The industry has been openly discussing the concern about weak microvia interfaces after IR reflow and the potential for an undetected open or latent defect that can escape after expensive components have been soldered to the board. A specific concern is for the reliability of stacked microvia designs on very complex panels that are often built-in low volumes. This type of build is typical of American and European OEMs who are using large and expensive BGA components in mission critical electronics. Due to the limited number of units made, this board segment of the industry is more vulnerable to weak interface failure than the HDI boards for mobile devices that are made with high levels of automation in mass production by fabricators in Asia. Further complicating the board design impact, the metallization process that is used can have very different reliability performance from different lines in different regions.

The goal for the metallization process is to form a continuous metallurgical structure to withstand the thermo-mechanical stress of IR reflow during assembly. The best condition consists of epitaxial growth of a thin electroless copper deposit on the target pad with a grain structure that recrystallizes with temperature and becomes indistinguishable from the target pad and electrolytic copper structures. There are multiple factors that influence the ability to form this recrystallized structure, which in turn affects the strength of the microvia interface. These include the circuit design, laminate material selection, type and settings of laser via formation, post-laser conditioning of the target pad copper, the desmear and electroless copper process processes, and the electrolytic copper via fill plating processes.

Through extensive auditing as a supplier of primary metallization and electrolytic copper via fill chemistries, and cooperative work with PCB fabricator customers to improve microvia reliability, a wide range of studies were conducted. Presented in this paper are potential areas of concern for microvia reliability with a specific focus on metallization processes and the factors stated above as well as testing on improvements. The approach taken includes low level DOE testing for process improvement as measured by a test panel using IPC TM-650 2.6.26A and TM-650 2.6.27, otherwise known as IST and simulated IR reflow testing. Experience in failure analysis techniques, limitations on some commonly utilized inspection methods, and a review of overall best practices for plating are also discussed.

Testing was augmented with SEM, FIB, and broad-beam Ion Milling techniques to evaluate various the various structures. Current induced or air to air thermal cycling were utilized to determine the level of microvia survivability and judge process improvements.

The electroless copper deposit must form a metallurgical bond between the target pad and electrolytic copper deposit to survive reflow assembly. 4-wire resistance measurements conducted on PWBs and coupons during reflow assembly revealed that thermal excursions fractured microvias. Cross-section analysis subsequently located fractures at and in the vicinity of the electroless copper deposit. Fortunately, IPC test method 2.6.27 allows the industry to contain the problem while investigations proceed to identify the causes and solutions. Meanwhile the utility of stacked microvias and its deployment is tempered by the discovery of the weak interface and its susceptibility to fracture induced during reflow thermal excursions. The weak interface is a hidden reliability threat that is amplified by the proliferation of stacked microvia features acknowledged as too fragile for deployment in high hazard environments. This paper presents the results of an ongoing study that assesses the electroless copper deposit in microvias prior to electrolytic plating. Resistance measurements were completed on daisy-chained microvias fabricated using established production processes through electroless copper, skipping the electrolytic copper process. Measurements completed on the independent L1L2 and L3L4 microvia daisy-chains revealed variation in chain resistance attributed to the integrity of the electroless copper deposit in the microvia. Resistance measurement provided chain continuity assessment that is impossible by weight-gain and backlight evaluation. Published electroless copper deposit thickness ranges from 0.3 to 3.0 µm for immersion times of 4 to 30 minutes. Calculated resistance for 0.3 to 1.0 µm thickness ranges from 14.3 to 44.3 ohm for chains containing 1300 microvias of 75 or 127 µm diameters and 75 µm deep. Measured chain resistance ranged from 12.00 ohm to electrically open defined as 9.99 x 108 ohm, undetected by weight gain, backlight, and cross-section analysis. Variation in chain resistance implied deposit variation bringing the process into question because microvia fractures were associated with the electroless copper deposit. Deposit variation confounds metallurgical requirements. Sufficient for subsequent plating as noted in IPC-6012E 3.2.6.1 is vague and sets low expectations for deposit performance. No specification exists that covers the visual assessment of cross-sections adequately that predicts microvia reflow survivability. This is why the myriad efforts devoted to visual assessments fail to address the problem. No method focuses on the influence of the electroless copper deposit. The ultimate goal is to eliminate PWBs as the source of field-failures while the immediate task is to address the need for stacked microvias, therefore measured resistance is recommended to baseline electroless copper processes with respect to completing the path to least resistance necessary for subsequent plating.

