Products
Integrated Circuits
Clock/Timing
Data Acquisition
Embedded
Interface
Linear
Logic
Memory
Power Management (PMIC)
Connectors, Interconnects
AC Power Connectors
Backplane Connectors
Banana and Tip Connectors
Barrel Connectors
Blade Type Power Connectors
Card Edge Connectors
Circular Connectors
Coaxial Connectors (RF)
Contacts
D-Sub, D-Shaped Connectors
FFC, FPC (Flat Flexible) Connectors
Fiber Optic Connectors
Filters
Relays
Whether you are making DYI surface mount PCBs or a PCB prototype factory that makes SMT PCBs, solder paste is always used. It is applied to the SMT pads and then passed through a reflow oven to solder the electronics firmly on the copper surface. Aiming at applying solder paste on the SMT pads, the most economical way is to use SMT stencils. The question at hand is how to punch holes in these SMT stencils. In this section, we will discuss three different methods of making SMT stencils.
In this method, you could choose to use a thin 4-mil stainless steel sheet. After completing the PCB design, turn off all other PCB layers except the SMT pad layer (the isolation layer). Exporting to a .dxf file or any other format you know can easily make a 2D CNC file. Most CNCs will understand G-code. Then, you can use your favorite software to generate the G-code file. A good enough choice is the Mach 3 program. Alternatively, you can search for an online G-code file converter or prepare your G-code according to your SMT pad size. The CNC should be well set up with a suitable PCB cutting tool. From this point, once the G-code is loaded on the CNC, it will comfortably cut away the steel specified by the G-code to make through holes in the steel plate. In this way, you can easily use CNC to make SMT stencils.
Another way to make SMT stencils is by using chemical etchants. In this case, we will still use a 4-mil stainless steel sheet for SMT stencils. In this method, instead of exporting a .dxf file from the PCB Design software, you can export a .png file with a real 1:1 scale. Unlike the CNC .dxf file, it is suggested that you make the SMT pads white and other parts black. This file represents your mask. Don’t forget to mirror this .png file if needed. Various methods can be used to transfer this mask to the steel plate. These methods include toner transfer and UV exposure. Toner transfer is usually used when preparing small PCB prototypes, while UV exposure is preferred for large-scale PCB prototypes and can also be used for small prototypes. It is recommended to wash the steel plates with acetone or alcohol before they transfer the mask to the steel plate. After the mask is well transferred into the stencil, the rest is covered with tape to protect it from corrosion. The sheet is then dipped into a hydrogen peroxide solution, which eats away at the exposed areas; the SMT pad areas are white. In a short time, a perfect SMT stencil aperture is created, and the SMT stencil is ready for use after cleaning.
This method is similar to that used in CNC, except that the heat from the laser is what is used to remove the material. The heat generated by the laser is enough to vaporize or melt the material. The benefit of using a laser is that you have freedom of choice for a range of materials for your SMT stencil. For plastic sheets, you can choose laser etching or laser engraving. Extra care should be taken to avoid degradation of the finished hole quality due to the laser heat-affected zone. This can be achieved by balancing the laser current with the laser cutting speed.