Component placement process

This chapter covers why manufacturers inspect printed circuit assemblies, how they have implemented and enhanced visual inspection, what automated inspection systems they are using, and how they have implemented these antomated systems. The scope of this chapter includes only inspection of printed circuit assemblies during the assembly process, as typically shown in Fig. 53.1. Thus, it includes inspection of solder paste after the paste printing process step, components after the component placement process step, and solder joints after the solder reflow process step. Not included, however, is incoming inspection of components and the bare printed circuit board (PCB). The focus of this chapter is on prodnction nse of inspection, not the collection of measurements dnring process development in a research and development (R D) environment. 

Post-Reflow Solder Joint Measurements. Solder joint measurements, such as fillet heights, average solder thickness across the pad, void volume, and pm-to-pad offsets, as shown in Fig. 53.8(c), provide information about the paste printing process, the component placement process, and the solder reflow process steps. Attribute measurements, such as solder bridges, opens, or insufficient solder, are most common. Quantitative measurements of solder... 

In general, there is little impact on the com-ponentplacementprocesswhenimplementing a lead-free process. Components will have a lead-free finish, however the shape and size remains the same. One area of concern in the component placement process is placement accuracy. Several studies indicate that components that would center back to the PCB pads in a tin/lead process will not center back as well in a lead-free process. We will have to monitor the placement process to ensure components are placed onto the PCB pads. 

As far as the production of these samples is concerned, the outlay involved in the lasering, metallization, and component placement processes for MID components is comparable to that for the manufacture of other electronic parts such as printed-circuit boards. The costs incurred for preparing the plastic blanks as the MID bodies in real-part quality, by contrast, are considerably higher because an injection-molding tool is necessary. 

Conventional printing processes are less useful for applying conductive adhesives or solder paste. Dispensing of single dots in a complex geometric application is therefore necessary. Three-dimensional circuit carriers also decrease the freedom of component placement, which leads to restrictions for placement systems. 

Nagelsmit MH, Kassapoglou C, Giirdal Z. Fiber placement process for AP-PLY composite components. In Proceedings SAMPE SETEC, Leiden NL 2011. p. 195—202. 

For processing fully consolidated tape materials the process simulation tool ProSimFRT has been developed at IVWProSimFRT enables simulation of the component quality depending on the void content and the surface roughness. The simulation can be used for tape winding as well as tape placement processes using a compaction roller. 

Surface mount techniques have also been successfully used to assembly hybrid circuits. The metallized ceramic substrate is simply substituted for the conventional printed circuit board and the process of screen printing solder, component placement, and reflow solder is identical. 

As noted previously, substrate (typically PCB) design has an effect not only on board/component layout, but also on the actual manufacturing process. Incorrect land design or layout can negatively affect the placement process, the solder process, the test process, or any combination of the three. Substrate design must take into account the mix of surface mount devices that are available for use in manufacturing. 

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. 

Armordon is also a polypropylene based self-reinforced plastic manufactured by Don Low Ltd. The production process is similar to the production of PURE and combines two different types of PP ensuring a strong affinity between the two components of the tape. Further processing steps may use the tapes directly for winding or tape placement processes or they can be woven into fabrics. Typical applications can be found in the security industry as armor, since Armordon shows an excellent ballistic performance. Additionally, applications in the automotive sector and consumer products such as luggage are imaginable. 

Elimination of manual component placement and insertion reduces process time, defects, and process cost. 

Cell (Batch) Process. The cell process routes the circuit boards in batches between the different steps. The cells or workstations are not always in immediate proximity to each another and can be entirely manual, semiautomated, or fiilly automated in terms of the actual process step. For example, component placement may be fully automated, but require several machines for inserting the different component types. Circuit boards are typically loaded and unloaded by hand between machines. Table 40.1 lists the advantages and disadvantages of the cell process. The cell or batch process is best suited for a facility that assembles a high mix of low production volume products (e.g., prototype development or high-reliability circuit boards) where flexibility is necessary on the factory floor. 

An important consideration about the pin transfer technique is that it requires an open bath of the adhesive or flux. Adhesives readily absorb water from the air. Fluxes lose vehicle (water or alcohol) and possibly other constituents through evaporation. By either mechanism, the material properties change, which affects the quantity of fluid retained on the pin and deposited at the site (including the fhp-chip process previously described). Adhesives must have sufficient wet strength, and the fluxes must have enough tack to hold the component in place for the duration of component placement activity as well as subsequent handling of the circuit board on its way to the curing oven or reflow oven. 

Vision system limitations are determined by the speed with which the computer can process information (e.g., circuit board coordinates, component geometries, defects). The more information to be processed, the slower is the component placement step. For products requiring the placement of thousands of parts per circuit board, even an additional few tenths of a second per component can add up to a significant loss of production throughput. 

Component Packaging Formats. Odd-form components come in a variety of sizes, geometries, and I/O configurations that determine the ease with which the component can be automated into the assembly process line. Another important variable is the component placement machine. Typical component packaging formats for the placement machine include tape and reel (radial and axial leaded), extruded tube, tray, continuous strip, and bulk containers. Each of these formats has benefits and drawbacks, as noted in the following subsections. Industry specifications have standardized nearly all of these packaging schemes. For example, there is EIA-468-B for radial deviees and EIA-296-E for axial components. 

Matrix Trays. Matrix (or waffle) trays are an inexpensive means to snpply components that are generally too large or heavy for tube or tape packaging and must be individually separated from one another. See Rg. 40.28. An important consideration is that the trays themselves may not be a suitable starting point for the automated placement processes especially for very small components. Rrst of all, the components have too mnch room to move about in each bin to estabhsh coordinates for initiating the pickup and insertion process. Also, component leads are not always oriented correctly in the insertion plane. 

Successful control of the placement process, whether a highly automated chip shooter step or the manual placement of odd-form components, must meet the following objectives ... 

But there are also quality control reasons to smooth out the flow of bottlenecked assembly processes. For example, if the component placement machine has a throughput of 100 circuit boards per hour, but the reflow machine can only process 90 circuit boards per hour, then printed and stuffed boards will spend time in the open factory air awaiting entry into the reflow step. The consequence is a degradation of paste properties, resulting in the increased likelihood of solderability defects and a drop in product yield. Such technical ramifications must also be addressed when considering equipment utilization for an assembly process. 

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. 

Soldering flux is applied prior to component placement and soldering. Flux choice should be tailored to the hot-bar process.The flux can be either liquid or paste. TTie flux chosen should be tested to resist charring and the development of polymerized decomposition products, or lacquers, which adhere to both circuit board and hot-bar. Residue build-up on the bar can adversely inhibit hot-bar performance by diminishing thermal transfer. The residue can also become thick and uneven enough to prevent the hot-bar from squarely contacting component leads. Baked-on aqueous-clean flux residues can make flux cleaning difficult. With no-clean flux, overheated residues may detract from the visual appearance of the printed circuit assembly. 

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