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TABLE 52.6 PTHs with Component Leads: Minimum Acceptable Solder Conditionsa Criteria A. Vertical fill of solderb,c Note: Less than 100 percent older fill may not be acceptable in some applications: e.g., thermal shock. The user is responsible for identifying these situations to the manufacturer B. Wetting on primary side (solder destination side) of lead and barrel Target for Classes 1,2, and 3 is 360 wetting present on land and barrel C. Percentage of land area covered with wetted solder on primary side (solder destination side) The land area does not need to be wetted with solder on the primary side D. Fillet and wetting on secondary side (solder source side) of lead and barrel E. Percentage of land area covered with wetted solder on secondary side (solder source side) Class 1 Not Specified Class 2 75% Class 3 75%
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Notes: a Wetted solder refers to solder applied by the solder process. b The 25 percent unfilled height includes both source and destination side depressions. c Class 2 may have less than 75 percent vertical hole fill.
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FIGURE 52.23 (1) Vertical fill meets requirements of Table 52.6 (2) Solder distination side (3) Solder source side. (IPC)
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52.6.2 Solder Balls or Solder Splash Solder balls and splashes that violate minimum electrical design clearance, are not encapsulated in a permanent coating, or are not attached (soldered) to a metal surface are unacceptable (see Fig. 52.24). 52.6.3 Dewetting and Nonwetting Dewetting is a condition that results when molten solder coats a surface and then recedes to leave irregularly shaped mounds of solder that are separated by areas that are covered with a
ACCEPTABILITY OF PRINTED CIRCUIT BOARD ASSEMBLIES
FIGURE 52.24 Solder ball causing a short circuit between pins in SOIC.
thin film of solder and with the basis metal not exposed (see Fig. 52.25). Nonwetting is the partial adherence of molten solder to a surface that it has contacted, while the basis metal remains exposed (see Fig. 52.26).
FIGURE 52.25 Example of dewetting. (IPC)
FIGURE 52.26 Example of non wetting. (IPC)
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Dewetting and nonwetting of solder joints is generally caused by contaminants on either the component leads or on the PCB PTH or pads. A minimum of dewetting is allowed on solder joints assuming that the solder joint meets minimum requirements as defined in Table 52.6 and Sect. 52.4.2 by component package type, and good wetting is evident on the portion of the solder joint that does not display dewetting. Nonwetting is unacceptable since adequate wetting is not achieved and indicates a serious solderability problem on the component or the PCB. 52.6.4 Missing and Insufficient Solder Missing solder is an obvious unacceptable condition since solder provides electrical continuity and some measure of mechanical attachment of the component to the PCB. Insufficient solder becomes an unacceptable condition for SMT components when the minimum solder fillet requirements defined in Sec. 52.4.2 are not achieved. Insufficient solder becomes unacceptable for PTH components when the minimum requirements of section 52.6.1 and Table 52.6 are not achieved. 52.6.5 Solder Webbing and Bridging Solder bridged between electrically noncommon conductors creates a shorting condition and is an unacceptable condition (see Fig. 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).
FIGURE 52.27 Solder connection across conductors that should not be joined. (IPC)
For components with leads, with the exception of criteria stated in sec. 52.4.2.4, solder cannot come into contact with the component body or end seal. Solder contacting the component body or end seal is generally unacceptable. 52.6.6 Lead Protrusion Problems Measurement of lead protrusion is defined as the distance from the top of the PCB land to the outermost part of the component lead, which can include any solder projection from the lead.
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