Pin-in-paste

CALCULATION OF SOLDER FILLET VOLUME FOR PIP PROCESS (PIP = PIN IN PASTE)... 

Pin in paste (PIP) technology, also know as alternate assembly and reflow technology (AART) by Universal Instruments and others, allow the use of THT parts with solder paste and reflow soldering. Paste is deposited onto the board at through-hole locations, the leads of a THT part are placed through the paste, and at the conclusion of the placement process the board and components are reflowed. The two major concerns with the use of the PIP process are the deposition of an adequate volume of solder paste and the use of THT components whose body can withstand reflow soldering temperatures. In the wave solder process, only component leads were raised to soldering temperatures, but in the reflow process the entire part must withstand the process temperatures. 

Vicari, D. Pin-in-Paste, or Alternative Assembly and Reflow Technology. Universal Instruments white paper, at http //www.uic.com/wcms/WCMS.nsf/index/White Papers 3.html pmpaste... 

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). 

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. 

Solder Volume. The inabiUty to apply enough solder paste to meet standard through-hole solder-joint acceptabUity 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. 

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. 

FIGURE 47.18 Solder-joint defects can result from uneven solder distribution (a) A solder-starved pin-in-paste solder-joint showing large void (b) a good pin-in-paste solder joint showing nearly 100 percent barrel fill and no voids. (Courtesy of Hewlett-Packard). 

Stencil designs that preclude solder paste from entering the PTH also result in a significant reduction of solder paste on the top surface and in the PTH. These reductions may make the pin-in-paste technique impractical for some boards. If the solder volume is too low, the available solder distributes unevenly around the PTH pin and the PTH barrel, resulting in voids... 

Pin-in-Paste on Thick Boards. Since the solder volume increases proportionately with increasing board thickness, pin-in-paste soldering is best suited for thin PWBs (<1.6 mm). The technique is limited by available solder and required solder volume for proper barrel fill. There are two ways to deal with the added volume. 

FIGURE 47J0 Application of solder pre-form foil washer to augment pin-in-paste solder volume. Note that the flux from the solder paste is sufficient for pre-form soldering. 

Temperature Compatibility. If pin-in-paste soldering is to be used, check that components are temperature-compatible with the oven reflow process. The high temperatures and long exposures associated with oven reflow soldering may cause unsuited molded component bodies to melt or warp. Connector contact normal force may be affected if the molded connector body softens or distorts. Solder joints or wire bonds internal to some devices may become disbanded, and some, such as electrolytic capacitors, may leak or even explode as a result of an oven reflow cycle. Check the component manufacturer s specification for thermal hmits and compatibihty with oven reflow soldering. 

FIGURE 47.21 Cross-sectional micrograph showing pin-in-paste buried intrusive soldering. Note that very little solder has drained. 

DCB double cantilever beam (test specimen ) PIP pin-in-paste... 

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