld under repeated pressure cycling using pneumatic pump systems. In general, a very careful design and tight control of parameters discussed in this work has to be implemented in order to succeed in ultra-fine dot solder paste dispensing.
lead-free, pb-free, piston, Archimedes, fine dot, pump, SMT, surface mount, flux, solder paste, dispensability, dispensing
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Dr. Ronald C. Lasky , Paul Socha , Carol Gowans
Although many have predicted the demise of through-hole components, they
are alive and well with tens of billions assembled each year. In many cases
these components are assembled by wave soldering. However, in many mixed
product technology (i.e. SMT and through-hole on the same board) products,
it makes sense to consider assembling the through-hole components with the
pin-in-paste (PIP) process. PIP has been successfully used for several decades
now; however in many cases it is not possible to print enough solder paste to
obtain an acceptable solder joint. In addition to this “solder starved” condition,
the large quantity of solder paste used to form the though-hole joint results in
excess residual flux. This residual flux can lead to difficulties in in-circuit testing
and potential surface insulation resistance concerns.
In light of the above need, solder preforms have been developed. These slugs of
solder typically come in the same sizes as 0402, 0603, and 0805 passive components.
The solder preforms are placed by the component placement machines
onto the solder deposit. This additional solder assures that an adequate solder
joint is formed with a minimum of solder paste and its residual flux.
Although PIP was an early application of solder preforms, more recently other
“solder starved” applications have emerged such as radio frequency (RF)
shields and connectors. In addition, the use of ultra thin stencils in the assembly
of miniaturized components can result in some other components being solder
starved and, hence are candidates for solder preforms.
This paper will cover the design and assembly techniques for using of solder
preforms in the “solder fortification™” needs described above. Several successful
applications will be presented. In some of these applications, defects were
reduced by 95% after implementing solder preforms.
Apex 2011, solder starvation, flux, PIP, pin-in-paste, through-hole, SMT, solder fortification, solder paste, solder preforms
[Permanent Link to this Paper ]
Posted on 11 Apr 2011
by Mario Scalzo , Todd O'Neil
Although many predicted the demise of through-hole components, they are alive and well with tens of billions used each year. In mixed SMT/through-hole PCBs, through-hole components, and especially connectors, are often used for their mechanical robustness. A typical example would be a USB connector in a laptop PC. Typically an SMT connection just doesn't have the mechanical robustness needed to support multiple connector plug-in and removals. However, performing a full wave soldering process to assemble a few through-hole components on a mostly SMT PCB doesn't usually make economic sense and may damage the PCB. In such situations, the best option is often to assemble the through-hole components and connectors with a selective soldering process.
This paper touches on identifying favorable flux properties, down-selecting low solids fluxes for lead-free selective wave soldering, the selective soldering process itself, and testing criteria. Topics reviewed will be the flux selection, optimizing the selective soldering process by varying the flux concentration, pre-heat parameters, soldering temperatures, and dwell time. The paper will finish with a summary of the work and a systematic process to select a flux and optimize the selective soldering process for high yields and quality.
flux, pb-free, lead-free, selective soldering, SMT, through-hole, PCB assembly, Apex 2011
[Permanent Link to this Paper ]
Posted on 11 Apr 2011
by Dr. Ning-Cheng Lee
In this article, the interconnect infrastructure for SMT, BGA, and
flip chip are reviewed, with particular emphasis on the bonding technology. Interconnection technologies are the vital part of electronic packaging. Obviously, interconnections of SMT industry, from components to boards to board-level assembly methods, are the most mature and well established technology. BGA, on the other hand, intelligently utilizes the knowledge of SMT interconnections and re-engineers the design through combining the strength of various interconnect technologies and successfully comes up with a great family of versatile packages. Flip chip interconnects, while also trying to incorporate existing technology, place a good deal of emphasis on the polymeric systems, and very much develop a new arena of interconnect concepts and processes. The impact of flip chip interconnect progress is expected to ripple through the rest of electronic industries in the near future.
