Double-sided preheating influence on solderability

In this paper, the performance of a complete halogen-free (HF) board assembled with five ball grid array components using daisy chains on an 8-layer high density interconnection printed circuit board (PCB) was examined using bending tests. The solder joint was composed of the SAC405 solder ball melted with the SAC105 solder paste on the Cu pad surface of either organic solderable preservative (OSP) or electroless nickel immersion gold (ENIG), and was tested to examine the extent of intermetallic compound (IMC) formed. A board-level cyclic bending test was executed to the as-reflowed assemblies according to the JESD22-B113 standard to evaluate the reliability of various surface finished HF PCB assemblies under a recommended load condition. By Weibull analyzes the characteristic lives of the assemblies were 99098 and 130290 cycles on OSP soldering system and ENIG system respectively, indicating that the ENIG had a higher bending fatigue endurance as compared to the OSP. The dominant failure mode was the fracture of Cu trace within the micro-via caused by the crack of the resin coated copper layer. At the interface of the solder joint, the IMC formed on both OSP and ENIG was with various Ni contents of 3.9 and 20.1 at%, and different thicknesses of 5.6 and 1.1 , respectively. Numerical simulations showed that both composition and thickness of IMC influenced the stress concentrations and characteristic life of the HF test vehicle as they could slightly increase localized solder joint rigidity and result in more serious pull and drag behaviors at the corner joint. Although the on the ENIG was stiffer than that on the OSP, the thicker IMC layer instead increased the stress on the OSP system due to size effect, and resulted in a lower characteristic life.

Board-level drop testing is an effective method to characterize the solder joint reliability performance of miniature handheld products. In this study, drop test of printed circuit boards (PCBs) with a four-screw support condition was conducted for a 15 mm times 15 mm fine-pitch ball grid array (FBGA) package assembly with solder ball compositions of 36Pb-62Sn-2Ag and Sn-4Ag-0.5Cu on printed circuit board (PCB) surface finishes of organic solderability preservative, electroless nickel immersion gold, and immersion tin. Finite element modeling of the FBGA assembly was performed to study the stress-strain behavior of the solder joints during drop test. The drop test results revealed a strong influence of different intermetallic compound formation on soldered assemblies drop durability. The lead-based solder supersedes the lead-free composition regardless of the types of surface finish. Joints on organic solderability preservative were found to be strongest for each solder type. Other factors affecting drop reliability such as component location on the board and thermal cycling aging effects are reported. Finite element modeling results showed that a solder joint is more prone to failure on the PCB side, and the predicted solder joint stresses are location dependent. Predicted failure sites based on simulation results are consistent with experimental observations.

Board level drop testing is an effective method to characterize the solder joint reliability performance of miniature handheld products. In this study, drop test of printed circuit boards (PCB) with four-screw support condition was conducted for a 15 mm /spl times/ 15 mm fine-pitch ball grid array (FBGA) package assembly with solder ball compositions of 36Pb-62Sn-2Ag and Sn-4Ag-0.5Cu on PCB surface finishes of organic solderability preservative, electroless nickel immersion gold and immersion tin. The results revealed a strong influence of different intermetallic compound formation on soldered assemblies drop durability. The lead-based solder supersedes the lead-free composition regardless of the types of surface finish. Joints on organic solderability preservative were found to be strongest for each solder type. Other factors affecting drop reliability such as component location on the board and thermal aging effects are reported.

