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APRIL 2022 I DESIGN007 MAGAZINE 69 by Happy Holden The semi-additive processes (SAP) are not new. I first used them with a novel process back in 1978. MacDermid had a novel SAP process called PLADD II (plated additive). It was an anodized aluminum foil applied to laminates that we could easily etch off after drilling and continue with a special electroless copper for thin metallization. In his Tech Talk column, Karl Dietz wrote about SAP many times from 2000 to 2010. 1-5 In a 2010 col- umn, Karl included a table to show the relationship between copper thickness, resist thickness, and resolution capabilities of processes. 5 Karl devoted many Tech Talks to "fine-line imaging" and to related topics such as photoresist adhesion, developing, fine-line etching, stripping, and pattern plating. Semi-Additive Processes The older mSAP processes used thinned cop- per, usually to nine or five microns, from half-ounce foil or very thin copper foil (usually with a peelable process is oen better than a subtractive pro- cess for conductor shape consistency. How- ever, even with the additive process, there can be concerns for trapezoidal shaped conduc- tors. It is usually a flipped trapezoidal shape as compared to the same concern when using the subtractive process. Another item which can assist in consistent conductor shape and etching control is using thinner copper. inner copper will naturally give less trapezoidal shape and controlling the etching of the conductor width and space is easier. Rogers Corporation has several high frequency circuit materials which are used in mmWave applications with a laminate available with one-quarter ounce (9 µm) copper. Using this laminate with very thin copper can be ben- eficial for the PCB fabricator to have more con- sistent conductor shape, width, and spacing. Millimeter-wave applications are more sen- sitive to circuit feature variability, as well as small circuit anomalies, which may have been acceptable for lower frequency applications. e PCB fabricator is challenged to control conductor widths and spacing to a tighter requirement and the shape of the signal con- ductor also needs to be more consistent. Addi- tive processing at the PCB fabricator can be beneficial for these mmWave circuit require- ments and using a laminate with thin copper can also be advantageous. DESIGN007 John Coonrod is technical marketing manager at Rogers Corporation. To read past columns or contact Coonrod, click here. protection). They would usually have a flash copper strike and may use tin plating as the etch resist. The IC substrates (for flip-chips) were always the leading edge of this technology, from 2005 onward, but quickly converted to the use of the Ajinomoto build-up film (ABF) from Japan that was additive electroless on the bare-etched, vacuum laminated dielectric films. The advanced-modified SAP (amSAP) processes did away with the copper strike and further exploited the thin-copper foils. Averatek's new A-SAP TM (or pure additive) starts with a treated copper foil on the laminate, but after drilling, the copper foil is etched away. This allows a new generation of nanoparticle Liquid-Metal Ink TM catalysts to be used to prepare the surface for fine- grain electroless copper application. After pattern electroplating and resist stripping, this electroless copper, being only around 0.7 to 1.2 microns thick, permits a flash etch without any etch resist. To read this entire column, which appeared in the January 2022 issue of PCB007 Magazine, click here. Happy's Tech Talk Semi-Additive Processes and Heterogeneous Integration

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