Technical Articles

Minimizing BTC Voids

And why waiting until the last process step is a bad idea.

BTCs, or bottom termination components, are a class of package referred to by a variety of acronyms and abbreviations. Different component and packaging companies may use different nomenclature (FIGURE 1), but almost all these components share one common, ugly characteristic: large pads that are prone to solder voiding. By design, these large thermal or ground pads require a defined percentage of contact with the solder and PCB to properly conduct heat and/or electricity. Excess solder voids can impact performance and reliability of the package.

The voiding problem has given both designers and manufacturers heartburn for the better part of two decades. Some of the drivers for this frustration are that voiding rates are highly variable, and numerous factors can impact their formation. While there is still...

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Tin Whiskers Revelations: Part III

Tin Whisker Study Conclusions.

Silver is a known whisker promoter but small amounts of bismuth mitigate the problem.  Alloy composition has a strong influence on tin whisker production and has proven to either mitigate or exacerbate the propensity for a solder to whisker. In our year-long study of the relationship between alloy composition on tin whisker formation, SAC 305 notoriously produced more and larger whiskers than any of the other alloys tested.

Sn0.6Ag0.6Cu3Bi, labeled “Alloy #69-2” in our test matrix, a tweak on one of the original nine alloys, produced the least and smallest whiskers. It also performed well in the other electronic solder suitability tests such as wetting, oxidation resistance, melting temperature and mechanical properties.

In this final round of tin whisker growth tests we aged the samples at 85%RH and 60°C for 5000hr, or roughly seven months. We...

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Tin Whiskers Revelations: Part II

Phase 2 of our study involved wetting balance, spread, DSC and mechanical tests.

We’re continuing our summary of a year-long experiment aimed at identifying lead-free solder alloys that mitigate tin whisker growth. This study cut right to the chase, beginning with an elimination round. The first phase knocked out two-thirds of the nine original candidate alloys based on their whisker production.

The results of the tests were both intriguing and enlightening, and forced us to rethink what we thought we knew about tin whisker propagation. The investigation focused on the conventional wisdom that tin whiskers are the result of compressive stresses caused by electroplating, bending, intermetallic growth, and thermal expansion mismatches.

It assessed the influence of these most commonly cited potential causes by creating...

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Assessing the Impact of T4 Solder Powder on Print Transfer Efficiency

Abstract. The drive to reduced size and increased functionality is a constant in the world of electronic devices. In order to achieve these goals, the industry has responded with ever-smaller devices and the equipment capable of handling these devices. The evolution of BGA packages and leadless devices is pushing existing technologies to the limit of current assembly techniques and materials.

As smaller components make their way into the mainstream PCB assembly market, PCB assemblers are reaching the limits of Type 3 solder paste, which is currently in use by most manufacturers.

The goal of this study is to determine the impact on solder volume deposition between Type 3, Type 4 and Type 5 SAC305 alloy powder in combination with stainless steel laser cut, electroformed and the emerging laser cut nano-coated stencils. Leadless QFN and μBGA components will be the focus of the test utilizing...

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Tin Whiskers Revelations: Part I

A multi-part study found plenty of whiskers, but not where they were expected.

THE ELUSIVE TIN WHISKER is only 1/100th the width of a human hair, but this tiny, single filament protrusion can wreak havoc with all sorts of electronics, and has even been cited as the cause of some sudden acceleration failures in cars. Tin whiskering is not a new phenomenon, however. It has been documented as far back as the 1940s, so why the heightened concern now? Lead was a great mitigator of whiskers and its removal from electronic solder opened the door for increased whisker proliferation, perhaps with sometimes deadly consequences.

Despite years of study, a single root-cause mechanism on whisker formation has yet to be established. Common wisdom suggests that they are a form of stress-relief, compensating for the compressive stresses induced by...

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J-STD-004B: A New Twist on an Old Standard?

Before rejecting a flux, be sure you understand the standard.

