Pastes, deposition

Edwards R, Sedwick PN, Morgan V, Boutron CF, Hong S (1998) Iron in ice cores from Law Dome, east Antarctica implications for past deposition of aerosol iron. Ann Glaciol 27 365-370 Filer JM (2001) Oxygen isotope variations of basaltic lavas and upper mantle rocks. Rev Mineral Geochem 43 319-364... 

Grotaers, L. D., 1995, The Buchan Caves Gravels implications for past depositional environment, B.A. (Honours) thesis, Deakin University, Geelong, Australia (unpublished). 

Sediments and biota remain priorities for future CP monitoring according to risk assessments. Analysis of dated sediment cores from Europe and from Asia would help assess the current and past deposition in aquatic environments. Temporal trends of CPs in biota need to be studied and there are a number of wildlife and human tissue banks that could supply suitable samples. While there has been much focus on levels in biota in aquatic environments, more work is needed on levels in terrestrial biota to follow up the early work that demonstrated high concentrations in herbivores. This should include more measurements of human tissue samples (blood, mother s milk) given the potential for human exposure via house dust, vegetation, and meat of herbivorous animals. 

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. 

Deposition of conducting salt on glassy carbon or mixed with carbon paste Deposition of conducting salt on glassy carbon or mixed with carbon paste... 

Accuracy of solder paste deposition location and deposition volume... 

Solder paste deposition problems for example, wrong volume or location... 

Syringes older paste deposition system takes translated Gerber file and dispenses appropriate amount at each pad centroid. 

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. 

Archimedes screw pump dispensing utilizes an Archimedes screw to push the adhesive or solder paste out of a nozzle (see Fig. 40.18.) The speed and duration of a turn, as well as the size of the opening, determine the quantity of adhesive or paste deposited on the site. As in the case of the time-pressure technique, different deposit quantities can be realized by multiple spindle (heads) or a computer program that alters the screw speed or turn duration to change the amount of material dispensed from the same orifice. All other considerations with respect to the roles of adhesive or solder paste viscosity and shelf-life also apply to this technique. 

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. 

Coplanarity. Coplanarity of component leads, BGA balls, or CCGA columns is a must. All component leads should share a common seating plane within the specified tolerances associated with the component vendor s specification. The coplanarity requirement also depends on the dispensed solder paste height and paste deposit uniformity, as the solder paste accommodates some small differences in lead-to-lead coplanarity both as a paste and also as the solder assumes the characteristic dome shape upon melting. 

Solder Paste Deposition for Pin-in-Paste Soldering. To prepare a board for pin-in-paste soldering, you deposit generous amounts of solder paste over or adjacent to the circuit board s targeted PTH sites. This is done during solder-paste stenciling in preparation for SMD component placement and reflow (see Fig. 47.15). 

FIGURE 47.15 Solder paste stenciling in preparation for SMD component placement (a) a PWB cross-section prior to solder paste deposition (b) the same PWB after solder paste deposition. Note that SMD pads and PTHs have received solder paste deposits. In the case of (b), paste has been forced into the PTH by the squeegee during solder paste printing. 

Of course, stencil apertures have to be created in the surface-mount stencil to accommodate paste deposition on PTH sites for pin-in-paste soldering. If solder volume is not a concern, it is desirable not to dispense paste into the FTH.This is discussed later in this chapter. 

FIGURE 47.17 Ao, Bq, Q, and Do are solder paste apertures cut into a conventional stainless steel stencil. Apertures Bq and Do are designed to prevent solder paste from being forced into through holes. Corresponding paste deposits are indicated as Ai, lii, Ci, and Dj. C,JCi and Ao/Ai are usual round and square apertures and deposits, and solder paste is allowed to enter corresponding PTHs. 

Automated 3-D inspection of solder paste depositions has the following major advantages ... 

It is sometimes too slow to cover aU paste depositions on an assembly. 

Automated optical inspection systems also image only a small portion, or view, of the assembly at a time. These systems normally can use somewhat bigger views than the 3-D solder paste systems because the features being extracted often do not require as much magnification as do measurements of solder paste depositions. However, inspection of components, such as 0402, 0201, and 01005 passive components, or very-fme-pitch deposits, can require the same level of magnification and therefore views as small as the 3-D solder paste inspection systems. 

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