September 2023
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Page 49 Why PCB Cleaning is Critical in the EV Market Continued from previous page
number of circuit boards in the electric vehi- cle and e-mobility world are of the highest re- liability.
Military and avionics, the traditional
leaders in high reliability circuit board man- ufacturing, provide some insight to best prac- tices though they don’t come without a cost, literally. Functional safety in these markets has focused on building redundancies into their systems; using mil-spec components, washing boards up to eight times in the man- ufacturing process and, finally, conformally coating the boards to protect against harsh environments. The automotive manufacturing industry
is one of the most price competi- tive markets and has neither the space nor ability for built-in re- dundancies to systems using high-cost mil-spec components. It also must manufacture thou- sands, if not tens of thousands of boards a day, not thousands of boards a year. The most cost-ef- fective way to prevent electro- chemical migration and increase reliability is to clean the ionic residues from the circuit board after reflow and then coat the board with some type of confor- mal coating.
Dendritic Growth The three conditions re-
quired for dendritic growth are ionic residues, humidity and elec- trical bias. Cleaning removes the ionic residues from the boards and coating reduces the amount of humidity exposure to a circuit board. Since there is no way to prevent a bias on a live circuit board, minimizing the remaining two factors has proven to greatly increase reliability. A common argument against
cleaning has been that since the invention of “no-clean” solder pastes, cleaning is not critical be- cause all the ionic residues on a board are encapsulated. In theo- ry no-clean solder pastes do an excellent job of encapsulating ionic residues, but only when their specific reflow profile is fol- lowed. As circuit boards assemblies
become more complex and heavi- ly populated, maintaining a con- sistent thermal profile over the entire area of the board becomes increasingly difficult. If there is a deviation from the solder paste manufacturer’s recommended re- flow profile, locally or generally, there will be active ionic residues on the board. These active residues are
why almost all high reliability printed circuit boards are still cleaned regardless of if the solder paste used is a no-clean, RMA, or water-soluble. The cost of clean- ing is less than the cost of a recall and far less than the cost of fail- ure on a board that causes a crash or leads to a thermal run- away event. In fact, according to documents filed with the
NHTSA, major OEMs have already begun to have recalls related to insufficient circuit board cleaning on electric vehicles.
In one instance, a “Printed Circuit Board
(PCB) did not undergo a sufficient cleaning process during sub-supplier production.” This could cause “a shutdown of high-voltage electrical power [that] would result in a loss of propulsion, increasing the risk of a crash.”
Cleaning Standard? However, it isn’t only high reliability cir-
Dendritic growth can be caused by ionic contamination, humidity and bias.
cuit boards that should concern the industry. Overall reliability has been a metric widely monitored by the automotive industry for decades. Those of us old enough to remember the 1980s and 90s will remember that Japan- ese automakers overtook the lion’s share of
Continued on next page
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