SMT, BGA, Flip Chip, CSP, Interconnection, surface mount, ball grid array, Packaging, assembly, soldering, pb-free, lead-free
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Mike Bixenman D.B.A., Steve Stach, Dr. Ning-Cheng Lee
Ionic Cleanliness testing machines are designed to determine the total ionic content extractable from the printed wiring board for purposes of process control. The conductivity of the extract solution is measured and the results are expressed as sodium chloride equivalence per unit area. The problem with this method is two fold: 1.) Many of today’s low residue flux and lead-free flux residues are not soluble in the extract solution. 2.) Contamination of concern is with site specific components, from which contamination does not correlate to the area of concern. The purpose of this study is to research low residue and lead-free flux structures, identify solvent compositions that will dissolve these residue types, and offer options for performing both bulk and site specific ionic cleanliness testing methods.
lead-free, Cleaning, flux residue, flux, soldering, solder, solder paste, SMT
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Dr. Ning-Cheng Lee
White Paper Video
Lead-Free Flux Technology and Influence on Cleaning
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Lead-free flux technology for electronic industry is mainly driven by high soldering temperature, high alloy surface tension, miniaturization, air soldering due to low cost consideration, and environmental concern. Accordingly, the flux features desired included high thermal stability, high resistance against burn-off, high oxidation resistance, high oxygen barrier capability, low surface tension, high fluxing capacity, slow wetting, low moisture pickup, high hot viscosity, and halogen-free. For each of the feature listed above, corresponding desired chemical structures can be deduced, and the impact of those structure on flux residue cleanability can be speculated. Overall, lead-free flux technology results in a greater difficulty in cleaning. Cleaner with a better matching solvency for the residue as well as a higher cleaning temperature or agitation are needed. Alkaline and polar cleaner are often needed to deal with the larger quantity of fluxing products. Reactive cleaner is also desired to address the side reaction products such as crosslinked residue.
lead-free, flux, flux residue, solder, soldering, cleaner, Cleaning, SMT
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Dr. Benlih Huang, Dr. Ning-Cheng Lee
Tombstoning has plagued the surface mount assembly industry for decades. While the problem seemed under control, it has begun creeping in again due to the miniaturization of discretes such as 0402S and 0201S. This article studies tombstoning behavior on a series of SN AG CU Lead-Free Solders and attempts to find a way to control the problem.
lead-free, pb-free, tombstoning, solder, solder paste, SMT, solder alloy, pasty range, soldering, reflow
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Dr. Ning-Cheng Lee
The reflow profile is engineered to optimize the soldering performance based on defect mechanisms analysis. In general, a slow ramp-up rate is desired in order to minimize hot slump, bridging, tombstoning, skewing, wicking, opens, solder beading, solder balling, and components cracking. A minimized soaking zone reduces voiding, poor wetting, solder balling, and opens. Use of low peak temperature lessens charring, delamination, intermetallics, leaching, dewetting, and voiding. A rapid cooling rate helps reducing intermetallics, charring, leaching, dewetting, and grain size. However, a slow cooling rate reduces solder or pad detachment. The optimized profile favors that the temperature ramps up slowly until reaching about 180°C. The temperature is then gradually raised further up to 186°C within about 30 seconds, then raised rapidly until reaching about 220°C. After that, the temperature is brought down with a rapid cooling rate. The conventional profile was developed due to the limitation of past reflow technologies. Implementation of the optimized profile requires the support of a heating-efficient reflow technology with a controllable heating rate. Vapor phase reflow can provide a rapid heating, but has difficulty to control the heating rate. Infrared reflow can regulate the heating rate, but is sensitive to variation in parts features. Emergence of the forced air convection reflow provides controllable heating rate. In addition, it is not sensitive to variation in parts
features, thus allows the realization of the optimized profile.
