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86 The PCB Magazine • May 2017 Introduction Several defects that are related to the desmear process are presented in this month's Trouble in Your Tank. Inadequate or excessive desmear will lead to several PTH defects and failures. Resin smear, in- effective texturing of the resin, and even over- ly aggressive desmear will contribute to poor plating, adhesion failures and a myriad of other non-conforming defects. However, proper trou- bleshooting protocol dictates that the engineer also looks at drilling as the contributor to these and other defects. As an example, drilling can cause torn out glass bundles, extremely rough hole walls as well as excessive smear. Poor drill practice may also lead to wedging at the B-stage- to-copper foil interface. These are just a few of the defects that are presented in this column. Root cause of these defects and the subse- quent effect on PTH quality and reliability are also presented. 1. Excessive Etchback Description: Excessive glass and excessive positive etchback. Characteristics: Can lead to plating folds, barrel cracking, glass voids, and uneven copper plating in the holes. Possible Causes: a) Temperatures of the solvent and permanganate steps are too high. b) Dwell times in the solvent and permanganate steps are too long. c) Concentrations of the solvent and/or permanganate steps are too high. d) Excessive in feed rates (too high of a chip load in drilling). e) Drill is punching its way through the stack. Besides the excessive etchback on the sec- tion at the right, there is concern about long- The Desmear Defect Guide TROUBLE IN YOUR TANK by Michael Carano RBP CHEMICAL TECHNOLOGY Figure 1: Excessive etchback (right) and glass fiber bundle protrusions (left). (Source: IPC-9121 Process Effects Guide)

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