NEWS // FEATURES // NEW PRODUCTS In both cases, considering the solubility characteristics
is key to choosing the right cleaning agent / equipment combination to have a well-designed cleaning process. As mentioned in the previous section, because the solubility characteristics of pastes and adhesives are different, we would need to consider a different type of chemistry to be effective cleaning both of these soils.
II. Substrates Obviously, stencils are the substrates. We need to consider the foil material and if the stencil is attached to a frame, we will need to take into account compatibility with the mesh, epoxy and the frame itself (typically aluminum). Occasionally, there will be a few misprints. In this
case, we also need to consider the PCB surface fi nish and whether they are A-side misprints (no refl owed components) or B-side misprints (refl owed components). A-side misprints can be typically cleaned in any standard
stencil cleaning process with a DI water rinse. I recommend a De-Ionized water rinse to eliminate any possibility of ionic residues being left on the surface. If they are B-side misprints, my recommendation is to clean them in your PCB cleaning (defl uxing) process, as the stencil cleaning process would get too complicated and less cost effective.
III. Chemistry (Rs) In chapter two of this series, we learned that for any cleaning process we need to consider the Process Cleaning Rate (Rp) which is equal to the chemical energy (Rs) plus the mechanical and thermal energy (Rd). We also learned that it is critical to understand the balance between them to create and to maintain a stable cleaning process over time. The
chemistry to be considered needs to satisfy several parameters, including matching well with existing cleaning equipment. If the user is purchasing new cleaning equipment, it is my recommendation that you choose the equipment type that allows the most fl exibility to evaluate
24 EIU
a wider range of chemistries to achieve the optimal results. In many cases, the user already has existing cleaning
equipment. If this piece of equipment is not designed to do what is needed to meet the cleaning requirements, then we will have to rely more on the chemical energy. What this means is that our cleaning process may
not have the optimal conditions to produce the cleaning results we need at the lowest cost of ownership. Cost of ownership is not only related to the price of the chemistry, but it involves other factors like time needed to clean (throughput), energy costs, waste disposal cost, environmental factors, etc. Although solvent-based chemistries are still used
in automated stencil cleaning machines, most of the chemistries are water based. The use of solvent-based chemistries requires the use of equipment with more robust safety (i.e., explosion proof, etc.) and environmental (limited exhaust, etc.) features.
IV. Equipment (Rd) Cleaning equipment is the second part of the equation. It will provide the mechanical and thermal energy needed to achieve the expected cleaning results. As mentioned above, the most common types of
stencil cleaning equipment use Spray-In- Air or Ultrasonics. While some will provide thermal energy (temperature) too, most stencil cleaning processes are done at ambient temperature (20°-25°C). Rinsing is not always present. Since the rinse cycle, is designed to remove any cleaning agent and residues left on the surface of the stencil (or misprinted PCB), the absence of this cycle will limit the choices we have in terms of cleaning agents. It will also, prevent us from cleaning misprinted PCBs unless we add a separate DI water (or solvent) rinse.
www.electronicscomponentsworld.com /
www.electronicsproductionworld.com January 14th 2012
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