Issue link: https://iconnect007.uberflip.com/i/1489778
12 DESIGN007 MAGAZINE I JANUARY 2023 between the package and feature sizes of 484- ball 1 mm pitch BGA (U100) and a 100-ball 0.4 mm pitch BGA (U101). First, we look at the 1 mm pitch part, then evaluate what it will take to escape all balls to the outside perimeter of the BGA. For this evaluation, we will only consider the use of stacked microvias. Staggered microvias will also work, but require significantly more board area. Additionally, the stackup will assume dual stripline for all internal signal layers. e first challenge in routing the BGA is to define how many HDI (sequential lamination/ microvia) layers will be needed. Figure 2 shows a structure with every layer of a 28-layer board as an HDI layer. is, however, is not produc- ible with current HDI fabrication techniques— more on this later. e basic technique to escape route the BGA is the quadrant method. With this method, we will divide the BGA into four quadrants and then use a standard pattern in each quadrant. Figure 3 shows the basic quadrant pattern. Once the quadrants are defined, we will route the BGA in two-row patterns. e out- side-most rows will be routed straight. e inside row will be routed in the direction of the quadrant, and then straight. Here the two outer-most rows will route on the top layer. e next two rows will route on the first internal signal layer, and so on. Figures 4 and 5 show the routing of first internal signal layer. Figure 2: A structure with every layer of a 28-layer board constructed as an HDI layer. This is not possible with today's fabrication processes. Figure 3: A quadrant pattern used to escape route a BGA.