The soldering of THT components through solder waves is a thermal process. However, current design rules, guidelines and guideline catalogs do not map the soldering heat requirement of a solder joint. Therefore, this approach cannot ensure sufficient solder fill according to IPC-A610 at the design stage. This requires objective models for evaluating the design data concerning manufacturability. These models have considerable potential, both technologically and economically. In addition to automated manufacturability checks, the automatic and model-based determination of optimized soldering programs results in potential reduction of scrap, shorter process development and more robust processes. Extensive studies of experimental, numerical and analytical models for the prediction of minimum solder fill are the basis for successful artificial intelligence (AI)-based modeling of THT-selective wave soldering. On this basis, it is possible to train meaningful AI models and actually validate the solder fill. The mentioned approaches will be highlighted in this paper and it will be shown how they can be profitably applied in practice from electronics design to manufacturing.

An In-containing mid-temperature solder paste (MTS) has been developed and successfully used in mobile phone board-stack soldering with a 200°C peak temperature profile [1]. It is now being tested for a wafer-level package (WLP256) application using reflow profiles with peak temperatures ranging from 200°C (P200) to 240°C (P240). SAC305 was used as the control, which was reflowed using a traditional lead-free soldering profile (P240). With the constant paste-to-ball volume ratio of 1:4, the joint morphology changed with the reflow profiles. Under the 200°C peak reflow, hybrid joints were formed, in which the mixing zone, dominated by the MTS, was present at the PCB side while the area above the mixing zone maintained the original SAC305 morphology. Inside the mixing zone, In was present in the form of a InSn4 (γ) phase, Sn(In) solid solution, and likely, In4Ag9 particles. Increasing the reflow peak temperature to 210°C (P210) and above, the homogeneous joint was formed, which indicated the fully-merged SAC305 ball with the MTS paste during reflow. The homogeneous joint morphology was similar to the traditional morphology of SAC305, in which an Sn dendrite was surrounded by a Ag3Sn precipitate network. In most likely existed in In4Ag9 participles and the Sn(In) solid solution. The temperature cycling test (TCT) was conducted with a -40/125°C and 20-minute dwelling profile. Regardless of the reflow profiles, the MTS outperformed SAC305 in TCT. The P210 profile forming the homogeneous joint, resulted in the best TCT performance, which was more than a 30% improvement than its counterpart using SAC305 paste. The other profiles also improved the characteristic life at least 11% more. Using the same reflow profiles, the MTS had also demonstrated that the drop shock performance (g-force >6000g) was at least comparable to or significantly better than SAC305, in which the P220 (220°C peak temperature) reflow, forming the homogeneous joint with the paste-to-ball volume ratio of 1:2, led to the best drop shock performance—more than 90% enhancement than SAC305. Although the reflow profiles impacted the performance, the failure modes remained similar to SAC305.

There has been increased interest in reducing energy consumption during SMT assembly over the past few years. Increasingly, the environmental, financial, and regulatory effects have been demanding new innovations. Low-temperature solder alloys have also been of increased interest to the SMT industry for a variety of reasons including component sensitivity, step soldering, and reduced energy consumption. However, while the reduced temperature and subsequent energy during reflow have often been listed as a benefit, it has rarely been publicly quantified. In this paper, we quantify the effect of reduced temperature on oven energy usage by recording the actual consumption rate of an oven under different reflow conditions. The demonstrated reduction in oven energy consumption may then be used by manufacturers to evaluate tradeoffs and benefits of low-temperature solders.

In the last few years there have been concerns in the industry especially in the products requiring high reliability when using microvia structures. As a result many fabricators have been mandating push back on complex high layer count designs which has resulted in very conservative rules for designers to use and meet the fabricators capabilities. Some fabricators have also struggled to ensure even simple structures are built reliably and with repeatability. This study was to look at materials which would ensure that more complex structures could be built reliably. Traditional thin fiberglass reinforced dielectric layers can have issues with the lack of resin movement through spread glass necessitating higher resin to glass ratios for the manufacture of stacked microvias. Current industry practice has been to limit designs to staggered vias or to 1-2 layers of stacked microvias. This work extending a previous study presented at Apex 2022 1 will show how a thin hydrocarbon dielectric layer can be used and optimized for stacked microvias that demonstrates solid thermal reliability up to 5 levels of HDI. It also shows there seems to be no indication yet of a ceiling on how many layers could be used and introduces buried vias and how they affect the reliability for offset versus direct attach of via structures.