Documentation shows that there are latent reliability issues with stacked filled Microvia designs for complex printed circuit boards. This issue is broadly defined as a weak interface between the plated copper and the blind via the target pad. When thermally stressed, the generally weak interface fractures (1). This is manifested especially during forced convection assembly reflow. While many of the studies of Microvia Interfacial fracture focused on conventional electroless copper as the PTH choice, there have been no recent studies that compared Direct Metallization to Electroless Copper.

Summit Interconnect and RBP Chemical Technology started a joint project to provide additional insight into the weak interface defect. The boards were processed in either a graphite-based direct plate or conventional electroless copper using specially designed test vehicles. The testing protocol followed IPC-TM-650 test method 2.6.27B as the method of choice along with OM Testing. IPC D coupons are populated on the test vehicle with 3, 4, 5, 6, and 8 mil diameters blind vias and two different dielectric thicknesses.

A second objective of the project related to the reliability of the blind vias as diameters decreased. If smaller diameter vias could be manufactured with increased interfacial strength, this would allow for micro BGAs with smaller diameter Microvias and increased space for routing density. A third objective is to compare the various diameter blind vias' reliability and two different aspect ratios with test vehicles constructed with spread glass weave prepreg and standard weave glass. The final objective is to determine if two other hole preparation processes before Metallization influenced the thermal reliability of the blinds vias.

The Company has developed and industrialized an advanced technology of plastic-integrated structural electronics. Key benefits are 3-dimensional shapes, reduced thickness and weight as well as simplified assembly. The benefits are especially suited for automotive interior use cases. Automotive OEMs have stringent reliability requirements and product validation is made accordingly. Therefore, the Company has technology verification process to ensure that electric components and materials, such as polymer substrates, functional inks and surface mounting adhesives, form a reliable solution. This paper describes motivation for selecting specific reliability tests. It presents also example cases from technology verification and automotive product validation.

The automotive industry is experiencing significant change in design and performance expectations as it moves to the future. Synonymous with high reliability in harsh conditions, today the automotive industry is also being linked to advanced electronics. The market is learning how to incorporate function originally created for other industries such as high speed, high processing power and even the use of radio frequencies into a robust vehicle.

Current automotive mega trends require the use of electronics, many of which are sophisticated in design and function. This includes hardware to support Connectivity, Autonomy, Shared ride services, and Electrification (CASE). Certain performance characteristics have been adopted from consumer, telecommunications and aerospace markets which include a wide range of performance characteristics and design considerations. The automotive industry once used less complex electronic systems now must adopt this wide range to be successful. The semiconductor packages, circuit boards, and assembly techniques will support traditional systems, established for automotive decades ago through those currently used in the handheld and telecommunications markets. Understanding material performance, durability and consistency has become extremely important.

The IPC has taken steps to specify test requirements specific to the automotive industry through the IPC 6012DA Automotive Addendum. In addition, material suppliers and fabricators understand that the use of established, proven processes is important to the automotive supply chain. There are a host of process considerations and performance characteristics to investigate on the path to creating vehicles of the future.

When looking at the wide range of performance expected from the four mega trends, it is easy to understand that the required electronics and the expectations from those designs will also span a significant range. This paper will investigate all aspects of the electronic build. It starts with how increased performance influences the semiconductor packages, how that then affects design for PCB fabrication and finally the influence on assembly materials to join all pieces together.

Specifically, it will explore how, as more processing and performance is required of the package, the need for highly robust interconnect features to deal with miniaturization, signal routing, and increased thermal dissipation becomes greater. It will propose one chemical process set for the filling of blind vias that includes capabilities from large via sizes for robust construction and will also satisfy high density construction as designs shrink. Lastly, it will illustrate how solder alloy type and flux chemistry will provide robust reliable solder connections for the full range of automotive needs.