This study is part three in a series of papers on voiding in solder joints and methods for mitigation of voids. In this study several new variables were tested and compared to previous data on voiding. A new circuit board design was used which is different than the circuit board used in previous studies. The new circuit board design includes two sizes of QFNs (Quad Flat No Lead), BGA's (Ball Grid Array), and LGA (Land Grid Array) components which are susceptible to voiding. The following variables were evaluated with respect to voiding: Solder powder size was varied using a no-clean lead-free solder paste and IPC Type 3, Type 4 and Type 5 SAC305 solder powders. Solder alloy was studied using SAC305 alloy, SN100C® alloy, SN100CV® alloy and a mixture of SAC305/SN100C alloys. Surface finish on the circuit board is thought to have an impact on voiding. Organic solderability preservative (OSP) surface finish is more difficult to wet with solder than other surface finishes and in theory should produce higher voiding levels, and this was evaluated with multiple solder pastes. The performance of a new “low voiding” no clean solder paste was compared to other industry standard no clean solder pastes. Several new stencil designs on QFN thermal pads were tested. Data was gathered and statistical analysis performed to compare and contrast the voiding data. What was learned from this work is summarized below: Solder powder particle size affects voiding for some solder pastes. Voiding tends to decrease as solder powder size becomes smaller. Solder alloy affects voiding and seems to follow a trend with melting range. Wider melting ranges tend to generate lower voiding levels. Organic solderability preservative (OSP) surface finish gave higher voiding than electroless nickel immersion gold (ENIG) surface finish for a no clean lead free solder paste that was tested. Two different water soluble lead free solder pastes showed similar voiding levels for OSP and ENIG finishes. New “low voiding” solder pastes are becoming more common in our industry. In this work, low voiding solder paste F showed lower voiding than an industry standard solder paste C. Stencil design has an impact on voiding. In this study, area of printed solder paste coverage was varied. Voiding levels were much higher for 50 to 60% solder paste coverage than for 70 to 80% solder paste coverage. Based on the results of this work, the following recommendations were made to reduce voiding. Use of smaller solder powder sizes can reduce voiding with certain types of solder paste. Use of specific solder alloys may help mitigate voiding. Use a solder paste that works well with the surface finish to minimize voiding. Use “low voiding” solder pastes to reduce the overall potential for voiding. Optimize the stencil design for the components to allow for complete wetting and gas escape routes. Applying these recommendations may create issues on the printed circuit board assembly (PCBA). For example, use of smaller solder powder particle sizes may lead to increased random solder balling and/or graping. Increasing the volume of printed solder paste may lead to bridging for micro BGA and ultra-fine pitch components. Reduction of voiding may be important enough to risk these potential issues. This is left to the reader's judgement. It is the intent of this work to analyze general trends in voiding and give recommendations to help the reader to “Fill the Void.

Corrosion on automobile printed circuit broad

One of the most pressing challenges facing today's electronics packaging industry is to identify reliable and cost-effective solder alloys to replace toxic lead containing solder. Besides evaluating new alloy compositions and improving the soldering process, it is important to understand how surface finishes applied to the copper metallization affect the joint characteristics. The characterization of intermetallics during the early stages of nucleation and the growth is hindered by the nanoscopic grain size and layer thickness. This study investigates the impact of PCB finish and solder type on the interfacial intermetallics. Five types of solder and four types of finishes were used: Sn37Pb (SP), Sn3.5Ag (SA), Sn3.5Ag0.7Cu (SAC), Sn2Ag0.5Cu4Bi (SACB) and Sn3Bi8Zn (SBZ) solders, combined with immersion Silver ( I – Ag ), electroless Nickel-immersion Gold (ENIG), Organic Solderability Preservative (OSP) and immersion Tin ( I – Sn ). It was shown that both the SP and SAC solders follow a parabolic growth model and that the surface finish has a significant effect on the intermetallic morphology and growth kinetics. A combined SEM and XRD investigation was shown to be a suitable method for characterizing nanoscale intermetallics.

The long term reliability of lead-free Sn 96.5 Ag 3.0 Cu 0.5 (SAC305) has been assessed under accelerated temperature cycling (ATC) conditions. Test vehicles consisted of printed circuit boards (PCBs) with three different Pb-free surface finishes Organic Solderability Preservative (OSP), Immersion Silver (IAg) and Electroless Nickel Immersion Gold (ENIG). These boards were connected to the ball grid arrays (BGAs) and 2512 chip resistors by a lead free solder joint. The components were subsequently thermally cycled from 0°C to 100°C with 30 minute dwell times at the upper and lower temperature limits. They were monitored for electrical failures which were defined by a resistance reading of >1000. The life times of the BGAs and chips were predicted using 2-parameter and 3-parameter Weibull analysis as a function of board finish and alloy. Life times were substantially influenced by board finish therefore optical and scanning electron microscopy (SEM) were performed on as-received and post temperature cycling samples in order to establish their effect on failure mechanisms and on microstructural evolution as a result of thermal cycling.

