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Were the two boards abutting, the heaters would try to compensate for a much larger thermal mass and overheat. Heater performance would be much different than during steadystate manufacturing and not give a good picture of heater performance during thermal profile. This rule also applies to thermal profiling at wave soldering. 47.3.11 Pin-in-Paste Soldering (Intrusive Reflow Process) 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. 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. 47.3.11.1 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).
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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.
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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 PTH. This is discussed later in this chapter. 47.3.11.2 Solder Volume. The inability to apply enough solder paste to meet standard through-hole solder-joint acceptability criteria is one of the major shortcomings of pin-in-paste reflow. That is why this technique is usually relegated to boards 1.6 mm (0.063 in.) thick. Requisite solder volume is dictated by component pin pitch and the available printing space between component leads; stencil thickness, which is generally limited by the smallest or finest pitch components on the PWB; and PTH volume and associated component lead displacement volume. Since requisite solder volume for pin-in-paste reflow is a function of the ratio of PTH volume to component lead displacement, it follows that reducing PTH barrel size is advantageous,
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especially in the case of thick PWBs. But as the volumetric annulus between lead and barrel is reduced, there is a tendency for excessive void formation. Solder pastes formulated for surface-mount applications consist of solder spheres in a creamy matrix of organic chemicals composed of soldering flux and other materials to help with printing and component placement as required for SMT processing. Voids in the solder are caused by vaporization of solder paste organic components during the reflow process. In an unlikely twist, enlarging a platedthrough hole in a thicker board may also benefit the pin-in-paste technique. The paste printing process drives solder paste into the PTH, effectively increasing the solder volume available for joint formation.A double-print cycle forces more paste into through holes. If solder paste is in the PTH, some quantity of it will be displaced as the component s lead is inserted (see Fig. 48.16a). Upon reflow, the solder generally wets back up the pin (see Fig. 47.16b). Sometimes, though, the solder will form a bead on the pin tip, as seen in Fig. 47.16c).Although electrically it is of no
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FIGURE 47.16 Board sections illustrating pin-in-paste process: (a) A PTH component pin inserted through solder paste squeegeed in and around PTH. (b) At reflow, the solder has melted and wet to pin and component leads. If solder volume is correct, surface tension effects will draw the solder between pin and barrel and a good solder joint will form. (c) The solder paste displaced by the pin insertion has melted into a ball around the pin tip. So much solder was displaced that voids are apparent between the pin and the PTH barrel.
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consequence and well adhered since it is soldered, it can interfere with critical clearances (such as aboard to chassis) or impede proper board seating at in-circuit test or in other fixtures. A solder paste stencil can be made to occlude solder deposition on top of or in the board s PTH, as can be seen in Fig. 47.17.
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FIGURE 47.17 A0, B0, C0, and D0 are solder paste apertures cut into a conventional stainless steel stencil. Apertures B0 and D0 are designed to prevent solder paste from being forced into through holes. Corresponding paste deposits are indicated as A1, B1, C1, and D1. C0/C1 and A0/A1 are usual round and square apertures and deposits, and solder paste is allowed to enter corresponding PTHs.
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