Solder bridge

Solder masks (solder resists) are protective polymer coatings used on printed circuit boards (PCB s) to prevent solder bridging between conductors during soldering (1-3). The selective application of the solder mask limits the amount of conductor area in contact with the molten solder, minimizing solder usage and slowing the rate of conductor metal contamination in the solder pot. 

Such a positrol log contains complex steps, each of which might well be kept in its own log. For example, the reflow soldering operation could have a number of what steps, such as belt speed furnace zone temperatures for preheat soldering zone cool down zone atmosphere chemistry control atmosphere flow speed visual inspection for missing, misaligned, or tombstoned parts solder bridges and solder opens. 

Becanse of the extra operations and exposure of components on the secondary side to molten solder, this type of assembly is prone to many assembly defects. It is important to note that soldering fine-pitch parts on the secondary side using wave soldering results in excessive solder bridging and should be avoided. In addition, active components, such as ICs, may be damaged by exposure to excessive heating. 

Solder mask webs below manufacturing capabilities could result in solder mask breakdown and poor product yields. This problem could result in a PCB assembler with solder bridging defeets. 

S/M coverage Features left open that could result in solder bridging Figure 20.14g... 

Solder bridging Small gaps between soldered features without S/M that could bridge solder Figure 20.14h... 

Reworkable May contain Pb Thermal damage to board Solder bridging Plngged PTHs... 

PTH Hole Fill Exposed Copper after Soldering Post-Assembly Corrosion of Exposed Surfaces Handling Defects Solder Bridging Plugged Holes Equipment Safety Paste misprints Black-Pad, Black-Line Nickel Brittle Fracture Solder Mask Attack RF Signal Loss Post-Assembly Corrosion of Exposed Surfaces Tin Whiskers Ionic cleaning Failures Solder Mask Attack Post-Assembly Corrosion of Exposed Surfaces Post-Assembly Corrosion of Exposed Surfaces Tarnish Solderjoint Microvoids Electrochemical Migration... 

Initial Ramp. In this step of the reflow time-temperature profile, preheating of the boards, components, and solder paste is initialized. The solder paste begins to lose some of its volatile components and the flux becomes chemically active (activates). If the ramp gradient is too steep, volatiles will be given up too rapidly, boiling will result, and the solder paste whl spatter. This can cause explosive solder bah formation and decrease local solder volume, and thus impair bond rehabihty. Unattached solder balls may form solder bridges between two closely spaced conductors and produce an electrical short circuit. 

When exposed to the wave, the adhesive-attached surface-mount devices pick up solder on their metal contacts and the solder bridges from the contact to its corresponding bonding pad on the bottom side of the PWB. In the case of solder-tail components, the molten solder is drawn by capillary action between the lead and the FTH barrel. If the barrel and lead are hot enough and well fluxed, the solder fills up the barrel and wick to form aiets from pin to barrel. As the board continues past the wave, it cools, solder sohdifies, and joint formation is complete. 

The hot-air knife, as previously described, is sometimes useful in removing unwanted solder bridges from between component leads while the solder is still molten. This is especially useful in connectors and PGAs with interstitially arrayed pin fields (see Fig. 47.24). 

Laser soldering is not prone to solder bridges and if component leads are held down to solder-coated PWB lands, then soldering defects are rare. Perhaps the most common characteristic... 

For EGAs and other area-array devices, following a visual check, use transmission x-ray to inspect for solder bridges. Sometimes this method also reveals lack of solder. Lastly, if in-circuit testing or functional testing is done on the first-build product, then it should be applied to the reworked product also. 

X-ray inspection can provide a visual depiction of the solder connection that must not contain solder bridges, incomplete reflow, attachment fractures, misahgnment that violates minimum electrical clearance, or missing or damaged spheres. It s also considered defective for solder voids to displace more than 25 percent of a sphere (see Fig. 52.19). 

Solder bridged between electrically noncommon conductors creates a shorting condition and is an unacceptable condition (see Rg. 52.27). Solder webbing is a continuous film of solder that is parallel to, but not necessarily adhering to, a surface that should be free of solder. Solder webbing is also an unacceptable condition (see Fig. 52.28). 

Solder resist would be ineffective and considered a defect when there are blisters, scratches, or voids that bridge noncommon circuits, or if the exposure has the potential to allow solder bridges. It could be considered a process indication if bhsters or flaking expose bare copper. 

Visual inspection after the reflow and wave-soldering process steps can also be jnst a qnick scan for obvious defects to detect a process condition outside of control limits. In this case, the operator visually inspects for solder bridges, large solder balls or solder splashes, lifted leads, and a... 

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