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AUGUST 2019 I PCB007 MAGAZINE 79 to the electrical connection needed for electro- lytic plating and any leads for electrical con- nection will remain as a part of circuitry like an appendix. This could result in some para- sitic elements that can disturb the circuit per- formance. The SAP uses a thin copper base layer for the electrolytic plating. This provides an ad- vantage compared to a fully additive electro- less plating method. There are various means of achieving thin base copper, as described in Table 1. The etched copper foil provides the easiest accessibility (Table 1, #1). This copper foil is sold by many manufacturers with 12-micron foil the most commonly used, balancing cost and foil thickness. Before patterning, the foil is uniformly etched to reduce the overall start- ing thickness. A sulfuric acid with hydrogen peroxide system is a good etchant for this purpose. The foil has enough length of tooth to get adhesion for most of the resin system; however, this tooth length limits the etched down copper thickness to around 3–5 microns as a minimum. Many high-density intercon- nect (HDI) PCB designs use this method for consumer products, such as cellphones, moth- erboards, laptops, desktop PCs, etc. The ultrathin copper foil is also available as a commercial product (Table 1, #2). It is usu- ally less than 5 microns and is handled with a carrier material. The carrier material, such as copper foil or aluminum foil, will be removed from the surface when the foil is laminated to the substrate. The foil tooth for adhesion is necessarily limited to get good thickness control of the base foil, so the physical adhe- sion is not like regular foil. Also, tooth prepa- ration could limit the achievable thinness of the copper foil due to the foil manufacturing process. An alternative adhesion improvement is a primer coating underneath the foil. This promotes adequate copper adhesion, but it could be subject to additional UL testing and affect electrical properties because of its direct contact to the conductor surface. This foil is mainly used for package substrates because of the cost and achievable performance balance; it does not fit for most consumer products for this same reason. Electroless copper plating (chemical copper deposition using wet process) can be used for the SAP [1] (Table 1, #3). The copper plating process is the same as conventional electroless copper plating in the PCB process, but treat- ment to the base laminate is necessary to get good adhesion in general. A chemical desmear process is commonly used. Once the substrate surface has prepared texture for the adhesion, the catalyst for the electroless copper plating is applied followed by electroless copper plat- ing. The electroless copper needs to be 1.0 mi- cron or more for good current distribution in the panel during the subsequent electrolytic copper plating process. The chemically depos- ited copper has a higher etching rate than the electrolytic copper [2] and control of the etch amount is important to prevent undercutting of the electrolytic copper circuit. The sputtering for the base copper deposi- tion is not generally used for the SAP (Table 1, #4). Although this method provides the thinnest copper for the base conductor, the process becomes a hurdle for PCB manufac- Table 1: SAP base copper types showing advantages and disadvantages.