Fluxes Dispensing

FIGURE 40.14 Flux dispensed on the balls of a flip-chip component (a, b) The solder balls are immersed into the flux reservoir (c) the component is removed from the reservoir with the solder balls coated with flux (d) the component is placed on the circuit board. 

Corrosion from Repair. Very often, even water-clean boards are repaired with no-clean formulations to avoid another aqueous cleaning step. Sometimes after a board is totally assembled, components of the board may not be suited for exposure to water. Although no-clean residues after soldering are generally benign, care must be taken to minimize the flux dispensed during board repair. There is a chance that the pooled flux will not be... 

AH the flux dispensed has reached a near soldering temperature... 

The presence of fluxes such as NaCl or Na2S04 during the decomposition of ammonium dichromate ensures that Cr203-pigments are formed in addition to nitrogen and water, instead of extremely fine powders. The isolation of ammonium dichromate can therefore be dispensed with by calcining mixtures of sodium dichromate and ammonium chloride or ammonium sulfate at 800 to 1000°C. 

Using a thin wire (22 or 24 gauge wire works well) and dispense a drop of flux on PZT about 0.5 mm from the free end. 

Solder paste consists of solder powder suspended in a flux. The material is dispensed as a slurry, then heated to its reflow temperature, and finally cooled to create a soHd solder joint. The liquid flux serves multiple purposes ... 

Figure 5.10 Dispense patterns for no-flow fluxing underfill.

Staychip 2078E/ Cookson Epoxy anhydride, fluxing imderfiU N/A Ceramics, laminates Automatic dispense or stencil Area array on PWBs using solder-reflow cycle for underfill curing... 

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). 

FluxFill 2000/ Loctite No-flow, fluxing underfill (volcano reflow) Not applicable CSP (ceramic), FCOB (silicon with passivation) and OSP -copper, copper-nickel-gold, and PWBs Dispense, 18-gauge needle 10 psi Fluxing CSP or flip-chip on board (volcano cycle)... 

Amicon E1330LV/ Emerson Cuming Unfilled, no-flow fluxing underfill (reworkable) Not applicable Nickel-gold and OSP finishes Syringe, jet dispense, or stencil print Flip-chip, CSP, EGA modest post-reflow cure (offline) required. Reworkable, 300 second reflow cycles 225°C peak. Variable fi-equency microwave curable. 

For the loading step, Eq. 4 is subject to the following boundary conditions C, = Cjo at the sample reservoir and zero flux boundary conditions, n.VC, = 0, at all the other boundaries across the chip, where n is the unit normal to the surface and C,o is an initially applied concentration for the ith species. The initial conditions for Eq. 4 are C = C,o at the sample reservoir and C, = 0 at all the other places across the chip. For the dispensing step, the boundary conditions are similar to that for the loading step however, the initial conditions are the concentration distribution of the loading step when it reaches steady state, C = C/ ioading ... 

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