The space- and other high reliability markets are continuously driven towards an increased use of deeply integrated electronics. The increasing demand for complexity and functionality results in the use of large package components with a high number of I/Os. In order to allow the use of components with high pin counts up to 1752, complex high-density interconnect (HDI) printed circuit board (PCB) technology is required. Reconciling the use of multiple laser-drilled microvia levels in a stacked configuration with the reliability requirements for space is challenging when using heritage dielectric materials. The use of ceramic filled low CTE material allows the manufacturing of complex HDI PCB technology with a high reliability.

The work presented in this paper is part of an ongoing European (Horizon 2020) “COMAP-4S” project on components and macro components packaging for space. The Project is coordinated by SAFRAN ELECTRONICS AND DEFENSE with partners ACB, TUB and NANOXPLORE. The most complex PCB technology targeted within the project is four levels of microvias, requiring the use of low CTE laminate material. The reliability of different microvia configurations from all four levels staggered to all stacked was evaluated using test methods as described in ESA’s ECSS-Q-ST-70-60C standard for qualification and procurement of printed circuit boards. The test results of various material-level reliability tests, interconnection stress testing (IST) and reflow simulation combined with rework and traditional thermal cycling are provided to demonstrate a high reliability of the different via configurations and overall PCB technology.

Every layer interconnect (ELIC) printed wiring boards (PWB) were sourced after D-coupon evaluation per IPC-TM-650 2.6.27B. U.S. and off-shore D-coupons were tested to assess reflow survivability, a prerequisite that demonstrates reliable copper interconnects. Tested chains mirrored configurations designed into the actual 12-layer ELIC PWBs including 11-stacks. The structures, test results, and reflow simulation models are discussed. Evidence of the weak microvia interface was presented to the IPC Technical Activities Executive Committee early 2017. Resistance-temperature measurements recorded during convection reflow assembly identified stacked microvia open-reconnects. Reflow tested daisy chains were cross-sectioned, viewed by optical microscopy, scanning electron microscopes (SEM), and focused ion beams (FIB). Copper fractures were located primarily between the target pad and electrolytic copper-fill. Stacked microvias which fractured during reflow imitated mechanical switches. Opens triggered generally above 215C, remained open, and then reconnected when cooled below 215C. Fractured microvias deceptively appear to function normally, are undetectable at ambient temperatures by in-circuit-tests (ICT), conventional air-to-air (ATA) temperature shock or cycle (TS/TC), and therefore introduce a hidden reliability concern when deployed.

Fractured microvias are simply physical contacts, not metallurgical bonds. Unlike stacked structures, staggered configurations formed using the same process withstood the reflow thermomechanical strain. Layer-to-layer staggered microvia paths provided a reliable high-density interconnect (HDI) alternative to stacked structures as long as there was room to fit the configuration. This provided a temporary window, a brief opportunity, to solve the weak stack dilemma that, unfortunately, was squandered and is now closed. Testing to date suggested U.S. suppliers, in general, are not prepared to supply ELIC constructions with greater than 3-stack microvias, let alone 11-stack microvias, that consistently survive reflow. Stacked microvias with weak copper interfaces are subject to reflow-induced fracture which, therefore, precludes their use in mission-critical, high-hazard environments. Consequently, the U.S. supply base is at a technology disadvantage plagued with issues sourcing ITAR compliant HDI PWBs necessary to address circuit congestion set in motion by increased silicon content and smaller footprints.

This paper presents two key aspects of reliability of ball grid array (BGA) technologies: one aspect for high-reliability and the other for commercial applications. For high reliability, it presents the effect of isothermal aging at the BGA part level to address burn-in requirements for die screening per Mil standard specification. Pull and shear behavior of BGAs with tin-lead and SAC305 solder balls were characterized for a number of components before and after two burn-in conditions (125℃ and 150℃ for 240 hr). The burn-in and fresh BGAs were assembled onto PCBs and then subject to thermal cycling (TC) between –40℃ and 105℃ to determine cycles-to-failures (CTFs) for comparison. Pull and shear strength test results of tin-lead and SAC305 solder balls were presented for CABGA208, CTBGA228, and CVBGA360 with 0.8, 0.5, and 0.4 mm pitches. CTFs in Weibull plots were presented for the SnPb CABGA208 for fresh and burn-in conditions.

