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FIGURE 47.35 For laser soldering to be effective, it is necessary to have lead-to-pad, pad-to-pad, and lead-to-pad coplanarity: (a) noncoplanar component leads on constant-volume (coplanar) solder pads; (b)a coplanar lead array on noncoplanar PWB solder lands.
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surface-mount packages is the body-push method (see Fig. 47.36), in which a force is applied to the component body, springing the leads just slightly.As the solder melts below the lead, the lead drops down to board level, where it is frozen in the cooling solder. It is necessary to control the amount of push such that the solder joint is not stressed by a lead frozen in compression. This is particularly the case for coarse-pitched component leads, where much force may be required to overcome a coplanarity problem. When board pads and component leads are reasonably coplanar, then this method is adequate for high-yield, high-reliability soldering as long as the noncoplanarity is in a negative direction, as indicated in Fig. 47.36a.
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FIGURE 47.36 The body-push method would be effective in (a), where a lead is below the level of adjoining leads. In the case of (b), too much force would be needed to push the component body down to make the high component lead coplanar with adjacent leads.This could result in too much stress in the solidified solder joints.
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A compliant hold-down method has been reported that is inexpensive to fabricate and easy to implement.This consists of a silicone rubber foot and an aperture that allows the beam unobstructed access to the lead-pad combination. This method has been proven effective for TAB as well as for rigid leaded surface-mount device bonding. It obviates the problems associated with transparent hold-down methods in terms of flux build-up. The compliant foot holds leads to pads and can accommodate lead-to-pad noncoplanarity differences.
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Flux for Laser Bonding Requirements are much the same as for other reflow methods. Flux must be active enough to remove oxides from the component lead, bonding pad, and the solder. Its optical characteristics in terms of reflection, absorption, and transmission must not interfere with the solder process. If too absorptive, the flux could char, becoming more absorptive, and then overheat and wreak damage to the underlying circuit board. Also, if denatured, it will lose its chemical effectiveness as a fluxing agent. In laser soldering, particularly in the scanned methods, the flux adds a path of heat transfer to the board and preheats the next joint to be soldered. In the absence of a liquid flux, the PWB is more likely to char if the laser impingement is not stringently controlled. During soldering, the laser beam s output is quick and intense. The high energy of the process results in exceptionally high processing temperatures for very short durations. The high processing temperature aids in flux activation, making even the mildest of no-clean fluxes an effective soldering agent.
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Fluxless Laser Soldering There have been many reports of fluxless laser soldering. As used for inner-lead bonding to silicon die, these utilize a high-peak-energy, short-duration pulse. Under these conditions, when the mass of the lead and the bonding pad is small enough, the process is more akin to welding with the component lead melting and alloying with an underlying metal such as tin. To accomplish conventional reflow soldering by means of a laser, or any other technique, it is necessary to employ a liquid solder flux or gaseous analogue pretreatment to bond effectively. Plasma cleaning and inert storage may be one such method. Ultrasonic assisted laser soldering can be effective, but is slow, requiring precise placement of an ultrasonically agitated head and running the laser beam either through the ultrasonic tool or precisely adjacent to it. The ultrasonic agitation breaks up the oxides surrounding the molten solder and the component lead, allowing joining of the liquid solder to the component lead metallurgy. The use of gaseous phase carboxylic acid fluxing with laser bonding has been demonstrated.15 This precludes the need for conventional liquid fluxes and leaves no visible residues on the circuit board. No cleaning is required and the assembly reliability is not degraded from the use of a gaseous fluxing approach.
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Laser Solder-Joint Characteristics There are very few differences between solder joints prepared by laser as compared to those formed by other methods. Laser soldering results in intermetallic formation, but the layer is extremely thin if the laser soldering cycle heating is kept short much thinner than that found in solder joints manufactured by more conventional methods. Since the heating is highly localized, cooling is rapid resulting in exceptionally fine solder grain growth. The fine grain growth leads to greater joint strength initially. The strength advantage, although significant in magnitude, tapers off as the solder joint ages and metal grain coarsening occurs. Coarsening rate depends on ambient temperature and time.
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Solder Sources and Defects Associated with Laser Reflow Solder requirements are the same as for any other process. There are no alloy composition requirements specific to laser soldering, as this soldering method is compatible with leaded or lead-free solder alloys. Even high-temperature alloys can be soldered by this technique. When single-point laser reflow is applied, the board quality and integrity are not compromised if parameters are chosen and adequately controlled. Laser soldering is not prone to solder bridges and if component leads are held down to soldercoated PWB lands, then soldering defects are rare. Perhaps the most common characteristic
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