lead-free, pb-free, SMT, vapor phase, infrared, convection, soldering, solder paste, flux, defect, profile, reflow
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Dr. Ning-Cheng Lee
Reflow soldering of solder paste is the primary interconnection method used in SMT assembly process. The major issues which plague the reflow soldering performance include, but not limited to, bottom-side-component-holding, bridging, dewetting, low-residue, opening, solder balling, solder beading, solder-fillet-lifting, tombstoning, defective balling for BGA, and voiding. The mechanisms, causes, and cures for each issue are briefly discussed in this article.
lead-free, soldering, solder paste, SMT, bridging, dewetting, opening, solder balling, solder beading, solder-fillet-lifting, tombstoning, balling for BGA, voiding, pb-free, reflow
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Dr. Yan Liu, Pamela Fiacco, Derrick Herron, Dr. Ning-Cheng Lee
Consideration and selection of dip transfer fluxes and solder pastes for PoP assembly are described, based on process considerations. The crucial properties vital for successful dip transfer include homogeneity, open time on the flux/paste bed, volume and consistency of the dip transferred material, open time after the dip transfer before reflow, and solder joint formation. For each property, one or more practical, recommended test methods are described. Overall, this work should provide the assembly house with an easy way to select a flux or solder paste adequate for dip transfer of PoP assembly applications.
PoP, package-on-package, flux, solder paste, dip transfer, soldering, SMT
[Permanent Link to this Paper ]
Posted on 24 Jan 2011
by Dr. Ning-Cheng Lee , Paul A. Jaeger, Wanda B. Hance
Solder beading is a special phenomenon of solder balling when using solder paste in certain SMT applications. In brief, solder beads are large solder balls near components with very low stand-off (see scheme below). With more attention being drawn to no-clean paste applications due to CFC concerns, a better understanding of this event becomes indispensable. In this study, the data indicate solder beading was caused by flux outgassing which overrode the paste cohesive force during the preheat stage. The outgassing promoted the formation of isolated paste aggregates underneath the low clearance components. At reflow, the isolated paste melted and , once emerged from the underside of the components, coalesced into solder beads. Processingwise, this problem can be remedied by slowing down outgassing via a milder preheat profile, or by reducing print thickness. Materialwise, solder beading can be corrected by enhancing the paste cohesive force via cold welding of solder powders during the preheat stage. This in-turn can be accomplished through the use of lower activation temperature flux, coarser solder powder, higher metal load, and solder powders with lower oxide content. Other parameters which could affect the performance will also be discussed.
lead-free, pb-free, solder balling, SMT, flux, solder paste, beading, solder beading
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Dr. Ning-Cheng Lee , Gregory Evans
This paper focuses on many of the problems facing process engineers today. The experiments used in this study were designed to find the true causes of the problems and headaches which plague SMT assembly today. Data indicate that wicking is caused by a relative hotter component and is aggravated by non-coplanarity. It can be reduced by slower heating rate or more bottom-side heating. Bridging is caused by slumping, and is aggravated by smaller pitch dimension and slower flux wetting speed. Tombstoning is a result of uneven heating. It can be reduced by optimizing pads spacing and by using fluxes with slower wetting speed, or by a smaller print thickness. Problems such as slumping, clogging, solder balls, and white residue are also discussed.
lead-free, pb-free, white residue, solder balling, tombstoning, bridging, wicking, clogging, slumping, SMT, solder paste
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
by Paul Socha
Paul A. Socha, Indium Corporation, reviews the types of solder preforms and their uses. He offers 10 basic steps to determining if an assembly needs preforms. Solder preforms can be used on a mixed SMT and through-hole PCB or to fortify solder paste on a difficult joint. Most solder preforms can be flux coated. When incorporating preforms, be sure to consider possible effects on reflow, cleaning, and RoHS compliance.
solder preforms, solder paste, SMT, PCB assembly, through-hole, solder fortification
[Permanent Link to this Paper ]
Posted on 9 Mar 2010
by Karl Pfluke
Increasingly, electronics manufacturing services (EMS) providers specializing in SMT are seeking to diversify and fill capacity. Photovoltaic (PV) solar cell module assembly is becoming a popular choice to meet those goals. PV cell stringing in solar module assembly is achieved using many common SMT materials and processes. Solders, fluxes, and common reflow technologies produce electrical interconnects in both a-Si and c-Siphotovoltaic technology.
