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HowClean IsCleanEnough? Continued from page 55
the circuit board for electrical access to volumes between component and board. In electronic manufacturing,
solder paste deposited on a PCB is heated to reflow solder for electrical connections. The process causes sol- der flux to volatize and vaporize. During volatilization, the flux
removes oxides and permits the sol- der to chemically bond a PCB and component. Ideally, only solder is left behind, but more practically, a flux residue remains. This flux residue, depending on its chemical con-
Residue Cocktail Solder
Exposed Sensor Traces Pad
ferent compositions of materials, with different risks. A significant risk lies in the presence of ionic com- ponents near solder terminations. These compounds have the capability of migrating under a voltage bias, which produces a current. An even more dramatic result of ionic com- pounds is the creation of dendrites. Dendrites serve as semi-permanent conductive paths between previously unconnected PCB pads. They are formed of ionic compounds that remained on a PCB and, under volt- age bias, created a continuous chain of electrically conductive molecules between terminals, resulting in cur- rent leakage and irregu- lar performance for a final product. In understanding the
Sensors can be placed under bottom terminations to gauge contamination.
stituents, can have varied impacts on a final assembly. Further, because
importance of electronic circuit cleanliness, a first hypothesis poses that some components of solder flux residues are more detrimental than others. The most commonly-pres- ent ionic constituents for solder flux residues have been narrowed down to a
handful of ionic compounds. Some are more detrimental than others; howev-
Faster circuit speeds can be achieved via denser circuit designs, tighter pitch, and shorter line spacing. But speed and performance also require clean circuits, and
dirt and residues present on circuit top surfaces and bot- tom terminations can hinder performance.
flux is designed to attack metals, flux residues can pose problems for PCBs. Solder flux can be formed of dif-
er, the impact of each can be quanti- fied. The table offers a list of com- pounds, broken into groups of ionic
compunds (anion and cation) and weak organic acids. Regardless of the
compound, a fluid carrier system must be present in order for an ionic com- pound to mobilize. The most viable carrier system is water. Water, without con-
August, 2015
the effects of conductor spacing can be evaluated for 0.005-in., 0.015-in.,
Board Terminations
Metal Ions Liquid Center
taminants and in a normal air environment, tends to exhibit resistance on the order of 1 to 5 MW. Suf - ficient moisture allows for an ionic species to mobilize and provide repeatable resistance values often greater than 10 MW. Overhydrated compounds tend towards a resistance equivalent to water, masking the effects of a contaminant. A consistent hydration method is necessary to pro- vide repeatable hydration levels for an ionic compounds present on a PCB.
Reduced Spacing A second hypothesis
poses that reduced spac- ing between test locations correlates to lower SIR values. To explore this hypo -
thesis, a test coupon incor- porated exposed, parallel sensor traces. The traces were 0.005-in. (0.127mm) wide and separated by 0.005-in. Therefore, test- ing adjacent sensor traces (7 to 8, 8 to 9, etc.) provides insight into the SIR over a 0.005-in. gap. Skipping a trace (7 to 9, 8 to 10) provides insight into the SIR over a 0.015-in. gap (0.005-in. space + 0.005-in. trace + 0.005-in. space). With a total of 13 conductors,
SIR sensors were used here under terminated components.
will allow for the sensitivity of clean- liness levels to be associated with the density of the components on a cir- cuit board. A more densely populated circuit board or one with fine pitch BGAs, for example, would have a more stringent SIR requirement than a circuit board with only through-hole technology parts. The
Continued on next page Migrating Metal Ions
Active residues from solder flux can lead to leakage currents in circuits.
0.025-in. … 0.115-in. spaces. It is expected that the SIR for 0.005-in. spaces will be substantially lower than that for a of 0.075-in. space sim- ply because there is a larger gap. The capability to vary conductor spacing
Sensor #7 on Component F
Pin #7 for Component F
Aven Tailors Inspection Systems For The Perfect Fit
Tailor Your Own System at
www.aveninc.com or Call Us at 734-973-0099
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