J-STD-004 was updated about five years ago, but the comingling of designations from the old (A) and new (B) revisions in industry literature has created much confusion among users. What changed, what didn’t, how does it affect flux selection criteria, and what does a user need to know? Here’s a quick overview:

Flux designation has three components. The first two letters, RO, RE or OR, represent the basic chemical composition: rosin-based, resin-based or organic acid-based. Nothing has changed there. But the next component to flux designation, L, M or H, which describes the flux’s activity level as low,...

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The Reflow Limbo: How Low Can We Go?

Sn/Cu/Ni soldering performance at low temperatures.

Nickel-modified tin-copper solder, known commercially as SN100C®, is a leading lead-free alloy for PTH soldering, rework and hot air leveled PCB final finishes. Because it contains no silver, it is much more economical than SAC alloys containing 1, 3 or even 4 percent of the precious metal, and it produces smooth, shiny, easy-to-inspect solder joints. Why has it not gained major acceptance as an SMT alloy? In large part, fear. Fear of full compatibility with SAC reflow processes.

SnCuNi melts at 227°C. SAC305 begins to melt at 217°C, reaching its fully liquid state at approximately 220.6°C. Recommended reflow temperatures are typically at least 13°C higher than melting temperatures; hence the SAC305 peak temperature window of 233 – 255. Applying the 13-degree guideline to the...

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Production Testing of Ni-Modified Sn/Cu Solder Paste

ABSTRACT.  Low-silver and no-silver lead-free PCB solder alloys represent substantial cost savings over SAC alloys that contain 3% silver. Many wave solder operations use silver-free alloys for through hole soldering, and SMT operations are also beginning to adopt the alloy for surface mount soldering.

This paper reviews two case studies that test Ni-modified SnCu solder paste on mixed technology circuit assemblies which currently use SAC305 in production. The major differentiator between the two cases is the reflow profile: In one case, the SAC profile was considered nearly perfect for the SnCuNi alloy, peaking at 245°C. In the other case, thermally sensitive components constrained the profiles to peaking near 235°C, only 8 degrees above the silver-free alloy’s 227°C melting temperature. This important case is used to explore the lower limit of reflow capability and compare it...

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Whats on Your Rework Bench

Every seasoned SMT process engineer has at least one nightmare story about bad rework chemistry. Whether it’s the wrong flux sneaking into the operation, underprocessed flux remaining on the board, or improper residue removal, they all risk the same inglorious fate: field failure. While rework operators and process engineers rarely get a glimpse of the long-term damage of inappropriate materials or processes, the Failure Analysis engineers see it all. They report that the vast majority of solder-related issues are on reworked solder joints, and the lion’s share of those is due to the rework chemistry.

How does the wrong flux work its way into rework? Simple human error. In some cases, operators who experience difficulty using approved rework fluxes on parts with poor solderability bring in more active materials from their personal collections. In auditing rework stations, we’ve found all sorts of unapproved soldering fluxes...

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Conformal Coating Over No Clean

Introduction. As the proliferation of modern day electronics continues to drive miniaturization and functionality, electronic designers/assemblers face the issue of environmental exposure and uncommon applications never previously contemplated. 

This reality, coupled with the goal of reducing the environmental and health implications of the production and disposal of these devices, has forced manufacturers to reconsider the materials used in production.

Furthermore, the need to increase package density and reduce costs has led to the rapid deployment of leadless packages such as QFN, POP, LGA, and Micro-BGA. In many cases, the manufacturers of these devices will recommend the use of no clean fluxes due to concerns over the ability to consistently remove flux residues from under and around these devices. 

These concerns, along with the need to implement a...

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Not All WOA (Weak Organic Acids) Are the Same

Abstract.  In an effort to reduce volatile organic compound (VOC) emissions within our environment, policymakers have encouraged and/or mandated that electronics manufacturers change from alcohol-based VOC-containing fluxes to water-based VOC-free flux alternatives. As a result, the use of VOC-free fluxes is growing throughout North America, Asia and Europe.

