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88 SMT Magazine • April 2014 can completely dissociate into negative or posi- tive particles. This changes the overall conduc- tivity of the solution. If water is applied, it be- comes ionized and the change becomes either positive or negative. Ionic residue examples include surfactants, flux activators, perspiration, and plating chem- istry, among others. Once one of these residues takes either a positive or negative ion charge, it then changes the cleaning solution's conduc- tivity. Thus, the cleaning process becomes dif- ferent from what it should be and special care must be given to the type of cleaning technolo- gies used for certain PCB applications. In terms of non-ionic residues, these aren't conductive, mostly insolvent in nature, and aren't prone to cause problems. They're usu- ally organic species having a role in fabrication and assembly and include mostly resins, oils, greases, or hand lotions. They don't necessarily change the solution conductivity, but they are just resins that need to be cleaned after a board is assembled. Most assemblies are cleaned using de-ion- ized or DI water. DI water is generally heated to a temperature in the range of 100–120°F. It usually contains 3–5% chemistry, and the rest is water. Most contaminants are cleaned using de-ionized water. When it comes to special agents that cannot be cleaned at a specific temperature, in some cases, cleaning agents need to be changed. This involves changing the concentration of the cleaning agent, the wash cycle temperature, and the speed of the rinse cycle to achieve better re- sults. There are different permutations that can be made to make DI water purer and cleaner. In some cases, there are restrictions where DI water cannot be used for cleaning, because cer- tain components don't react to water very well. Special washing techniques sometimes prescribed by the OEM customer can be used to clean resi- dues, flux remnants, and unneeded paste. But in some cases when those washing techniques are not specified, the assembly house has to use spe- cial chemistries, as shown in Figure 2. Rosin mildly activated (RMA) flux, a hybrid RMA, vapor de-greasing, foaming agent, and rosin flux are among the more prevalent special chemistries. RMA flux is mildly activated and not very aggressive. It contains fewer activators compared to water-soluble fluxes. This is an ef- fective flux if a PCB is mildly contaminated. If too much flux and residues are left after assem- bly is finished, RMA flux is good. However, due to its multiple types of rosins, it requires a sa- ponifier heated to 100–150°F, meaning it works in a hot environment when it comes to clean- ing. RMA provides the option of a final rinse with DI water. figure 1: Array of residues and contaminants left on a PCb. figure 2: batch cleaning, used when the assembly provider uses special chemistries. TiGhTEr SCrUTiNY NEEDED FOr pCB ClEaNiNG aGENTS continues Zulki'S PcB nuggeTS

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