PCB and EMS Process Engineering: The Man Behind the Curtain

“Pay no attention to the man behind the curtain.”  This famous quote from The Wizard of Oz, conjurs up the image of Dorothy, the Tin Man, the Cowardly Lion and the Scarecrow discovering that the great Wizard of Oz isn’t as grand or as magical as he seems.  He is in fact just a guy operating a bunch of controls behind the green curtain.  Today, references to “man behind the curtain” imply someone making decisions and making things happen behind the scenes.  Process engineering at EMS companies and PCB fabricators could be considered “the man behind the curtain”.

I was speaking with Holly Olsen from Electronics System Inc, discussing the fact that customer’s visiting and touring their PCB fabricator or EMS supplier to learn the processes and challenges encountered when building this custom engineered product is something that does not happen nearly as often as it used to.  Without these visits, knowledge of some of the behind the scenes decisions that are made day in and day out to help ensure the best yields are easy to overlook.  Holley and I thought it would be interesting to touch on a few of the key processes and decisions that are made behind the scenes and are often invisible to the customer.

Panelization:

The deliverable is an assembled printed circuit board.  But, throughout processing at both the EMS provider and the PCB fabricator, product is manufactured in larger panels and then broken down to the final product.

Most EMS providers prefer to specify their own panelization.  This allows them to determine fiducial size and location, tooling holes and break off points that best suit their unique process and equipment needs.   The size of the board, components that overhang the edge of the board, and board shape and thickness all play a part in the design of the ideal panel for processing.

Similarly, the deliverable to the EMS company is an array of parts for their further processing.  During the PCB fab process, the size of the manufacturing panel and the placement of parts, or arrays within the manufacturing panel is one factor that is adjusted depending on technology of the design.  A typical fabrication panel will be 18” x 24” or 21” x 24”.  As technology requirements increase, the panel size used will decrease.  Panel size is typically reduced to 12” x 18” when tight features are required or when the process requires tighter registration than standard processing is expected to meet.  With the smaller panel size, there is less impact from standard material movement.  When processing on a 12” x 18” panel does not yield the anticipated results due to material movement, process engineering may suggest using the center or “sweet spot” on the panel to minimize that impact even further.

Processing Thin Materials:

Again, this is an area that both fabrication and EMS work their magic behind the curtain.  From the EMS perspective, PCB’s that are less than .031” or flex materials require additional support for processing.  Thin materials will flex and move during SMT.  A common method to stabilize the array is to create a SMT pallet.  Pallets will cycle through and be reused in the process.  The number of pallets needed is determined by the manufacturing lot size and SMT cycle time to ensure proper manufacturing flow is maintained.

SMT pallets must be made to withstand high temperatures and cannot conduct heat allowing them to go through the reflow process.  CDM Durapol ESD is a commonly used composite material that can withstand the high temperatures and includes static dissipative characteristics.  Tension pins are designed into the pallet to align with the tooling holes on the PCB panel securing the board in place through processing.

Similarly, fabricators take special precautions with thin materials.  Often special carrier panels are used to transport the product from location to location and operators are trained to handle materials picking up from opposing corners to eliminate flex in the material.  Any dents or ding in the copper will have a high probability of creating scrap as the circuit patter is created.  Because automated equipment is not specifically designed for thin materials, leader panels are often taped to the manufacturing panel to provide additional support moving through automated equipment.

Stencil Design:

“The screen printing process is one of the most critical steps in the SMT process”, says Kevin Buffington, Manufacturing Engineer for Electronic Systems, Inc., “The combination of a good stencil and solder paste inspection is vital to the outcome of placement and reflow.”  The screen printing process begins with a well-designed SMT stencil.  Proper volume and placement of solder paste is crucial to the reduction or elimination of solder defects such as insufficient solder, shorts, and solder balls.  This is achieved by choosing the right foil thickness and aperture size for the mix of components on the printed circuit board.  Stencil design parameters are developed based on aspect ratio and area ratios of components.  These ratios are a calculation of the size of the stencil opening and the stencil thickness that allows the solder paste to release.  While not preferred, in limited cases where some very large components are included on a design with very small, fine-pitch components, step stencils may also be used to ensure that enough solder paste is deposited for the larger components.  Step stencils, as the name suggests, step up the stencil thickness in a specific area to place a greater amount of solder paste.

Framed stencils can either be fixed frame or universal frame.  Stencils will typically range from 15” x 15” to 23” x 23” and use either a solid or hollow aluminum frame.  As the name suggests the fixed stencil is permanently fixed to the frame.  The universal frame, the stencil is held within the frame which makes it possible to make changes as they are needed.

 While IPC specifies best practices for component spacing in design, today’s designers are expected to do more with less space and more often need to push the envelope of standard processing.  Fabricators and EMS providers are continually refining their processes to meet tighter pitch components and tighter pitch trace and space.   Involving your PCB fabricator and EMS provider early in the design helps to ensure that manufacturability is designed into the product.  Even better, plan a facility tour and gain a little insight into what goes on behind the curtain when manufacturing PCB’s and PCBA’s!

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