Trouble in Your Tank: The Desmear Defect Guide


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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, ineffective texturing of the resin, and even overly aggressive desmear will contribute to poor plating, adhesion failures and a myriad of other non-conforming defects. However, proper troubleshooting 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 subsequent 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.

 

To read the full version of this column which appeared in the May 2017 issue of The PCB Magazine, click here.

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