APPLICATIONS ENVELOPE FOR PEMs

Liquids, Gases, and Particles:

Environments which contain liquids, gases, and particles, as may apply in system assembly or use, should be guarded to obtain optimum use of PEMs. In the case of liquids used in processing, the concentration of halides and other anionic species and their residuals (Cl^-, F^-, Br, SO₄^--) in aqueous solutions should not exceed 50 ppm. Alkali ions, salts and residuals (Na⁺, K⁺, Li⁺) in aqueous solutions should not exceed 10 ppm. Volatile gases, such as SO₂, NO_x, O₃, CO, etc., should be minimized, although, standards for plastic-encapsulated devices are not available. It must be pointed out that high humidity levels (>85%RH) greatly accelerate the activation of the deleterious effect of the presence of these volatiles. Also, the size and quantity of particles should be few enough and small enough so as not to collect between leads or bridge the lead spacing over the useful life of the PCB. Bias and humidity are accelerators which increase sensitivity to this mechanism. This applies to both PEMs and HSMs. Particle sensitivity becomes even more critical with fine pitch leads as found in SMDs. Best practices would ensure the elimination of conductive particles from the environment. Conformal coating of the PCB will also keep particles away from packages as well as provide extra protection against airborne contaminants and liquid moisture ingress.