OEMs and CMs designing and building electronic assemblies for high reliability applications are typically faced with a decision to clean or not to clean the assembly. If ionic residues remain on the substrate surface, potential failure mechanisms, including dendritic growth by electrochemical migration reaction and leakage current, may result. These failures have been well documented.

If a decision to clean substrates is made, there are numerous cleaning process options available. For defluxing applications, the most common systems are spray-in-air, employing either batch or inline cleaning equipment and an engineered aqueous based cleaning agent.

Regardless of the type of cleaning process adopted, effective cleaning of post solder residue requires chemical, thermal and mechanical energies. The chemical energy is derived from the engineered cleaning agent; the thermal energy from the increased temperature of the cleaning agent, and the mechanical energy from the pump system employed within the cleaning equipment. The pump system, which includes spray pressure, spray bar configuration and nozzle selection, is optimized for the specific process to create an efficient cleaning system.

As board density has increased and component standoff heights have decreased, cleaning processes are steadily challenged. Over time, cleaning agent formulations have advanced to match new solder paste developments, spray system configurations have improved, and wash temperatures(thermal energy) have been limited to a maximum of 160ºF. In most cases, this is due to thermal limitations of the materials used to build the polymer-based cleaning equipment. Building equipment out of stainless steel is an option, but one that may be cost prohibitive.

Given the maximum allowable wash temperature, difficult cleaning applications are met by increasing the wash exposure time; including reducing the conveyor speed of inline cleaners or extending wash time in batch cleaners. Although this yields effective cleaning results, process productivity may be compromised.

However, high temperature resistant polymer materials, capable of withstanding a 180°F wash temperature, are now available and can be used in cleaning equipment builds. For this study, the authors explored the potential for increasing cleaning process efficiency as a result of an increase in thermal energy due to the use of higher wash temperature. The cleaning equipment selected was an inline cleaner built with high temperature resistant polymer material.

For the analysis, standard substrates were used. These were populated with numerous low standoff chip cap components and soldered with both no-clean tin-lead and lead-free solder pastes. Two aqueous based cleaning agents were selected, and multiple wash temperatures and wash exposure times were evaluated.

Cleanliness assessments were made through visual analysis of under-component inspection, as well as localized extraction and Ion Chromatography in accordance with current IPC standards.

Keywords: Aqueous based cleaning process, PCB defluxing, wash system cleaning parameters, thermal energy, chemical energy, mechanical energy.

Electronic assembly processes range from simple to complex and involve a wide range of materials to produce. Each step in the process and each assembly material used will have some impact on the assembly, most affecting the cleaning process window. The first step in developing the cleaning process is to determine the solubility parameters of the residues present post soldering with available cleaning agents. The first step quantifies wash concentration, wash temperature and wash time. The second step determines cleaning energy required to deliver the cleaning agent under low profile leadless and bottom terminated components. The second step determines optimal impingement sources to create a flow to wet and dissolve residue under component bodies. The third step is to test rinsing effects. The third step tests for the presence of cleaning agent and ionic residues left under component bodies. The fourth step is to check materials compatibility with the cleaning agent and process. The fourth step provides the process engineer with awareness of material compatibility risks when cleaning electronic hardware. The fifth step is to validate the acceptability of electronic assemblies using both electrical and chemical test methods. The purpose of this research paper is to determine the feasibility of this five-step method for developing the cleaning process window followed by validating acceptability of the electronic assembly.

Since the 1970s, ROSE testing was used to determine “clean enough.” In 2015, the J-STD-001 committee assigned a team to develop the next generation of “cleanliness” requirements. Section 8 defines the key concepts that drove the need for developing new cleanliness requirements.

•ROSE testing for product acceptance (pass-fail) is an obsolete practice for determining acceptably clean

•ROSE testing for process control is perfectly acceptable, but the numbers have to MEAN something. And those values need to be scientifically/statistically determined

•No one set value defines the line between acceptably clean and unacceptably dirty

•No one method determines acceptably clean and unacceptably dirty

A qualified manufacturing process should be determined using some form of temperature-humidity-bias sort of testing (such as SIR). Qualifying a manufacturing process through chemical analysis alone (e.g., ion chromatography) does not tell you the effects of the residue under humid conditions, which is where electrochemical failures occur. Companies that have come up with ionic standards by IC also use temperature-humidity-bias (THB) testing somewhere in their qualification process.

