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www.us-tech.com For Electronic Assemblies, Reliability Rules Continued from previous page
accurate. At present, the most commonly
removed (cleaned) flux is no-clean flux. More assemblers clean electron- ic assemblies reflow ed with no-clean solder paste than any other type of solder paste. Some of the assemblies
reflowed with no-clean solder paste arguably do not need to be cleaned while others ab - solutely must be cleaned. There are many factors
to determine if an assembly should be cleaned. Factors include the cost of failure, the climatic environment for the assemblies, and the geo- graphic component layout.
Cost of Failure If a product fails due to
the presence of harmful residues (or any other fatal defect), what happens? While some assembly failures can lead to very dramatic results (death, destruction, etc.), oth- ers can lead to loss of profits and reputation. While some failures may be considered helpful (planned obsolesces), most do not fall into that category. Some assemblies can han- dle residues without failing and oth- ers can’t. How can a label on a solder paste jar know whether or not an assembly can handle residues creat- ed by a no-clean process? The fact is the label is misleading. As noted ear- lier, a more appropriate definition would be “low residue.” It is then a matter for the technology owner to
determine how much residue is acceptable on a specific assembly.
Climatic Environment Residue-related assembly fail-
ures are usually the result of three combined factors: electrical current, moisture, and residues. The right
corrosive or conductive residues. While an effective circuit-board bak- ing process can eliminate moisture, unless an assembly is encapsulated, moisture can return to the assembly, causing the previously described fail- ures. A common belief is that to pre- vent the reintroduction of moisture, an assembly can be conformally coat- ed or potted. While this is true when assemblies are deeply potted, it is not true when assemblies are conformal- ly coated. A conformal coat- ing, while providing an effec- tive barrier to fluids, is often permeable. Small amounts of moisture can penetrate a con- formal coating, resulting in undercoat corrosion and elec- trical migration and leakage. The third factor, resi -
Trident automatic defluxing system.
combination of these three ingredi- ents will result in electrical migration (dendritic growth) or electrical leak- age, neither of which is desirable. To eliminate any chance of electrical migration or leakage failures, all one has to do is eliminate one of the three factors. Obviously, eliminating elec- trical current is not an option. The second factor, moisture,
acts as a catalyst for electrical migra- tion or leakage when it comes into contact with electrical current and
dues, can be eliminated. With a residue-free assembly, elec- trical current and moisture alone do not have anything to form a current path with. No current path means no elec- trical migration or electrical leakage.
Component Layout For component layouts,
the component-to-component and component-to-board spacings must be considered. As the spacing decreases, so too does the volume of tolerable residue. For those who may wonder how
to determine how much residue is contained on an electronic assembly, the answer involves both good news and bad news. The good news is fast, easy, and affordable testing systems
IPC approved resistivity of solvent extract (cleanliness) tester.
The volume of acceptable, toler-
able residues back in the 1970s was many times more than would be acceptable today. Some have sug- gested that a new test for circuit- board cleanliness is required. Perhaps a better concept is to
internally reject the current (and obsolete) cleanliness pass/fail stan-
Continued on page 68
Electronics, Inc.
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are available to determine how much residue is on an assembly. There are also well-defined cleanliness stan- dards, originally published as mili- tary standards, and subsequently morphed into (IPC) standards. The bad news is the cleanliness standard most used today was actually devel- oped in the 1970s. It was developed during a time before surface-mount- technology (SMT) components, and it has never been changed.
March, 2014
Extrusions
Electronic Components
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