CIG, flux, solder, photovoltaic solar cell assembly, solar module assembly, SMT
[Permanent Link to this Paper ]
Posted on 6 Jun 2011
by Chris Anglin, Ed Briggs
This paper is a summary of best practices in optimizing the printing process focusing on comparison of large and small apertures, square vs. round, not with the same area ratio but with similar or the same volume. This paper will definitively clear the air on the round versus square aperture debate.
SMT, circuit board assembly, stencil apertures, solder paste, stencil printing
[Permanent Link to this Paper ]
Posted on 21 Jun 2011
by Dr. Ning-Cheng Lee , Dr. Yan Liu, Pamela Fiacco
Head-in-pillow (HIP) is ailing the electronic industry when assembling BGAs or CSPs onto PCBs. It is caused by warpage of components or boards at reflow process, and is aggravated by oxidation. Methods for assessing the potential for occurrence of HIP are highly desired by the industry. Besides using BGA rework station followed by tedious dye and pry treatment, two other simpler methods are introduced in this work, Tiny Dot Paste method and Ball Onto Paste method. The Tiny Dot Paste method is stressed on the assessment of oxidation barrier capability of solder paste, while Ball Onto Paste method assesses combined capability of oxidation resistance and excessive fluxing capacity. Both methods are quick, easy, and close simulation, with the latter being better in real process simulation. Prevention of HIP can be accomplished by (1) designing packages without warpage, (2) printing more paste, (3) dipping solder paste or flux, (4) using inert reflow atmosphere, (5) reducing reflow temperature, (6) placing heat shield on BGA or CSP, (7) avoiding using water soluble solder paste for BGA bumped with no-clean process, (8) using solder bumps or solder powder with oxidation resistant alloy, (9) using fluxes with high oxidation barrier capability and high fluxing capacity. Among all options listed above, using solder paste with high oxidation barrier capability and high fluxing capacity is considered the most easily implemented approaches.
head-in-pillow, solder, soldering, reflow, SMT, solder paste, BGA, CSP
[Permanent Link to this Paper ]
Posted on 24 Jan 2011
by Derrick Herron, Dr. Yan Liu, Dr. Ning-Cheng Lee
Quad-flat no-leads (QFN) package designs are receiving more and more attention in the electronics industry. This package offers a number of benefits including (1) small size, such as a near die size footprint, thin profile, and light weight; (2) easy PCB trace routing due to the use of perimeter I/O pads; (3) reduced lead inductance; and (4) good thermal and electrical performance due to the adoption of exposed copper die-pad technology. These features make the QFN an ideal choice for many new applications where size, weight, electrical, and thermal properties are important. However, the adoption of QFN often runs into voiding issue at SMT assembly. Upon reflow, outgassing of solder paste flux at the large thermal pad has difficulty escaping and inevitably results in voiding. It is well known that the presence of voids will affect the mechanical properties of joints and deteriorate the strength, ductility, creep, and fatigue life. In addition, voids could also produce spot overheating, lessening the reliability of the joints. This is particularly a concern for QFN where the primary function of thermal pads is for heat dissipation. Thermal pad voiding control at QFN assembly is a major challenge due to the large coverage area, large number of thermal via, and low standoff. Both design and process were studied for minimizing and controlling the voiding. Eliminating the thermal via by plugging is most effective in reducing the voiding. For unplugged via situations, a full thermal pad is desired for a low number of via. For a large number of via, a divided thermal pad is preferred due to better venting capability. Placement of a thermal via at the perimeter prevents voiding caused by the via. A wider venting channel has a negligible effect on voiding and reduces joint continuity. For a divided thermal pad, the SMD system is more favorable than the NSMD system, with the latter suffering more voiding due to a thinner solder joint and possibly board outgassing. Performance of a divided thermal pad is dictated by venting accessibility, not by the shape. Voiding reduction increases with increasing venting accessibility, although the introduction of a channel area compromises the continuity of the solder joint. Reduced solder paste volume causes more voiding. Short profiles and long hot profiles are most promising in reducing the voiding. Voiding behavior of a QFN is similar to typical SMT voiding and increases with pad oxidation and further reflow.
