Decapsulation Process Guide


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A tutorial for IC Decapsulation recipe development.

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Decapsulation Process Guide

  1. 1. Elite Etch Basic Decapsulation Process Guide 1
  2. 2. 2
  3. 3. Contents Characterization of Plastic Packages—4 The characterization process is comprised of only eight steps.—4 Acid Choice:—4 Acid Temperature: ––5 Determine Correct Fixturing: —5 Determine Pulse or Reciprocating:—5 Determine Heat-up Time:—6 Determine Etch Volume:—6 Rinse Time:—7 Etch Time:—7 3
  4. 4. Characterization of Plastic Packages This guide will show you in simple steps the art of characterizing and opening virtually any plastic package for decapsulation. New users and veterans alike share one common dilemma, sample etch characterization. Characterization is the process of determining the correct etch recipe and fixturing to successfully decapsulate a plastic package. The first thing to remember is: All plastic packages follow the same characterization process no matter what type of package you are trying to decapsulate. The characterization process is comprised of only eight steps. 1) Determine Correct Acid Type 5) Determine Heat-up Time 2) Determine Temperature 6) Determine Acid Volume 3) Determine Fixturing 7) Determine Etch Time 4) Choose Pulse / Reciprocating 8) Determine Rinse Time While everyone ultimately follows these eight steps, using the eight steps in the correct order greatly reduces the time of characterization. The following paragraphs will explain how each of the steps are performed and can be optimized for a successful decapsulation. Acid Choice: Every package has an optimum acid for decapsulation. The first rule in determining the acid type is to start with Fuming Nitric Acid. Nitric acid uses the lowest temperature of all of the acid types usable in the system. The lower the temperature the less chance of thermal stress being introduced to the sample. If decapsulation is not sucessfull with nitric acid, the next step would be trying the six mixed ratios. If decapsulation is still not sucessfull finally use either 98% Sulfuric or 20% Fuming Sulfuric Acid. For this first step any definition gasket that has an opening that is 50% of the size of the package may be used to determine the correct acid type to be used. Common Acids Used For Decapsulation: TYPE USES 90% Fuming Nitric Least expensive, Lowest grade that will decapsulate plastic packages. 98% Fuming Nitric Faster Etch Rate, Much less metal damage, Wider temp range. Red Fuming Nitric Will work in the system. Efficacy has not been tested. 96 to 98% Sulfuric Least expensive, Lowest grade that will decapsulate Hi-Temp packages. 20% Fuming Sulfuric Faster Etch Rate, Much less metal damage, 4
  5. 5. Determine Etch Temperature: The basic rule for determining the temperature is as follows: The closer the etch temperature is to the boiling point of the acid the more reactive the acid will be, or put simply the higher the temperature the faster the etch. Conversly, The higher the temperature the more likely there may be metal damage. Therefore the correct tem- perature would be high enough to have a fast etch rate with no metal loss. Standard starting temperatures are: HNO3 75 to 80c Mixed 80 to 90c H2SO4 225 to 250°c In packages where unpassivated metals are used, a different technique is required. The temperature range should be between 30 and 50°c in order to protect the metalization. The etch rate will be much slower, but the metal will be preserved. Determine Correct Fixturing: Once the acid type and temperature have been determined, the correct fixturing can be chosen. The definition gasket is determined by the acid type used for decapsulation. When nitric acid is use for decapsulation the etch will be isotropic. In other words the deeper the package, the larger the hole is going to be. With an isotropic etch the definition gasket hole must be slightly smaller than the die size. This also applies to mixed acid to a lesser extent. Sulfuric acid etches from the center outwards and therefore a gasket that is slightly larger than the die size is required. Whenever possible the use of a pocket gasket will optimize the fixturing creating a precice location and cavity definition in a single gasket. Determine Pulse or Reciprocating: The choice of pulse or reciprocating etch determines the relative angle of the side wall. A reciprocating etch will produce a relatively straight side wall, while pulse will produce a more rounded side wall. Reciprocating requires an excellent seal between the etch head, gasket, and device to be etched. Any leak in any of the mating surfaces will render reciprocating inoperable. Any leaks will be visible in the waste line. This can be seen because the waste acid will appear to be slowly moving toward the waste bottle while there is no pump operation. Reciprocating works with nitric acid, mixed acid, and non-fuming sulfuric applications. Recriprocating etch does not work with applications using only fuming sulfuric acid. Use Pulse Etch only with fuming Sulfuric Acid applications. 5