Tin-Silver-Copper (SnAgCu) lead-free solder on Electroless Nickel Immersion Gold (ENiG) and Immersion Tin (ImSn) surface finish printed circuit board was subjected to blast test. A variation of intermetallic compounds (IMC) layer, hardness and reduced modulus of soldered sample exposed to blast test were intensively investigated using optical microscope and nanoindentation machine. Formation of IMCs due to reaction between solder and substrate during blast test provided deleterious effect of metallurgical bond strength and reliability on the solder joint. Microstructural analysis was evaluated via Infinite Focused Microscope (IFM). The findings of these studies indicate that best surface finished for blast test performance was not necessarily the best surface finish for optimum reliability. ENiG and ImSn surface finish can be advantage or a disadvantage depending on the application, package and reliability requirements. As a result, most component assemblers are using ENiG and ImSn in order to improve solderability as well as the wettability between solder and the substrate and to meet various package requirements.

In 1999 The Immersion Tin surface finish was introduced to the market. The industry at that time was mainly using ENIG (Electroless Nickel), HASL (Hot Air Solder Leveling with SnPb) and OSP (Organic Solder Preservative). At that stage more than 60% of the market in terms of square meter used HASL, mainly for wave soldering with eutectic SnPb solder. But market conditions were changing over time and new requirements such as the growth of the Surface Mount Technology (SMT) and the introduction of new regulations such as RoHS and WEEE had resulted in the fact that alternative surface finishes were required to fulfill the new challenges towards more flat surfaces compared to HASL (Thickness 1-40μm) or for the assembly of SMD (Surface Mount Device) parts, which were getting gradually smaller and smaller. In addition to meet the new ecological requirements from RoHS and WEEE, lead free surface finishes become paramount in many industries. New surface finishes such as Immersion Tin and Immersion Silver entered the market and steadily gained ground. One of the major concerns regarding Immersion Tin in the past was the potential for whisker growth. In 2002 the new Anti-Whisker-Additive was introduced for Immersion Tin which solved this problem and new qualifications in the automotive industry, industry electronics, the server infrastructure, enhanced the growth of Immersion Tin over the years. When time passed by those OEM´s and assemblers who changed from HASL to Immersion Silver were confronted with corrosion failures form the field (creep corrosion and silver migration). Furthermore solder joint reliability issues caused by champagne voids. Both impacts was leading to the situation that the Immersion Silver market nowadays is not much growing and even for the future more and more assemblers are changing either to OSP or Immersion Tin whereas other OEMs still keep Immersion Silver as a surface finish by doing additional efforts like more properly sealing the electronics from the environment e.g. better sealed housing or by improved conformal coatings. It was not considered by some OEMs that their products are losing the corrosion resistance of the full assembled package when changing from HASL towards Immersion Silver. Immersion Tin did not suffer this problem because of the good corrosion resistance comparable to the HASL finish. Further developments of the Immersion Tin process such as continuous chemistry regeneration, the improvement of soldermask compatibility and other features lead to the fact that nowadays Immersion Tin is growing year by year. This paper will also provide an outlook to the future of Immersion Tin.

The technology of surface finishes for printed circuit boards is seeing a dramatic shift from the Hot Air Solder Levelling (HASL) towards alternative finishes like electroless Nickel - immersion gold (ENIG), immersion tin, immersion silver and organic solder preservatives (OSP's). This trend is mainly caused by the worldwide environmental pressure to ban the use of lead for electronic assemblies as well as the demands of modern assembly technology, which require a higher coplanarity of the surface finish for surface mount assembly. One of the main benefits of HASL is the extremely good resistance of the surface against ageing under high temperature conditions. Alternative finishes, due to their limited thickness, offer excellent coplanarity, but inferior surface protection. A detailed understanding of the ageing processes is thus a prerequisite for the implementation of alternative surface finishes in high yield PCB production. In this paper the ageing characteristics of immersion tin surface finishes are investigated and correlated with the reliability during soldering operations. Ageing of tin surface finishes takes place by intermetallic compound (IMC) formation at the Sn/Cu interface due to solid state diffusion. Hence the kinetics of the process is assumed to follow a simple law. The kinetics of IMC formation was investigated experimentally and correlated with data from literature. The results show that the simple kinetic model leads to an overestimation of the kinetics in the temperature regime below 60°C. Changes in the reaction mechanism with temperature as well as with time must be taken into account for a conclusive understanding of the ageing characteristics. The only parameter with relevance for practical solderability is the thickness of the immersion tin layer.