Electrical components in a car are predicted to double within the next years. Increased functionality per PCB, miniaturization and higher power density are coming in parallel with this trend. Keeping or increasing the reliability of assembled PCBs or other substrates (e.g. DCBs–Direct Copper Bounded substrates will be even more critical due to higher ambient temperature, more functionality and so on. At the same time, there are many factors influencing the reliability of electrical devices. This paper/presentation will show how the size of components can influence the lifetime of a PCB assembly (PCBA). Furthermore, the interaction of component size, pad geometry, solder alloy and PCB surface. The base of the study is a reliability experiment with 14 different solder alloys(13 lead free solders and a Sn63Pb37alloy as a reference), six different surfaces, six different SMD-chips, combined with three different pad layouts. In total,208 test boards with 37440 solder joints were produced. A thermal shock reliability test was used. The time to failure was evaluated with the Weibull analysis, followed by an analysis of variance. In addition to that investigation, it will be shown, how the critical factors of reliability can change due to different properties of soft solder alloys. For the solder alloy and the interconnection, itself, there are some fatigue life advantages when using Innolot(Sn, Ag, Cu, Bi, Sb, Ni [1-3]), especially for thermal cycles -40/+150°C. On the other hand, by using Innolot there is a relatively high thermo-mechanical stress induced which can create ceramics defects at the passive components. The paper will compare and discuss the differences of the alloy-based stress in ceramic components due to passive cycle tests, real customer tests and stress analysis based on finite element method. As an outlook it will be discussed about the influence of different solder alloys to the reliability at different temperatures as well as differences of lifetime at different types of substrates.

The corrosion mechanism in the aqueous cleaning process is poorly understood. Much of the prior work is theoretical, based solely on Pourbaix Diagrams. These diagrams are based on the thermodynamic properties of the metal and select salts in a hypothetical situation. The issue with the thermodynamic approach is that it makes no prediction about the rate (or kinetics) of the system. Just because thermodynamics says a reaction is possible, it does not tell if it will occur, there can be constraints on the system which limit the extent to which a chemical reaction, such as corrosion, can come to equilibrium. Furthermore, the cleaning process is a dynamic one, which generally precludes equilibrium.

The cleaning process introduces additional effects that would not be accounted for in Pourbaix diagrams. The typical spray in air cleaning process adds fluid flow, aeration, different metals, or alloys, which may change based on the assembly mix, changing cleaning agent concentrations, introduction of soils, changes in pH. All of which may upset the proverbial thermodynamic apple cart.

This study attempts to further investigate the reality of corrosion in the cleaning process. Comparative electrochemical studies will be undertaken between two types of aqueous cleaning agents. One of which has a pH closer to 7,and one is engineered to have a robust corrosion inhibition package. Mechanisms and key corrosion parameters will be compared.

Today, printed circuit boards used within electronic assemblies for high reliability applications are typically subjected to cleaning or defluxing processes. As assembly complexity has increased, that is, more densely populated with greater use of stacked and leadless components and with ever reducing standoff heights, effective defluxing is increasingly challenged.

Copper traces and pads are integral to PCB design. In order to protect these from corrosion and oxidation, the PCB is covered by a solder mask. This prevents performance degradation by providing a barrier between soldered joints and other conductive elements on the PCB. As detailed in IPC SM-840D, solder mask materials applied to the printed board substrate shall prevent and/or minimize the formation and adherence of solder balls, solder bridging, solder build-up and physical damage to the printed board substrate.

The solder mask material shall help impede electromigration and other forms of detrimental or conductive growth [1]. The solder mask is necessary for long term reliability of PCBs, but can its presence also impact cleaning process effectiveness? When incorporating a solder mask, the designer can specify the solder mask as either Solder Mask Defined (SMD), Non-Solder Mask Defined (NSMD) or No Solder Mask (NoSM). Although there are design considerations for using either solder mask approach depending upon component details, in general with SMD and NSMD, the component standoff height is slightly less when compared with NoSM which could impact the cleaning process effectiveness.

For this study, the authors wanted to assess the impact of different solder mask options on under component cleanliness. The solder mask specification for the substrates used in this study included SMD and NSMD as well as NoSM for comparative purposes. The solder mask used on the test vehicles employed for this study was liquid photo-imageable (LPSM or LPI) solder mask. The test vehicles were populated with numerous chip cap components with four solder paste types: no-clean tinlead solder paste (old generation), no-clean tin-lead solder paste (new generation), no-clean lead-free solder paste (old generation) and no-clean lead-free solder paste (new generation).

All test vehicles were cleaned in a spray-in-air (SIA) inline process utilizing two different water-based engineered cleaning agents, one alkaline and the other pH Neutral. Additional variables considered were wash exposure time and wash temperature. Thus, for each solder paste used, variables included solder mask type, cleaning agent type, wash exposure time, and wash temperature. The test plan employed full factorial analysis.

Cleanliness assessment was conducted by visual inspection per IPC TM650. All components were mechanically removed from the test vehicle thereby enabling thorough under-component inspection. Localized extraction and Ion Chromatography analyses were also conducted in accordance with current IPC standards. Keywords Bottom Terminated Components (BTC), PCB defluxing, solder mask, failure mechanisms, component standoff.

