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14 The PCB Magazine • November 2016 VIAS FOR DUMMIES • Type IV—Plugged and Covered: A Type III via with a secondary covering of material ap- plied over the via • Type V—Filled: A via with material ap- plied into the via targeting a full penetration and encapsulation of the hole (Figure 3) • Type VI—Filled and Covered: A Type V via with a secondary covering of material ap- plied over the via • Type VII—Filled and Capped: A Type V via with a secondary metallized coating cover- ing the via (Figure 4) These type identifications simply specify how the different via structures are catego- rized, even though the term 'via' is commonly used generically or in combination with the 'through,' 'blind,' or 'buried' adjectives men- tioned above. Additionally, the microvia is another via structure that is commonly used for board in- terconnections and it is, in essence, just a blind via with dimensional requirements. More spe- cifically, a microvia cannot have an aspect ratio greater than 1:1 and cannot have a total depth into the board greater than 0.25 mm. Figures 5 and 6 show examples of microvias. Switching gears, qualifying one's ability to manufacture reliable via structures is, obvious- ly, accomplished through testing. These tests generally focus on thermally stressing a spe- cially designed coupon in an attempt to 'open' a connection. After all, the main failure mode associated with vias is that of discontinuity…an interruption in the electrical signal that the via has been constructed to transfer. For reference, two common IPC test meth- ods for this type of investigation are: • IPC-TM-650, method 2.6.26A [2] — DC Current Induced Thermal Stress • IPC-TM-650, method 2.6.27 [3] — Thermal Stress, Convection Reflow Assembly Simulation The reasoning for these stress tests, as well as others that can be used, is to evaluate the ability of the via structure's connections to withstand expansion and contraction within and around the structure itself. The thermal cycling nature of the test/exposure could result in an open circuit that can be diagnosed elec - trically and/or a mechanical separation that can be observed visually (Figure 7). This visual Figure 3: Type V via, filled. Figure 4: Type VII buried via, filled and capped.