The purpose of this paper is two-fold: 1. Research on the development of temperature-humidity-bias instrumentation and test board designs for product acceptance. 2. Follow on DOE to better understand the impact of cleanliness at the bottom termination of the QFN/BTC component.

A nano-Cu pressureless sinterable paste was developed for joint formation applications. The material was further engineered in both chemistry and process for enhanced oxidation resistance. This oxidation resistance capability allowed the paste to exhibit a shelf life of 6 months when stored at -10°C. The paste was insensitive to either sintering atmosphere or surface finish type, and consistently exhibited a joint shear strength around 13 MPa. Nano-Ag pastes were observed to be sensitive to surface finishes and performed particularly poorly for OSP-OSP system. The shear strength of nano-Cu was sensitive to sintering temperature and time, with shear strength increased from 220°C/15 min, 220°C/30 min, to 240°C/15 min. The joint shear strength deteriorated with increasing TCT (-40°C/150°C) cycle number. After 3000 cycles, the ranking of shear strength can be shown below: Nano-Cu > Ag-N > Ag-H2 > Ag-H1 > Ag-K > Ag-D. The high Pb joint performed the best. The microstructure of Nano-Cu joint formed at 240°C/15 min sintering condition indicated a continuous sintering progress with increasing TCT cycles, with porosity diminishing and smooth area increasing. This strongly suggests that either a higher temperature or a longer time than 240°C/15 min is desired for improved sintering extent. Further homogenization at paste manufacturing process also resulted in significant improvement in joint strength.

Key Words: Nano Cu, paste, pressureless, sintering, die attach, high power

With the advancement in miniaturization, the die is getting thinner and the solder balls are getting smaller for BGAs. Consequently, the thermal warpage is getting more severe due to the mismatch of the coefficients of thermal expansion between die and molding compound and the decreased stiffness of thinner miniaturized packages. Increased warpage can result in non-wet-opens(NWOs) during BGA assembly. NWO has been ailing the industry for a long time, and costly rework is required to remove the problem. In this study, a “cold-welding barrier” method has been developed to suppress NWO.  At BGA assembly, after printing solder paste onto PCB, the BGA balls are dipped into creamy flux or solder paste prior to placing onto paste printed. This flux pickup effectively suppresses NWO by serving as cold-welding barrier. The efficiency of different material and process variations for suppressing the occurrence of NWO defects have been investigated by using dummy BGA components and PCBs. The results demonstrate that effective NWO defect suppression can be achieved using the proposed method.

Keywords: BGA, non-wet-open, NWO, thermal warpage, solder paste, creamy flux, solid flux coating, cold-welding-barrier

Solder powder size is a popular topic in the electronics industry due to the continuing trend of miniaturization of electronics. The question commonly asked is “when we should switch from Type 3 to a smaller solder powder?” Solder powder size is usually chosen based on the printing requirements for the solder paste. It is common practice to use IPC Type 4 or 5 solder powders for stencil designs that include area ratios below the recommended IPC limit of 0.66. The effects of solder powder size on printability of solder paste have been well documented.

The size of the solder powder affects the performance of the solder paste in other ways. Shelf life, stencil life, reflow performance, voiding behavior, and reactivity / stability are all affected by solder powder size. Testing was conducted to measure each of these solders paste performance attributes for IPC Type 3, Type 4, Type 5 and Type 6 SAC305 solder powders in both water-soluble and no clean solder pastes. The performance data for each size of solder powder in each solder paste flux was quantified and summarized. Guidance for choosing the optimal size of solder powder is given based on the results of this study.

Key words: solder powder size, solder paste performance, solder paste printing, reflow, voiding, solder paste stability

Interconnect reliability especially in BGA solder joints and compliant pins are subjected to design parameters which are very critical to ensure product performance at pre-defined shipping condition and user environment. Plating thickness of compliant pin and damping mechanism of electronic system design are key successful factors for this purpose. In additional transportation and material handling process of a computer server system will be affected by shock under certain conditions. Many accessories devices in the server computer system tend to become loose resulting in poor contact or solder intermittent interconnect problems due to the shock load from the transportation and material handling processes.

