SMT reflow

Loctite 3594/ Loctite Fluxing (no-flow) underfill NA Device passivations, OSP - copper, copper-nickel-gold Automated pattern dispense Flip-chip-on-board using SMT reflow profiles. Suitable for high-fi-equency applications ( - 2.9 at 100 kHz). 

FIGURE 11.26 Typical thermal profile for SMT reflow soldering (types I or II assemblies). (Source G)x, R.N. 1992. Reflow Technology Handbook Research Inc., Minneapolis, MN. With permission.)... 

Hot air solder leveling (HASL) or reflowed tin on copper bond pads possess a thin intermetallic layer between the applied surface finish and the pads. During surface-mount technology (SMT) reflow, the addition of molten solder on top of this preexisting intermetallic layer will further its growth. Therefore, the TAL should be limited to minimize IMC but sufficient for good solder wetting. 

The through-hole components (axially leaded parts, pin-grid arrays, solder-tail connectors, etc.) are inserted into their respective PTHs before or after surface-mount component placement. Once the surface-mount components and the solder-tail parts are placed, the pastebearing board is then passed through the SMT reflow oven. During the reflow process, the molten solder coalesces around the through-hole pins wetting between the pin and barrel. Surface tension and capillary action draws the solder down the barrel to complete the solder joint. 

PCX withstands SMT (surface mount technology) reflow soldering. 

Figure 5.1 Representation of SMT for (a) wave soldering and b) reflow soldering.

There is a large number of products on the market that are mixtures of water and water-soluble organic solvents. These media are recommended for the removal of solder paste, SMT adhesives and flux residues from stencils, misprints and populated reflowed PCBs."... 

Surface mounting. Surface mounting of components onto PWBs is commonly referred to in the industry as SMT or surface-mount technology. It consists of the attachment of electronic components by automated dispensing or screen printing of solder paste and adhesive onto a PWB, placement of the components, then curing the adhesive and reflowing the... 

Surface Mount Applications. Tin-lead solders (Sn-Pb) are the standard materials used to interconnect electronic components on PCBs. The most common reflow soldering process is SMT which uses tin-lead solder pastes (58-61). The pressure to reduce the industrial use of lead is growing, particularly in Europe, since it poses as a hazard to human health (62). 

It should be noted at this point that there are three distinctly different solder balls referred to in this chapter and in publications discussing SMT. The solder sphere test refers to the ability of a volume of solder to form a ball shape due to its inherent surface tension when reflowed (melted). This ball formation is dependent on minimum oxides on the microscopic metal balls that make up the paste, the second type of solder ball. It is also dependent on the abUity of the flux to reduce the oxides that are present, as well as the ramp up of temperature during the preheat and drying phases of the reflow oven profile. Too steep a time/temperature slope can cause rapid escape of entrapped volatile solvents, resulting in expulsion of small amounts of metal, which will form undesirable solder balls of the third type, that is, small metal balls scattered around the solder joint(s) on the substrate itself rather than on the tinned metal of the joint. 

Coleman W.E., Jean, D., and Bradbury-Bennett, J.R., Stencil design for mixed technology through-hole/SMT placement and reflow. Soldering and Surface Mount Technology, Vol. 12, No. 3, 8-12, 2000. 

An adjunct to the surface-mount process, this method, sometimes referred to as intrusive reflow, allows the soldering of some through-hole (solder-tail) parts into plated-through holes on the circuit board during SMT oven reflow. This process can eliminate or reduce the need for wave soldering—a step prone to defects. 

Because of safety and environmental concerns and comphance with the Montreal Protocol for the reduction of ozone-depleting chemicals, this soldering technique had fallen out of favor. With the advent of Pb-free soldering, there is increased interest in vapor-phase soldering for SMT applications, but it is likely that it will remain a niche application. Due to its continued diminished status, vapor-phase reflow is covered only briefly. 

This technique consists of directing a heated stream of air or other process gas such as nitrogen at the component to be removed. Modem hot-gas repair stations are sophisticated, with computer control, predictive reflow profiling, vision systems to assist in component alignment to bonding pads on the printed wiring board (PWB), preheating stations, and accommodations for component removal. Hot-gas repair is most often used for SMT components but can be adapted for PTH component removal. 

Oven reflow surface-mount technology (SMT) soldering... 

FIGURE 49.1 Four methods of connector attach to printed wiring boards (PWBs) (a) SMT applied by reflow... 

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