Recently there has been an increase in the evaluation of low temperature lead-free soldering materials, such as tin-bismuth, in a process known as “hybrid assembly” in which higher temperature SnAgCu BGA/CSP components are assembled using low temperature tin-bismuth based pasts at typical tin-bismuth paste reflow soldering temperatures. One of the concerns in terms of assembly and reliability is the mixing of the high and low temperature alloys and its influence on reliability. A study was done to compare a series of tin-bismuthbased solder pastes (Sn58Bi, Sn57.6Bi0.4Ag, Alloy A, Alloy B, Alloy C, Alloy D)assembled with higher temperature SnAgCu BGA daisy-chained components and SnAgCu paste with SnAgCu BGA components in terms of thermal cycle reliability testing from -40°C to +125°C. Thereliability test vehicles (RTV) were thermally cycled from -40°C to +125°C until failure up to 3,000 thermal cycles for the tin-bismuth paste assembled boards and until failure up to 5,500 thermal cycles for the SnAgCu paste assembled boards.

The tin-bismuth pastes assembled with SnAgCu BGA components showed 1stfails between 1,500 and 2,000 cycles. By 3,000 cycles most of the tin-bismuth paste test board components had full fails or partial fails. In comparison the 1stfails with SnAgCu paste assembled with the SnAgCu BGA components did not occur until 2,500 cycles with some of the SnAgCu paste assembled SnAgCu BGA components not having fails after 5,500 cycles.

A challenge during the failure analysis was that individual BGA solder joints on the RTVs could not be electrically measured to identify the specific solder joints which failed during thermal cycling. The failed thermally cycled test vehicles were, therefore, evaluated using non-destructive CT (computed tomography) inspection to investigate which specific solder joints in the BGA component showed solder joint cracking. Based on the CT inspection analysis, the locations where there was evidence of cracking in the solder joint were cross-sectioned to validate the results of the CT inspection.

There was good correlation between the CT analysis and cross-sectioning in terms of determining solder joints which had cracking from thermal cycling. In a majority of the boards cross-sectioned, 100% cracking in a specific solder joint was identified.  Most of the cracking observed was at the board side of the solder joint in contrast to SnAgCu joints, where the cracking would typically occur at the component side.

Solder joint cracking in the tin-bismuth paste with SnAgCu BGA component boards was found to occur at both the component and board side of the solder joint with most of the cracking at the board side of the solder joint. Solder joints with full fails had 100% cracking at the board side.

Drive towards lead-free electronics began in the early 2000s. Solder pastes based on tin (Sn), copper (Cu), and silver (Ag) were the initial replacement for the traditional SnPb solder. With the SAC alloys, several researches reported that one year of aging consumed more than 50%of the component life. Once the detrimental effects of aging were discovered, the industry started the search for better solder paste materials. The SAC based pastes were made better by adding elements such as Bismuth (Bi), Antimony (Sb), Nickel (Ni). Recently, all the leading manufacturers have introduced new solder materials that claim to have high reliability in harsh environments. Extensive tests are required to filter the best solder pastes. In the study, three high reliability solder materials from leading manufacturers have been selected and used for the test vehicle assembly. SAC305 paste is also included for comparison with the new materials. The test vehicle is a printed circuit board (PCB) of FR-4 laminate material with three CABGA208s (15x15mm) with SAC305 spheres, three LGA36s, and six SM resistors. Three surface finishes, namely electroless Nickel immersion Gold (ENIG), immersion Silver (ImAg), and organic solderability preserve (OSP), have been considered for the study. Immediately after assembly, all boards are aged for a period of twelve months at 125oC. All the boards are then thermally cycled for 5000 cycles from -40oC to +125oC with a ramp time of 50 minutes and dwell times of 15 minutes at high and low temperatures.

Two parameters Weibull analysis is used to quantify the performance of the different alloy materials. ANOVA analysis involving the different composition and surface finish is also done in order to get insight into the most influential factorson the component reliability. Generally, all the new alloys were found to outperform SAC305 paste. Materials with a high content of Bi, Sb, and Ag performed the best in the lot. The microstructure analysis showed that bulk solder failure was the typical failure mode with the crack propagating in bulk along with the intermetallic compound layer on the component side.

KEY WORDS: BGA, Reliability, Thermal Cycling, Surface Finish, Solder Join