In this paper, design variables such as pin hard-Au plating thickness, mother board locking mechanism and damping structure design are experimented and reviewed. Also a shock measurement device is used to real-time monitor the acceleration, duration and direction of shock in large stationary or moving systems in transportation and transferring process. Two transportation routes from Fushan China to Sezimovo Czech Republic through China and Russia border by train then return by sea cargo through Mediterranean, Arabic and South China Seas in which a product package is embedded with the shock measurement device. The collected force data of g force can be used to calculate the shock energy level ΔV. The comparison between the value of ΔV and shock energy tested in the lab can be used to judge whether a system design can sustain and cause contact interconnect problem in transportation and transferring process. These design variables and stresses can be evaluated by drop test or vibration test to ensure system functional integrity is achieved.

Increased demand for radar capabilities has required increasingly more efficient thermal management techniques. Simple cold plates using tubes or channels are frequently used to cool high power components but restrictions on size, weight, and power have restricted their effectiveness. This project analyzed several thermal management approaches and used additive manufacturing to dramatically increase performance with materials and structures that could not be fabricated using traditional methods.

Conventional cooling methods flow coolant parallel to the heat source surface in a serpentine path under components. In parallel flow, a thermal gradient develops across the channel which diminishes the heat transfer effectiveness across the channel width. In addition, the coolant temperature increases as it removes heat from each component, becoming less effective as it progresses.

Using multiphysics simulation software, a cold plate design using manifolds and microchannels was designed to maximize cooling efficiency by dividing the coolant into several parallel paths and directing the flow into microchannels at a direction perpendicular to the heated surface. After a short length through the microchannels, the coolant returns to an adjacent manifold and exits the plate. By increasing the surface area and minimizing the microchannel path length, very large heat transfer coefficients were obtained with a corresponding low cold plate thermal resistance (<0.05 [in2-°C/W], normalized for area).

Typically, microchannel cold plates have been expensive to manufacture requiring precision machining, solder, and brazing. Using additive manufacturing, cold plates have been made with direct metal laser sintering using copper, aluminum, and titanium. By designing to the tolerances of the additive process and using guidelines to eliminate internal support structures, low thermal resistance cold plates have been produced and tested.

Using simulation, the optimum microchannel spacing, coolant flow rate, and manifold sizing have been determined to maximize heat removal and minimize pressure drop. Thin cold plates (<4 mm thick) can be designed to achieve two-sided cooling in high power radar applications. This advanced manufacturing process can place the highest cooling power at the hottest components to mitigate thermal issues in current and future platforms.

With an increase in reliability and performance, reduced weight and cost, adaptive sizing, and the ability to respond to increased power demands, thermal upgrades can be designed, manufactured, and validated in months instead of years.

Embedded components technology has launched its implementation in volume products demanding high levels of miniaturization. Small modules with embedded dies and passive components on the top side are mounted in hand held devices. Smartphones have been the enablers for this new technology using the capabilities of embedded components. With this technological background another business field became interesting for embedded components – the embedded power electronics. The roadmap of the automotive industry shows a clear demand for miniaturized power electronic applications. Drivers are the regulations for the international fleet emissions which are focusing on three major trends.

The first trend will be higher efficiency of “classic” internal combustion engines, the second will be the efficiency of body application and the last trend will be the electrification of the drive train. For all targets a huge potential for embedded power electronics is visible. A European project was launched in 2013 as a development project for new power packages and power modules using die embedding technology. For the realization of this technology, copper termination on MOSFETs, IGBTs and power diodes is necessary. Equipment and processes have been developed in the supply chain to support the development of power modules ranging from 500 W to 50 kW. For enhanced thermal performance of power modules, a concept for double sided cooling has been developed. Beside full area contacting the MOSFET on the drain side with copper and contacting the embedded components in a power core with an isolated metal substrate with silver sinter paste, a very effective concept for reducing thermal resistances and stray inductance was shown. With the realization of the 500W demonstrator the proof of concept was realized. This paper will focus on the behavior of the power module for operational conditions of a PedEleC (Pedal Electric Cycle) application. The full reliability evaluation (thermal shock test, reflow test and power cycling) and switching behavior was shown. Furthermore, the thermal properties of the PedEleC power module were verified with the results from thermal simulation. In addition, more detailed investigation of the electrical properties before and after reliability treatment of a single chip  test vehicle was shown.

Keywords: power embedding, PCB technology, miniaturization, automotive requirement, electro mobility