Panel Plating

A) Coating of unit items (panels, plates, disks, etc.) B) Coating of semifinished articles (strips, sheets)... 

FIGURE 5.6 Key manufacturing steps in panel plating and pattern-plating methods. 

The panel-plating method is ideal for bare copper board. However, it is a difficult way to make padless via holes, which are becoming more popular. Generally, the conductor width of 0.004 in is considered to be the minimnm realizable by this method for mass prodnction. 

Althongh the use of the panel-plating method in the United States and western Enrope is limited, nearly 60 percent of the PTH boards in Japan are manufactured by this method. 

FIGURE 27.23 Via fill flow in a noncap plate process (panel plate/button plate). 

FIGURE 27.24 Via fill flow in a cap-plate process (button plate/panel plate). 

Panel Plate entire panel surface and holes. 7 mils minimum... 

Note Panel plating increases the overall thickness of copper for final etch. 

An innerlayer detail is essentially a thin double-sided printed circuit. The standard innerlayer process contains no plated holes because it is produced by usiug a priut-and-etch process. Bliud, buried via layers aud laminated cores contain holes that must be plated in either a pattern-plate or a panel-plate process. Figure 27.31a shows a typical flowchart for four innerlayer process options. Processes 1 and 2 support standard innerlayers, and processes 3 and 4 can be used for buried via iuuerlayers or core subassembhes. All four processes start with a bare copper-clad laminate and end with a patterned double-sided circuit.The patterned circuit must be iuspected and treated to enhance adhesion prior to further ML-PWB lamination. All four of these sequences work equally well with any of today s materials systems. 

There is no aspect ratio limitation and the need for controlled-depth drilling is completely eUminated.To be able to driU bUnd vias prior to lamination, a clad outer stack-up is required. One side of the outer component layer is patterned prior to lamination, while the other is patterned after lamination.This means that when the blind via is metallized, the unpattemed side is blanket-metallized. This is true for either pattern plate or panel plate. This outside layer is metalUzed again when the holes in the finished ML-PWB are metallized. The result can be very thick plating on the exterior of a bUnd via board. To minimize this problem, the fabricator should plate the blind via iimerlayer with the minimum possible current density on the blanket-metaUized side. 

Electroless copper The type chosen will depend on the method of image transfer as well as on the need for panel plating. These processes are readily automated. The largest percentage (estimates are as high as 95 percent) of printed circuit board manufacturers worldwide rely on the electroless copper method for hole metallization. 

The electrolyte compositions for via filling baths typically run with high concentrations of copper (up to 50 g/1 as metal) and lower concentrations of acid (approximately 100 g/1). These concentrations are juxtaposed to high-aspect ratio plating.Thus, these process usually are utilized in a panel plating mode for filling vias only. This type of process usually requires some type of planarization step post-filling to decrease the amount of copper plated on the surface. 

Horizontal plating gives snperior surface thickness distribution. Variation of less than 10 percent across a panel-plated surface is normal. This is due to the anode s close proximity to the cathode usually, the anode-to-cathode spacing is between 8 and 10 mm, or 0.3 and 0.4 in. 

To date, most horizontal eqnipment is designed aronnd panel plating. Pattern plating has its advantages and is being worked on by a series of eqnipment snpplieis and conld be available in the near fnture. 

A formidable challenge facing acid copper plating of printed circuit boards is thickness distribution on the surface of the panel (whether pattern or panel plated) and thickness distribution in the barrel of the hole or the via, namely throwing power. If the throwing power is 100 percent, then plating 1 mil of copper in the hole would result in plating 1 mil on the surface. 

At 50 percent throwing power, to achieve 1.0 mil in the hole would result in 2.0 mils on the surface, which would limit the etching capabihties in the panel plate and increase the thickness variation in pattern plate. 

Table 31.1 shows representative high-tech MLBs fabricated by means of additive technology. Innerlayer patterns of all MLBs listed in the table are made by the foil process. The out-erlayer patterns are formed by the panel-plate additive process, which is the subject to be discussed in the following section. 

Perform fully electroless copper deposition (panel-plate). 

It should be cautioned, however, that there is a limitation to this panel-plate additive approach, even though the thickness of deposited copper film is much more even across the panel. If very fine conductors are to be formed by this method, the conductor cross section becomes intolerably distorted from the ideal rectangular shape as the ratio of the width to the height of the conductor approaches unity, as shown in Fig. 31.6. 

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