Pastes deposition process

The materials used in the surface-mount, area-array assembly process include solder paste, stencils, components, flux, and the PCB. In wave soldering applications, solder paste and paste deposition processes are eUminated, and a cured adhesive is used to retain bottom-side components on a PCB during exposure to a solder wave. 

The process parameters key to the stencil printing process include the squeegee type and hardness, print speed, print pressure, and print gap. An optimal stencil printer setup provides a clean sweep on the stencil surface and a repeatable solder paste deposition process. Shore A scale polyurethane squeegee hardness level between 85 to 95 is appropriate to achieve repeatable solder deposits for lead-free solder pastes. Print parameters such as the print speed, print pressure, and the print gap are adjusted to accommodate the variety of solder paste rheologies available. There are no specific equipment modifications necessary to stencil printer lead-free solder pastes. 

The basics of the paste preparation were explained in Sect. 2.3.3. For the devices presented in this book, the paste was deposited onto cleaned chips using a dropcoating method [48,61]. The deposition was performed by the company Applied-Sensor (Reutlingen, Germany). A metal-wire loop is immersed in the paste and the tin-oxide suspension adhering to the loop forms a droplet, which is accurately positioned in the membrane center. After the drop deposition, the whole chip is put in a belt oven and annealed for 20 min at a temperature of 400 °C. This temperature is close to the elevated-temperature steps at the backend of the CMOS process. Consequently, we never observed a significant difference of the circuitry performance between coated and uncoated chips. The whole deposition process is, therefore, fully CMOS compatible, and no additional on-chip annealing is necessary. 

These trends in lithologic features of the sedimentary rock mass can be a consequence of both evolution of the surface environment of the planet and recycling and post-depositional processes. It has been argued (e.g., Mackenzie, 1975 Veizer, 1988) that both secular and cyclic evolutionary processes have played a role in generating the lithology-age distribution we see today. For the past 1.5-2.0 billion years, the Earth has been in a near present-day steady state, and the temporal distribution of rock types since then has been controlled primarily by... 

However, in the development of chemical and biosensors, new pastes must usually be developed. Polymer thick films can be screen printed on cheap polymer substrates with a thickness anywhere between 5 and 50 /xm. Importantly, no high-temperature steps are involved in the deposition process. The first commercial planar electrochemical glucose sensor (the ExacTech by MediSense) was a screen-printed sensor. A comparison of thick-film deposition against thin-film deposition is shown in table 3.10. 

As with most processes, numerous variables affect the final product. For solder paste deposition using stencils, Chouta and Heck identified 39 variables relating to materials, 32 variables to processes and equipment, and 10 variables to personnel and environment. Among variables critical for the successful stencil dispensing of adhesives are stencil design, the adhesive material and its rheological properties, and the printing process. 

In past few years, metallized plastics have received considerable industrial and academic attention because of their useful applications. They possess the properties of both the polymer and the metal. There are several techniques for the plastic metallization, but these can be divided into three major categories. First is a metal blending process in which the polymer is mixed with a metal powden Second is a metal deposition process in which the pt ymer film is coaled with a thin metallic layer Third is a metal complexation process in which the polymer is chelated with a metal salt Due to reduction of the mechanical properties of the polymer in the first process and the trouble as well as high cost of the second process which often requires several surface treatments to enhance the adhesion between the metal and the polymer metal-polymer chelates, which are often prepared by reacting the solutions of a polymer and a metal salt, have attracted significant interest because of their resulting solubility and easy processability into films and fibers... 

Assembly requirements also place constraints on the circuit board layout. A very high part density requires a large number of apertures in the solder paste stencil, which can cause the stencil to become locally too flimsy to control the solder paste deposit. A surface-mount circuit board with a very wide range of component sizes and package configurations may require multi-thickness stencils to properly control the paste deposit. Solder paste printing quality is a determining factor in solder-joint defects observed after the reflow process. 

Dispensing defects can be detected by visual inspection or automated inspection techniques. Such automated techniques include those based on visible light images as well as laser profilometry that determines the actual volume of the adhesive or solder paste deposit. However, inspection slows the process assembly line. The more joints that are selected for inspection (that is, not aU joints need to be inspected) and the greater the information detail required from of the inspection results (referring to the height profilometry data collection), the longer the delay in the process flow. 

There are a few other materials that need to be covered for this discussion. The first is solder paste. This is a mixture of minute solder beads, flux, and other materials to give it specific rheological characteristics for dispensing and chemical agents for metal surface preparation. For surface-mount applications, it is typically stenciled onto PWB bonding pads, and then the electronic component is placed upon the solder paste deposit. The paste holds the component in place during the reflow process.The second is flux, which, as mentioned previously, is a key component of solder paste. The flux is a heat-activated chemical agent used to clean solder-able surfaces. Both paste and flux will be covered in subsequent sections of this chapter. 

Pre-reflow AOI systems also have the ability to inspect 2-D solder paste however, this ability is utihzed only to inspect a small percentage of the solder paste deposits combined with the component misalignment measurements. Component misalignment measurements cover the passive components, whereas the solder paste measurements cover deposits for BGA, CSP, or fine-pitch QFP devices. Therefore, these systems are placed within production lines after the pick-and-place systems for passive devices but before the pick-and-place systems for the larger area-array and leaded devices. These systems serve the same purpose as those only meant for component placement measurement, both detecting defects and monitoring measurements within control limits to discover process drift as early as possible. 

In the solder paste printing process, defects typically are caused by poor alignment between the substrate and stencil, incorrect material selection (substrate, paste type and stencil design), or variations in the amount of paste deposited. Defect elimination relies on the engineer and operator to address these variables and monitor the process. 

Quenched and tempered AISI D2 cold work tool steel discs (28 mm diameter and 10 mm height) were used as substrate materials. The hardness of die discs were 59 HRC. ZrN and Zr(21% Hf)N coatings were deposited on substrates by a PVD technique after final polishing with a l)jm diamond paste. The process parameters of PVD technique are listed in Table 1. 

Based on the surface and interfacial stracture and an exploitation of potentiometrie titration, mieroeleetrophoretic mobility and maoroseopie adsorption data a modelling of the interfacial deposition process is able to provide an integrated picture of the interfacial chemistry for the deposition process [1,4]. The observed band wavenumbers and stmctural properties for the species deposited on the pastes conform to the previewed [4] respective speciation an exploitation of normalised Raman intensities of the main representative bands due to the duee speeies partieipating in the speciation is also in full conformity with the model prediction [4]. 

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