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Hot Cracking. Hot cracking, also known as partial melting, can cause previously sound solder joints to fail during the wave-soldering process. A typical mixed TH/SMT assembly is manufactured by assembling the surface-mount components to the top side, inserting the through-hole components, and wave soldering these components to the board from the bottom side. The first step of the wave-soldering process usually involves preheating the entire board. During the wave-soldering process, the SMT joints on the top side will be further heated due to conduction of heat through the board, particularly if there are many vias. If these solder joints reach the melting temperature of the solder (usually 183 C), the joints will begin to melt. If the joints melt completely, the assembly may be intact after reflow; however, if they only begin to melt, the surface tension of the solder is insufficient to prevent cracks from forming between the portions that are still solid. This type of failure is often detected as an intermittent in the field, since in-circuit test fixtures may bring the two halves of the joint into mechanical contact, causing the joint to appear electrically good. Solder Bath Contamination. Solder bath contaminant levels should be regularly monitored and limited to levels found in IPC-S-815. Many metals found on component terminations will dissolve into molten eutectic Sn-Pb solder. High Cu concentration is a relatively common occurrence that is associated with a rough solder surface and causes poor solderability. High Au concentrations can embrittle solder joints (see Sec. 57.5.1.3 for a discussion of this phenomenon). 57.4.2.4 Cleaning and Cleanliness. Improper handling procedures and improper selection and application of solder paste and wave-solder fluxes and their associated cleaning processes can cause ionic residues to be left on the board that result in low surface insulation resistance. Low SIR values can cause failures in and of themselves for some sensitive circuits and in other cases set up the conditions for further corrosion that eventually result in short circuits. Sodium and potassium ions and halide ions are the most commonly quoted culprits for these failures.The major source of sodium and potassium ions is handling, i.e., fingerprints. The primary sources of halide ions are soldering fluxes. The elimination of chlorofluorocarbons (CFCs) mandated by the Montreal Protocol has caused most SMT manufacturers to switch to water cleaning or a no-clean process. Water cleaning has been used by most printed circuit board manufacturers for some time, but outgoing cleanliness was not carefully monitored since the boards were cleaned again after assembly. Both the no-clean and water-clean assembly approaches must meet certain criteria to provide reliable assemblies. In a no-clean assembly process, there is no cleaning step after SMT or TH assembly. The finished assembly has whatever contaminants were present on the incoming board and components, plus any additional contaminants added during the assembly process. These contaminants are generally flux residues, both from the solder paste and the flux applied for wave soldering, although adhesives and fingerprints are other potential sources. A no-clean flux should have a low solids content so that it leaves little residue and be free of ionic contaminants such as halides that promote corrosion. Use of a flux that contains halides will result in low SIR readings and may result in shorting due to corrosion, particularly if the assembly is exposed to a humid environment. However, the incoming components and boards are cleaned, it is important that they are also free of halides when they arrive for assembly. Although SIR testing provides the best correlation with reliability, an ionic contamination test may be used for statistical process control. The measurement method may be found in MIL-P-28809. Solder balls may also be a problem on no-clean assemblies. Solder balls are formed during reflow of solder paste when some solder is left behind when the solder melts and beads up and by spattering during wave soldering. These solder balls are usually washed off by solvent or water cleaning; however, in a no-clean process they remain on the board. Solder balls can cause shorts by bridging the pads of small capacitors or resistors or the leads of fine-pitch quad flat packs. In a water-clean assembly process, the assemblies are cleaned with jets of deionized or saponified water after SMT and TH assembly. This process will work only if the flux residues
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