PCB007 Magazine

PCB007-June2022

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60 PCB007 MAGAZINE I JUNE 2022 Etch Rate Between the two etchants, alkaline etchant dominates in etch rate. Alkaline normally has an etch rate of 2–2.5 mils/min, whereas cupric chloride typically has an etch rate of 1–1.6 mils/min. 1, 2 is is one of the reasons why alkaline is favored more than cupric chloride. Since etching usually takes a large portion of time in the making of a PCB, having this reduc- tion in etch time can nearly double your pro- duction. For you to get the same production level with cupric chloride, you would need to invest in a longer etch section of your pro- cessing line, which would mean increasing the number of sequential etch chambers. Regeneration As seen in Table 1, both etchants can undergo a regeneration reaction to restore the etchant's quality. Although they both have the capabil- ity, cupric chloride has an upper hand because the regeneration reaction is simple. Regenera- tion for cupric etchant, in most cases, involves simply feeding your reagents into the etcher. For alkaline etchant, it is more complicated because the regeneration reaction is depen- dent on oxygen, which it gets from air flow- ing through the machine. is can be tricky because you need an optimal flow of air through your machine. For alkaline etchants, it is important to have a sustainable amount of ammonia in your solution (see "chemistry control" below). If air flow through the etch- ing machine is excessive, there will be a rapid loss of ammonia. On the other hand, if you do not have a high enough airflow, your etch rate will drop faster as you etch more pan- els. ere is another form of regeneration for alkaline etchant that is done through liquid- liquid extraction. 3 It is an efficient way to regenerate alkaline etchant, but it is not com- mon. is is because of the additional capi- tal cost and the required know-how to make it work properly. Oen it is considered eas- ier to send the used alkaline etchant to a com- pany that will regenerate it, with liquid-liquid extraction, for resale. Etch Factor Another matter where alkaline etchant is highly favored is etch factor, the ratio of down- ward etch to sideways etch. Alkaline etchant offers the benefit of a 4-to-1 etch factor (mean- ing it etches downward four times as much as it etches sideways). Cupric provides a standard 3-to-1 etch factor (Figure 1). Alkaline etchant's better etch factor opens the opportunity to maintain finer spaces and Figure 1: Comparison between alkaline and cupric etchant for the etching of one-ounce copper. Image is for purposes of demonstrating expected values from etching. Alkaline etch values are based on ideal operating conditions.

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