The purpose of this study is to explain several factors relating to the use of a VOC-free flux in the soldering process and their impact on testing and product reliability. These factors include; the effect of varying types of acids used in flux formulations and their impact on Ion Chromatography (IC) and Surface Insulation Resistivity (SIR) test results and weak organic acids (WOA) solubility and their influence on the electrical integrity of assemblies. This paper shall provide valuable insight into the outcome of acid-solvent interactions. 

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Lead-Free Alloy Development

Abstract.  When lead-free solders were first introduced to the electronics industry in the early 1990’s, the tin-silver alloy composed of Sn96.5/3.5Ag was the first investigated. The most obvious difference from the industry standard was the substantially higher melting temperature of 221°C versus 183°C (Sn63/Pb37). 

Although Sn/Ag had been used extensively in hybrid electronics, concerns over silver migration and silver creep drove the industry to investigate Sn/Cu and (S)n/(A)g/(C)u (SAC) alloys. Initial selection criteria deemed SAC to be the more reliable option. In early tests, Sn/Cu exhibited inferior reliability in thermal cycling (-65+125C) and was prone to in-process wetting issues. The three leading commercialized SAC alloys were introduced as SAC305, SAC387, and SAC405. (Note: the first number refers to silver content followed by the last number, which is the copper content. SAC305 is tin, 3% silver and 0.5% ...

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Overcoming the Challenges of the QFN

Introduction.  Over the last several years, the electronics assembly industry has witnessed a seismic shift toward component miniaturization. Consumer demands for more functionality in smaller packages have motivated OEMs and Contract Manufacturers to engage in new and innovative technologies. As technological advances usher in a more compact design, a decrease in available board space and more densely populated PCB’s have become increasingly common. In response to these demands, the industry has witnessed a proliferation of Quad Flat No Leads (QFN) packages. The goal of this study is to identify the advantages and challenges that the QFN package brings to the electronics assembly process. This paper will also discuss the results of data evidencing the fundamental steps required to successfully implement QFN technology into an assembly.

The QFN Package.  The QFN package, pictured in (figure 1), is a flat plastic package with perimeter leads underneath the device...

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Bar Solder Drossing Characteristics and Wave Contamination

Solder is a combination of metals that form an alloy with a melting point lower than any of the individual combined elements.

In the process of alloying, the metals are added and melted together and then cooled to a predetermined point above the melting point of the alloy. In the case of electronic-grade tin-lead (Sn63/Pb37) bar, this would be a point above 183°C (361°F) and for a leadfree alloy such as SAC305 (Sn96.5/Ag3.0/Cu0.5) it would be a point above 217-219°C (423-426°F). When an alloy is melted, the surface of the alloy is exposed to air. This interaction of the air on the alloy surface forms an oxide layer called dross. The density of the dross and the alloy are very similar, which causes a slow separation of the two. Typically, dross is not related to impurities but is more related to oxidation rates (although some impurities such...

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Controlling Copper Build Up In Automatic Soldering Equipment

Abstract.  Since “nothing solders like solder”, HAL (Hot Air Leveling) will continue to hold a significant place in the surface finishing industry. Furthermore, the wave soldering process will continue to be a viable means of electronics assembly. However, as automatic soldering processes using lead-free alloys have become increasingly prevalent, questions have arisen about copper dissolution into these alloys.                 

Introduction. The Sn/Ag/Cu family of alloys is the leading candidate for a lead-free alternative. The first part of this study was to determine if there is any significant difference between Sn/Ag/Cu alloys when used in automatic soldering equipment in terms of copper build-up in the system. The study compared two Sn/Ag/Cu alloys to determine if at processing temperatures one alloy would absorb less copper than the other alloy. 

The second part of...

