Issue link: https://iconnect007.uberflip.com/i/288471
90 SMT Magazine • April 2014 The hybrid RMA can also be used for low residues; however, it cannot be washed with DI water. Closed-loop processes are used to keep the discharge to a minimum. This same chem- istry can be used repeatedly. But hybrid RMA may not be able to remove all residues and contaminants from components with hard-to- reach areas such as the underside of LGAs or very-fine-pitch BGAs. Hybrid RMA sometimes requires processed chemicals or treated water. It cannot just be directly rinsed; there must be a closed-loop system, and in some cases, a special service may be required to take the water out of the PCB assembly facility to clean it and sepa- rate the chemicals. As for vapor de-greasing, the rinse tank is boiled, creating a vapor atmosphere. The vapor encompasses the board and removes the resi- dues. The advantage is the solvent is recycled. There is low waste, and less floor space is re- quired compared to in-line cleaning. It's a small cleaning unit and less expensive. The solvent could be costly and throughput is not as fast be- cause sometimes only one, two or a few boards can be processed at a time. Other special chemistries include foaming agents, rosin flux and several others used for extremely contaminated boards. However, the customer should keep several important points in mind. Those include understanding the dif- ferent types of fluxes available and how suitable they are for their PCB applications, the types of chemistries involved, the percentage of cleanli- ness their PCBs require, and how well their EMS provider or CM can quantitatively verify their percentage of cleanliness. reasons for Cleaning There are a number of reasons to provide customers with ultra cleanliness, and they all relate to maintaining high reliability. This is es- pecially true for mil/aero and medical electron- ics applications. For example, intermittent fail- ures can often stem from ionic contamination since it changes solution conductivity. In this case, dendrite growth resulting from ionic con- tamination can surface and cause shorts and opens. Dendrite growth is comprised of metal filaments that extend across small parts of a board. Dendrites are similar to tin whiskers that are created by chemical reactions and cause similar issues. Dendrite growth is particularly damag- ing when components are small and close to each other. For example, fine-pitch micro BGAs, micro CSPs, and other tight geometry packages are highly sensitive to dendrite growths. Testing EMS providers must contend with a long list of contaminants, and different tests are re- quired.. The most common is an ion chroma- tography meter. This system performs a high level of ionic contamination testing, as shown in Figure 3. It can qualify and identify ionic species to a remarkably precise level present on a PCB's surface. Other prevalent tests include the resistiv- ity of solvent extract (ROSE) test, also known as the solvent extract conductivity (SEC) test, IPC Standard TM650, and surface insulation resistance (SIR). The ROSE concept deals with resistivity. When ionic contamination levels in- crease, the resistivity of the circuit increases, as well. ROSE can measure that resistivity to deter- mine the amount of it on the circuitry, and then it instructs the operator on the steps to take to TiGhTEr SCrUTiNY NEEDED FOr pCB ClEaNiNG aGENTS continues figure 3: Ionic contamination testing required for different contaminants and different applied tests. Zulki'S PcB nuggeTS