Electroless Nickel Immersion Gold (ENIG) is one of the important final finish techniques that is used in the printed circuit board industry. Despite its relatively expensive cost compared to other final finishing processes such as OSP and immersion Tin etc., ENIG has many advantages. The low reactivity of Au can protect the underlying Ni and Cu surfaces from oxidation and keeps the board suitable for long storage time. It provides excellent surface planarity suitable for soldering, especially for tiny parts such as BGA and FlipChip, and the surface remains solderable even after multiple reflow cycles. It is useful for contact surfaces such as membrane switches and contact points and may also be used for wire bonding. The Immersion Gold (IG) bath provides the protective Au layer after the Electroless Ni (EN) process following the below equation: 2Au+ + Ni _ 2Au + Ni <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> . As an electrochemical process, the deposition rate (or Au thickness per unit of time) is highly dependent on the redox potential of Au <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> and electropotential of the Ni surface. The former can be considered as a fixed value in a Au plating solution while the latter will vary depending on the design of boards (pad size, connection of pad to the inner-layer Cu, etc.). The differences of surface potential, or so-called galvanic effect, makes the deposition of Au non-uniform, which may influence the properties of the board in many aspects. For example, chromatic aberration of the Au surface appears and some of the extremely thick Au pads may appear as red color; solderability may become uneven across all pads; and Au may be wasted due to the poor gold thickness distribution. More seriously, the galvanic effect may cause Au skip-plating, or black Ni because of Ni over-dissolution on specific pads, resulting in the solderability problem. Our new immersion gold technology can effectively reduce the galvanic effect by introducing a new chemical system. The electropotential differences were minimized among pads with various surface areas or connected to different inner-layer copper areas thus providing a uniform gold deposition. Meanwhile, the plated Au thickness can be steadily increased from less than 1 microinch to 3 microinches and metallic lustre of the Au surface remained normal, from the pale yellow of thinner Au to the lemon yellow of thicker Au. Ni corrosion resistance remained good for a range of Au thickness from 1 microinch to 3 microinches. Modified HNO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> porosity test yielded good results. Solderability test using SAC305 solder showed good reliability after 0×, 3× reflow and baking (150 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> C, 4 hours) conditions in ball shear and ball pull test.

In this study, the termination finish of Small-Outline Integrated Circuit (SOIC) chip components were converted from lead-free to tin-lead (backward conversion) and from tin-lead to lead-free (forward conversion). The motivation for these conversions is due to a combination of factors such as the supply chain constraints on component availability and the European Union's (EU) legislation on “restriction of certain hazardous substances” (or RoHS). Additionally, component leads with 100 weight percent matte-tin (100wt.%Sn) finish were converted to lead-free Sn-3.5wt.%Ag-0.5wt.%Cu (commonly referred to SAC305) finish. This type of conversion would be necessary to mitigate against the tin whiskers issue common in matte tin-plated components. All the conversions were performed by E-Certa, Inc using their “Robotic Stripping and Solder Dipping Process”. The converted parts were examined using x-ray fluorescence (XRF) technique to test for RoHS compliance for the metallic species. Electrical and die visual inspection utilizing optical microscopy, direct current (DC) pin-to-pin electrical testing and acid de-encapsulation were performed to inspect for damage on die, after the conversion process. There were no opens or short circuits found during the electrical test. The die was undamaged after the conversion process. As the final evaluation method of the robotic conversion process, the converted parts were assembled on a printed circuit board (PCB) using both tin-lead and lead-free processes. Three PCB surface finishes namely immersion silver (Im-Ag), organic solderability preservative (OSP), and electroless nickel/immersion gold (ENIG) were used. The microstructures of the converted components were typical in appearance and had well defined intermetallic compound (IMC) both along the bulk solder/PCB and bulk solder/component interfaces. Further, lead pull testing was performed on ‘as-assembled condition’ (AA) and ‘after thermal aging’ (ATA) components to determine the mechanical integrity of converted component termination finishes with respect to the original termination finish. The results indicate that the converted components have similar pull strength as that of the original components.