voiding, thermal pad, solder, solder paste, SMT, flux, Apex 2011, QFN assembly
[Permanent Link to this Paper ]
Posted on 11 Apr 2011
by Dr. Ning-Cheng Lee , Dr. Yan Liu, Derrick Herron
Quad Flat No Leads (QFN) package designs receive more and more attention in electronic industry nowadays. This package offers a number of benefits including (1) small size, such as a near die-sized footprint, thin profile, and light weight; (2) easy PCB trace routing due to the use of perimeter I/O pads; (3) reduced lead inductance; (4) easy PCB trace routing; and (5) good thermal and electrical performance due to the adoption of exposed copper die-pad technology. These features make the QFN an ideal choice for many new applications where size, weight, electrical, and thermal properties are important. However, adoption of QFN often runs into voiding issues at SMT assembly. Upon reflow, outgassing of solder paste flux at the large thermal pad has difficulty escaping and inevitably results in voiding. It is well known that the presence of voids will affect the mechanical properties of joints and deteriorate the strength, ductility, creep, and fatigue life. In addition, voids could also produce spot overheating, lessening the reliability of the joints. This is particularly a concern for QFN where the primary function of thermal pads is for heat dissipation. Thermal pad voiding control at QFN assembly is a major challenge due to the large coverage area, large number of via, and low standoff. Both design and process were studied for minimizing and controlling the voiding. Eliminating the via by plugging is most effective in reducing the voiding. For an open via situation, a full thermal pad is desired for a low number of via. For a large number of via, a divided thermal pad is preferred due to better venting capability. Placement of a via at the perimeter prevents voiding caused by via. A wider venting channel has a negligible effect on voiding and reduces joint continuity. For divided thermal pada, the SMD system is more favorable than the NSMD system, with the latter suffering more voiding due to a thinner solder joint and possibly board outgassing. Performance of a divided thermal pad is dictated by venting accessibility, not by the shape. Voiding reduction increases with increasing venting accessibility, although introduction of a channel area compromises the continuity of solder joint. Reduced solder paste volume causes more voiding. Short profiles and long hot profiles are most promising in reducing the voiding. Voiding behavior of a QFN is similar to typical SMT voiding and increases with pad oxidation and further reflow.
solder paste, reflow, SMT, solder, void
[Permanent Link to this Paper ]
Posted on 21 Feb 2011
by Wanda B. Hance, Dr. Ning-Cheng Lee
The mechanisms for void formation are investigated for applications involving solder paste in SMT. Generally the voids are caused by the outgassing of entrapped flux in the sandwiched solder during reflow. The voiding is mainly dictated by the solderability of metallization, and increases with decreasing solderability of metallization, decreasing flux activity, increasing metal load of powder, and increasing coverage area under the lead of the joint. Decrease in the solder powder particle size shows only a slightly negative effect toward voiding. The data indicate that voiding is also a function of the timing between the coalescing of solder powder and the elimination of immobile metallization oxide. The sooner the paste coalescing occurs, the worse the voiding will be. Increase in voiding usually is accompanied by an increasing fraction of large voids, suggesting factors causing voiding will have an even greater impact on the joint reliability than what shown by the total-void-volume analysis results. Preliminary data show that certain predry treatment and flux solvent with higher boiling point appear to cause increased voiding.
lead-free, pb-free, solderability, reflow, solder joint, SMT, voiding, void, flux, solder paste, soldering, solder
[Permanent Link to this Paper ]
Posted on 1 Jan 2009
