Chemical or laser etching coupled with electrical test is a formidable combination. by DON DAVIS
Decapsulation is a process that has been around for some time and has many different applications. Whether for analysis or verification purposes, the goal is the same: expose the die to perform the task at hand. A few other methods are available to achieve this. Some of the most common are manual etching, fully automated etching, plasma etching, and the newest (and more advanced) laser etching. My exposure to decapsulation has involved all the methods above; however, manual etching has become my primary choice for exposing the die. During more art form than skill, decapsulation has the advantage of leaving the device in question functional, unlike destructive methods such as micro sectioning, which literally saws the component apart. Many factors affect and ultimately determine success. A key aspect of using chemical etching decapsulation revolves strictly around temperature. Too much heat and the acid will react aggressively, destroying the part. Too little heat and the acid-to-mold compound reaction will be slow and messy. Next is acid deposition. Knowing exactly how much acid to use makes the process controllable, and predictable, and makes it much faster, too. Duration will always impact the functionality of a device once the die has been exposed. A final key element is device placement. Where and how the part is positioned ultimately sets the stage for a good or bad decap. While skill is important, exposing the die successfully does require certain items. My primary weapons of choice when performing decapsulation are a hot plate, fuming nitric, and/or fuming sulfuric acid. Both acids are equally aggressive, but not all package mold compounds are the same; therefore, knowing how to choose the right acid will ultimately determine success. So how do you know which one to choose? There is no single answer to that question, except trial by error. The acid could either be nitric or sulfuric, and the choice varies depending on the overall appearance of the mold compound surface. Some units require a mechanical approach to expose the die, while others will use slightly different acids. Whichever the case, visual interpretation ultimately regulates the outcome. Acetone and alcohol are used to help neutralize the chemical reaction between mold compound and acid. Whichever of the two acids is selected sets the stage for choosing the correct solvent. It’s a carefully scripted balancing act between acid application, a neutralizing rinse, and hand-eye coordination that makes manual etching somewhat difficult, but also amazingly unique. The entire pro- cess involves 100% control, and the yield, if done correctly, is easily repeatable. Time is solely based on experience, and can take only minutes to complete.