Breaking down the key steps of board lamination, including the cool down step.
Historical analysis of foil and cap lamination methods of manufacturing.
Additional design advantages and fabricator challenges of sequential design.
The high temperature and pressure of a machine press forms a printed circuit board
Lamination is one of the penultimate steps of bare board manufacturing, and as such, there is an increased emphasis on correct processes so as not to scrap a functional board so close to final inspection. The board lamination process itself is, physically speaking, a stress-inducing procedure board materials must undergo to coalesce into a final product. As much as any point in the build, material properties must be carefully weighed between performance and producibility, and designers and fabricators will need to consult closely to optimize the performance of the board without sacrificing yield.
An Overview of the Board Lamination Process
The board lamination process is a culmination of the work performed on the individual layers, and the process can be divided into two complementary jobs:
Layup - A physical representation of the board stackup designed at the outset of the layout. A technician will carefully build the board from the bottom up, starting with the unetched bottom layer, before alternating one or more prepreg layers to etched internal layers. Layers are held in place using alignment pins to prevent layers from shifting relative to one another. Layup preps the board prior to fusing the material layers in the following step.
Pressing - The stack undergoes a controlled heat and pressure press to melt the prepreg. The prepreg coats the etched copper layers, forming the insulative layer necessary for the electronic layers to function as expected in close proximity. Prepreg also acts as a bond for the layers of the board, hardening after a curing stage to create the mold of the PCB.
The pressing process must account for several factors. To save time and energy, multiple PCBs can be laid up between the pressing plates, provided that a separator is placed between individual boards. In order to prevent malformation of the boards during the press, the separator must be able to comfortably withstand the high temperature and pressure environment without significant changes to the physical shape or state. To prevent voids from forming in the circuit board that could inhibit the desired dielectric effect or contribute to a loss of structural integrity, a vacuum must be formed inside the laminating press.
The post-press environment must also be reckoned with; to avoid a quench at STP in atmospheric conditions, the pressed boards must be stored in a cool down press. As the press is heated, “cool down” might seem like a misnomer, but the press more slowly cools the boards compared to ambient temperatures to prevent warpage from thermal contraction.
A History of Lamination Told by Foil and Cap Methods
Lamination styles come in two flavors: foil and cap.
Foil is by far the preferred choice for a variety of reasons: it is a simpler, less intensive process with lower material overhead. Foil lamination builds the layup using foil on the top and bottom layers. Following lamination, they are then etched in the same process the inner layers were etched. While in adoption for decades of PCB production, it is in fact the newer of the two styles. Foil layers must be carefully selected to encourage a proper peel strength during removal. Further, operators should be mindful that the lamination conditions replicate the variables used by the laminate supplier. Much like the designer and operator collaborating to ensure the complexity of the design matches the shop’s capabilities, operators should contact material manufacturers so their production processes align.
Cap lamination, the original lamination method, uses a copper-clad laminate layer between the top and first inner layer as well as the last inner and bottom layer. This method is useful when the design calls for blind vias on the aforementioned layer pairs. Additional board performance constraints might require a specialty laminate between the outermost and adjacent layers.
Sequential Lamination Offers Advanced Via Designs
Blind and buried vias heavily influence the lamination process. Since drilling and plating occur following the lamination step, any vias that are not through hole must be added prior to the ultimate lamination step. This leads to sequential lamination or the process of laminating subsets of the board before final lamination.
Sequential lamination has to contend with far greater coefficient of thermal emission (CTE) issues, as single-step lamination does not have any voids or copper extending in the z-axis while undergoing the high heat and pressure necessary to fuse the board together. Notably, standard dielectrics possess a CTE value several factors greater than copper. When heated, this creates several high-pressure zones around the via barrel and pad, which can lead to delamination, surface cracking, and open circuits. Several design and material precautions should be undertaken to accommodate a successful sequential lamination:
- Z-axis CTE - The most important factor in reducing z-axis stress is procuring materials with lower z-axis CTE values to more closely match that of the surrounding dielectric.
- Glass transition temperature - The Tg value indicates a transition in the material (though not an outright phase change) whereby the hard crystalline structure gives way to a more viscous and deformable state. Generally, higher Tg values will help reduce stress, but beware the upper echelon of Tg values, as these may not perform well with the sequential lamination process.
- Copper retention - This design practice is encouraged for a standard lamination and even more so for sequential. The less removed copper on a layer, the more uniform the CTE expansion. Copper fills can be added to low copper density builds to compensate for the loss.
Whatever the needs of your board, the importance of proper material selection for both the designer and fabricator in the board lamination process is key. Cadence’s suite of PCB design and analysis software offers a wide catalog of tools to accurately analyze and design boards to meet the technological challenges of today’s dense and high-speed world. When you’re ready to begin a board, OrCAD PCB designer features powerful stackup tool integration to get your build off on the right foot.
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