The commercial aspect covered low temperature solder ball and solder paste for assembling of BGAs and other components. We evaluated two types of low temperature BiSn alloys with Cu/Ni or Ag additives and with melting temperatures between 139˚C solidus and 174˚C liquidus. The mixed hybrid configuration utilized SAC305 BGAs using SnBiAg alloy solder paste. The low temperature assemblies were subjected to two TCs (–40℃/105℃ and 0℃/105℃). CTF results were presented for CABGA208 with failure analysis performed by X-sectional evaluation.

Increasingly larger surface mount components are being developed in today’s SMT industry. With increasing footprints, maintaining acceptable warpage levels through reflow and/or real-world use is a growing challenge. Undoubtedly, efforts are made to mitigate warpage in both PCB and components. However, there are limits to these mitigation effects and they do not resolve sample to sample variation. Here the question is posed, what if 100% of components and corresponding PCB attach areas are measured for flatness prior to assembly? Could pick-and-place machines selectively pick a “best” matching component from those available, to place on the next PCB that comes down the line?

This paper and corresponding study only lays the foundation to answer these posed questions. It is hypothesized that matching shapes at room temperature based on minimizing gap between attaching surfaces is not the optimal way to make PCB to component matching decisions. Instead, it is suggested that predicting what these shapes will be at critical points in the reflow/reliability profile is the more critical shape matching to consider. In this study, a sampling of matching footprints of PCBs and components are measured under reflow temperatures via common full-field optical metrology techniques. Critical assembly temperatures are analyzed looking to optimize which component should go with which PCB by analyzing all possible combinations through software automation. Hypothetically, this data can then correlate back to room temperature shape combinations for the best overall surface mount reliability.

Raytheon Missiles and Defense, formed a L-Lead (Pb) to LF-Lead-free focus group to understand the metallurgical and manufacturing challenges to transition to a “pure” Pb-free metallurgical system for SMT surface mount technology CCA-circuit card assembly. To better prepare for this transition, a number of studies were initiated to evaluate baseline and next generation lead-free solder pastes. One such study used a temperature programmable heated or “hot-stage” installed on a current X-ray machine for real-time radiographic analysis under a simulated SMT reflow thermal profile. Representative PCB / CCA samples were solder printed with baseline and candidate LF solder paste alloys with and without BTC- bottom terminated components (QFN or MLF 100) devices, then transferred to the X-ray hot stage, which was programmed to run under the appropriate SMT Reflow thermal profile. During the hot stage reflow process, the devices and solder pastes were X-ray-video recorded with void density measurements made after reflow. Real-time radiographic imaging of hidden solder pastes under MLF 100s revealed solder joint and voiding formation mechanisms that differed from each of the various solder pastes evaluated. Radiographic results also demonstrated that solder paste wetting behavior correlated to both air + process void formation mechanisms. Moreover, a strong statistical correlation was observed between low void density versus high shock performance of the same baseline and LF solder paste alloy candidates evaluated in another parallel mechanical testing study.

Mainstream manufacturing testing strategies involve structural tests including optical inspection, structural defect finding, such as opens, shorts, missing and catastrophically defective components. This strategy, however, still leaves a great deal of potential faults, especially if the unit under test (UUT) is operating at high speed. Typically, high-speed testing, if performed at all, is left for a later stage of assembly. For example, while the structural tests can be performed on a panel containing several circuit boards, functional testing at high-speed is done only after the boards are separated, powered up and low-speed functional tests have passed. Moving high-speed testing up to an earlier test stage can save the costs of functional tests later. In many cases, if the high-speed controller is found to be faulty, it can be replaced during the structural test stage. This paper investigates ways that manufacturing defect analyzers (MDAs), in-circuit testers (ICTs), functional board testers (FBTs), and system level test (ST) automatic test equipment (ATE) can be augmented by a high-speed bus tester (HSIO) to provide at-speed tests in parallel with structural test. The article will discuss how this capability can be integrated into existing manufacturing test stages and examine the economic benefits of such an approach. It will also demonstrate the economic benefits of bringing high-speed test into the board test rather than perform those tests as part of the system level testing.