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Ink jets replace screen printing

Posted: 09 Mar 2009     Print Version  Bookmark and Share

Keywords:Screen printing  Solder masks  Ink jet printers 

Screen printing was developed in China roughly a thousand years ago. More recently, it has been the most frequently used method for printing various materials onto printed circuit boards. Its widespread use is based on its relatively low cost, simplicity and dependability. Although it is certainly not high technology, screen printing has been a key element in microelectronics assembly for decades.

The screen template is made by using photographic processes to develop the needed pattern, in a photo imageable material, pressed onto the screen. During use, a squeegee moves across the screen to push ink through the unpatterned areas onto the board. The ink deposited onto the board is allowed to dry. Periodically, the screen must be cleaned or replaced. The ability of screen printing to print in high resolution—i.e., to print extremely fine lines—is fairly good, but probably not good enough to keep up with shrinking printed wiring boards and their shrinking feature sizes.

Despite its long history, one of the problems with screen printing is the number of steps involved, as shown in Figure 1.

Figure 1: Screen printing process flow.

Screen printing is commonly used for printing three items onto the copper/laminate printed wiring boards: etch resists, solder masks, and legend (or ident) information. The etch resists are liquids (sometimes fairly viscous) that contain acrylic monomers, polymers, dyes. Solder masks are resist materials that protect the traces and prevent solder bridging during high-temperature assembly processes. In their formulation, solder masks are similar to etch resists, but have greater ability to withstand high temperatures. If a board is wave soldered, the solder mask prevents the traces from flowing together when heated. If a board goes through surface-mount reflow, the solder mask gives the same kind of protection.

Both etch resists and solder masks can also be laid down by the dry film process, where one of two protective sheets enclosing the dry film is removed, and the dry film laminated onto the board.

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