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PCB007-Mar2020

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58 PCB007 MAGAZINE I MARCH 2020 Feature by Chris Wall ELECTRA POLYMERS LTD. Introduction Electra Polymers has been involved with de- veloping and manufacturing solder mask for over 35 years, starting with two-component epoxy-based materials, through single-compo- nent UV-curing products to LPISM, which are still in use today. Most of these products relied on screen printing as a means of application with some formulated for application by HPLV air spray, electrostatic spray, or curtain coat- ing. In this article, I provide an overview of the development of inkjettable solder mask and the equipment to apply it. The first attempts to develop an inkjet solder mask were in the early '90s. The technology, however, did not really take off at that time, due to inadequate performance from the heads and printers. This performance difficulty was, in part, due to a lack of the computing power to drive the multiple nozzles needed. Only rel- atively recently has the hardware been avail- able to make the inkjet solder mask process viable. This article briefly sets out the methodology used to develop and formulate an inkjet sol- der mask, the advantages of the process over the existing LPISM process, and the results achieved. Understanding the Inkjet Application Process The first part of the method is to under- stand the inkjet application process. As men- tioned before, the first attempts to develop an inkjet solder mask were nearly 30 years ago; printhead and printer technology has moved on dramatically in that time. The solder mask printheads used today are a drop-on-demand (DOD) piezoelectric type. With this type of printhead, a voltage pulse is applied to each nozzle, which causes the nozzle walls to flex and eject a droplet of ink (Figure 1). Creating the right type of pulse—in terms of frequency, amplitude, and duration—depends on the acoustic characteristics of the nozzle chamber and the rheological properties of the ink; it's also critical to achieving good jetting performance. This calibration and tuning are usually done by the head manufacture/print- The Advantages of Developing, Processing, and Using an Inkjettable Solder Mask

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