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22 The PCB Magazine • November 2016 The panels are now ready for exposure or transportation. A finished panel after plate/ coat/expose/develop/etch is shown in Figure 4. Perfect Registration Now with the advent of direct digital im- aging (DDI) and board scanning, it is possible to have perfect registration on both sides of a board. There are DDI units that have nine cam- eras that scan a panel as it enters the machine from both sides. Then, as it starts to expose each side, the artwork is modified on-the-fly so that each land is perfectly registered to its hole. Without high-speed computers, scanners and the new software, this was impossible before. Novel Russian Technique At a recent PCB West conference, I had a con- versation with a Russian PCB fabricator. It seems the Russians also used landless vias after they saw them in Japanese computers. The Russian process was novel and I wish I had thought it up! They used the property of dry film photore- sist's sensitivity to oxygen. When they laminate the dry film to the panel, they have a shroud around the unit filled with oxygen. Thus oxy- gen is trapped in the holes by the dry film. They expose the panel with their normal artwork (but there are no lands for traces that will intercon- nect to the landless vias). They then do a short 100°C bake of the panels, remove the cover sheet on the dry film, and develop normally. The oxy- gen and heat has polymerized the dry film from the inside at exactly the same diameter as the hole. Thus when developed, the via has perfect dry film registration and is landless. More details, including figures that show how landless vias are more reliable than vias with small annular rings, can be found in the recently published article, "Against the Density Wall: Landless Vias Might be the Answer." [5] Swing Vias for BGA Breakout A new concept resulting from HDI is to ar- range the blind via rotation not in the normal north-east-west-south rotation but to move them into the same channel and stagger them. This arrangement forms a broad boulevard be- tween the buried vias 4x larger than the device pitch. This boulevard is 4X and can permit a vast freeway for global routing. The "swing"concept is explained further in an article I wrote with Charles Pfeil in 2011: "The principle is to 'swing' the vias away from the ball pads forming wide boulevards on the innerlayers for routing. Depending on the size of the micro-via, the pin-pitch and size of the ball pads, you may be able to align the mi- cro-vias into straight columns and rows. This will maximize the available space for routing." [6] Boulevards Enhanced by Via Structures Boulevards can be enhanced by the 3D via structure. The various depths of blind vias help to create larger boulevards. These structures can be created by skip vias, multiple build-up or sequentially laminated drilled and laminated vias. This is also a via structure you will want to have because it provides routing of high-speed critical nets using only the cross-over from hori- zontal to vertical of small low-inductance blind vias. These are the lowest inductance vias in the board and ideal for the highest-speed nets. These will also have a very high density because the crossover will be small blind vias, not the larger buried or through-hole vias. Boulevards Created by Via Placement A new concept in applying different types of fanout patterns for BGAs has been developed Figure 4: Landless via [0.010 in (0.25 mm) and .004 in (0.1 mm) traces] created with the liquid, electrophoretic, positive photoresist from PPG. (Source: Compunetics Inc.) INNOVATIVE USE OF VIAS FOR DENSITY IMPROVEMENTS