“A new high performance immersion tin incorporating a co-deposited organic provides a highly solderable PWB'surface finish. The process is described and includes solderability data after accelerated aging and extended processing. Studies of SIR, metallic dendritic growth and ionic contamination are included. A final section shows the benefits for fine pitch assembly, including the yield improvements seen at Reflow, ICT and final inspection. The reliability questions are answered and conclude that the flat solderable tin process provides a cost effective and yield-enhancing planar alternative to Nickel-Gold, HASL and OSPs.” "Nothing solders like solder" - so it is written. The application of molten Tin/Lead onto clean copper gives an excellent electrical and mechanical interconnection. The copper/tin/lead metallurgy is very well understood and documented, and the solder joints formed have a well-established track record for reliability. However, the use of the 63/37 tin-lead Hot Air Solder Level (HASL) process, which has been the finish of choice for several decades, continues to decline. HASL is a cost-effective finish but has several technical drawbacks related to the difficulty of application and to the assembly limitations with fine pitch technology. For this reason, it has already been superseded by a range of flat, so called “planar”, finishes which offer significant benefits with regard to the improved paste screening, the control of solder joint volumes and the fast, accurate placement of small components These selective planar finishes include several metallization processes such as Electroless Nickel-Immersion Gold and Electroless Nickel-Electroless Palladium which are not surprisingly at the high end of the cost spectrum. At the opposite end resides a whole group of substituted Imidazole type coatings that form the basis of the Organic Solderabiliy Preservatives (OSPs). These OSPs are relatively cheaper to apply than HASL and, in many cases, provide adequate solderability for the assembler. It is not surprising, however, that the unrelenting trend of miniaturization continues to challenge not just HASL but in fact all the finishes. The explosive growth of the Ball Grid Array technology, which has critical planarity needs, has seen its solder joint reliability plagued by incidences of interfacial stress fractures associated with Nickel/Gold. On the other hand, the use of mixed technology in the assembly process, (i.e. surface mount components on both sides of the printed circuit board together with through-hole interconnects), demands that the surface finish has a high tolerance to multiple heat cycles. This, in turn, is now challenging the OSPs which, in many cases, have limitations in withstanding the multiple thermal excursions, particularly when less aggressive and/or low solids fluxes are applied (an increasing trend). As a direct result, there has been a growing demand for further alternatives, and a new generation of thin immersion metallization processes has entered the market. These include both silver and tin technologies, both of which offer lower cost-of-use relative to gold and palladium. The modified immersion tin process offers a very novel solution to this problem, largely because it provides a very comparable copper/tin/lead metallurgy to that of HASL when used with tin-lead solder pastes and solder-wave. This paper goes on to review the case history of one such modified proprietary flat solderable tin process, and the benefits demonstrated to the assembler. Flat solderable tin, or any other new finish, has to unequivocally satisfy the stringent requirements of planarity, solderability, ionic cleanliness and surface insulation resistance. It is important that the new process can be readily integrated into the PWB fabrication operation. Coupled with this, an ability to deal effectively with the emerging volume production and assembly of fine pitch and area-array technologies represents the ideal. It is also extremely important to demonstrate that the tin finish, whether soldered or unsoldered, will not whisker and will not be subject to any corrosion, dendritic growth or any other mechanical or thermal breakdown which could conceivably affect the long term performance or reliability of the device.

Fatigue performance and microstructure of lead-free solder joints in BGA assembly at room temperature

Corrosion of electronics is a growing concern with the evolution of new product designs and their usage in complex environments. In this paper the focus is on the Printed Circuit Board (PCB) that is frequently reported for corrosion related problems. Sulfide films were grown on PCB's with Immersion Silver (ImAg) and Organic Solderability Preservative (OSP) surface finish. Time of Flight Secondary Ion Mass Spectroscopic (TOF SIMS) depth profiling showed significant moisture accumulation in the region below the corrosion product. In both the cases (ImAg and OSP) copper was seen to diffuse through the surface finish and react with the sulfide rich environment.