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A Comparison of Tin-Silver-Copper Lead-Free Alloys

Abstract.  As the electronics industry begins to focus upon the tin-silver-copper family of alloys as a viable replacement for tin-lead solders, research needs to be done to determine if any particular alloy is best suited for the broadest range of applications. The tin-silver-copper family of alloys has earned a great deal of positive response from various industry consortia and organizations in recent years and the majority of manufacturers plan on implementing one of these alloys. However, as there are several different alloy formulations within the tin-silver-copper family, background information is necessary to determine which alloy is best suited for the broadest range of applications. 

Introduction.  It is inevitable that lead will be eliminated from a great deal of electronics assembly. Whether ...

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A Study of Lead-Free Wave Soldering

Discussion.  As lead-free wave soldering becomes increasingly prevalent, questions have arisen about copper dissolution into lead-free alloys. Concerns have arisen over the use of alloys that may require more solder pot maintenance due to their high copper dissolution rates. 

The first part of this study was performed to determine if there is any significant difference between Sn/Ag/Cu alloys. The study compared Sn/Ag3/Cu0.5 (LF218) to Sn/Ag2.5/Cu0.8/Sb0.5 (CASTIN). The purpose was to determine if at wave soldering temperatures one alloy will absorb less copper than the other alloy.

Test Procedure.  Two pots of each alloy holding approximately 500 grams of metal were heated to 530°F (276°C). Copper strips were weighed, fluxed and then placed into the lead-free alloys. Thetemperature and strips were monitored every 5 minutes for any visual change. After 30 minutes changes...

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Specialty Solders: A Study of Indium/Lead

More complicated electronics will require the increased use of specialty solders. Specialty solders, such as indium alloys, offer advantages for gold soldering, step soldering and fatigue resistance.

Indium/Lead Solders on Gold.  Because they do not leach or dissolve gold as readily as tin/lead solders, indium/lead solders are recommended for soldering to gold.  As seen in fig. 1, the phase diagrams of tin/gold shows that tin dissolves approximately 18 percent by weight of gold at soldering temperatures of 225-250°C.1   Under the same conditions the indium/gold phase diagram shows dissolution of only 2 to 4 percent of gold in indium.  Thus, with a substantial reduction in the dissolution of gold, less intermetallics are formed and improved solder flow over the gold surface occurs.  

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A Practical Guide to Achieving Lead-Free Electronics Assembly

Abstract. To successfully achieve lead-free electronics assembly, each participant in the manufacturing process, from purchasing to engineering to maintenance to Quality/Inspection, must have a solid understanding of the changes required of them. This pertains to considerations regarding design, components, PWBs, solder alloys, fluxe s, printing, reflow, wave soldering, rework, cleaning, equipment wear & tear and inspection.

Introduction.  With the WEEE and RoHS Directive in Europe (in its most recent revision) potentially outlawing lead from electronics produced and imported in the EU as early as 2006 and foreign competition driving the implementation of lead-free electronics assembly around the world, additional questions regarding how manufacturers can successfully transition to lead-free assembly continue to arise. A great deal of consortia work and empirical data exists on lead-free soldering. What...

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Head-in-Pillow BGA Defects

Head-in-pillow (HiP), also known as ball-and-socket, is a solder joint defect where the solder paste deposit wets the pad, but does not fully wet the ball. This results in a solder joint with enough of a connection to have electrical integrity, but lacking sufficient mechanical strength. Due to the lack of solder joint strength, these components may fail with very little mechanical or thermal stress. This potentially costly defect is not usually detected in functional testing, and only shows up as a failure in the field after the assembly has been exposed to some physical or thermal stress.

Head-in-pillow defects have become more prevalent since BGA components have been converted to lead-free alloys. The defect can possibly be attributed to chain reaction of events that begins as the assembly reaches reflow temperatures. Components generally make contact with solder paste during initial placement, and start to flex or warp during heating, which may cause some individual solder...

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