  6. 6. Determine Heat-up Time: Heat-up time is the amount of time that is required to preheat the device to match the etch temperature selected. Efficient and reproducible etching will not occur until the part is at the same temperature as the acid. Programming of the heat-up time is dependent upon the temperature of the entire heat exchanger assembly and safety cover. Common indication of incorrect Heat-up Time: Decapsulation of first device opens correctly. Second device opens a little more than expected. Third device is almost over etched. Fourth or fifth device requires a reduction in etch time. The diagram below shows what is occuring and how to correct the problem. For this example, the first device opens correctly. Program Settings Heat-up Time 20 Sec. Etch Time 60 Sec. First Device: Everything except the etch head is cold. Device takes 60 sec to heat up. Etching takes 20 sec. Heat-up Time 20 Sec. Pumping Acid without Etching 40 Sec. Etch Time 20 Sec. Second Device: Entire Etch area is warmer. Device only takes 40 sec to heat up adding 20 sec to etch time. Heat-up Time 20 Sec. Pumping Acid without Etching 20 Sec. Etch Time 40 Sec. Third Device: Entire Etch area is Hot. Device only takes 20 sec to heat up adding 40 sec to etch time. Heat-up Time 20 Sec. Etch Time 60 Sec. Correct Setting Heat-up Time 120 Sec. Etch Time 20 Sec. By using 120 seconds of Heat-up Time almost all devices will start the etch process at Etch Time 1.* * Note: For all devices that have a heat sink, place the device on a hot plate at 85% of programmed tempera- ture for 30 minutes to ensure that the 120 second Heat-up Time will start the etch process at Etch Time 1. Determine Etch Volume: The etch volume is the amount of acid used in ( ml/min ) that is programmed in the etch time. Having too low of a volume requires a longer etch time than necessary. Having too high of a volume wastes the acid. The volume is determined differently when using nitric or nitric/sulfuric mixes when compared to sulfuric acid although both share a similar characteristic in that they each have waste material carrying capacity based on the reactivity and heat of the acid. The reactivity of a given acid is determined by the chemistry and by Etch Temperature. As a general rule when using nitric acid start with a volume of 3 ml/min. The color of the waste material will determine the correct volume. If the waste acid is light brown to clear the volume is too high. If it is dark brown and not moving freely the volume is too low. If the waste acid is brown and moving freely the volume is correct. Sulfuric acid requires a higher volume. The carrying capacity of the acid is influenced by the temperature of the acid, which reduces as the waste acid moves away from the etch head. Therefore if the color of the waste acid is light brown to clear the volume is correct. If it is very dark brown and not moving freely the volume is too low. If the waste acid is dark brown and moving freely the volume is still too low. 6
  7. 7. Etch Time: The etch time is the amount of time ( in seconds ) that the device is actually being etched. The easiest way to determine the etch time is to run a sample of the package for a short period of time. For SO and TSOP packages start with 10 seconds; for all other packages 60 seconds works well as a starting time. These are your base etch times. Examine the package opening. If the die is not exposed, decapsulate the package again using the above base etch time. Examine the package again. Continue to re-decapsulate the package changing the etch time to a shorter etch time only when you reach a decapsulation point at which it seems probable that an additional time increment (10 seconds or 60 seconds) will over etch the package. Next add all of the etch times together and subtract 3 seconds for each time the package was etched. ( The subtraction of the 3 seconds is to account for the rinse time. ) Run another sample package using the calculated etch time and make minor corrections as necessary. The Elite Etch operates with a waste cooling system which increases the back pressure to the heat exchancer. This back pressure will increase the efficiency of the acid by holding the acid on the device for a longer period of time. This is especially true of parts that are opened using 20% Fuming Sulfuric Acid where the etch rate may be up to five times faster than other systems. Rinse Time: The rinse time serves two functions. The first is to clean the part with cold acid and flush the part and waste line before nitrogen flush. The second, and very important function, is to make very small changes to the etch cavity. Start the characterization with the rinse time set to 12 seconds which is the middle of the programmable range. If the rest of the recipe is characterized you can make small changes to perfect the etch. For example if the pack- age is very slightly over etched, reduce the rinse time to make the cavity perfect. If the package is dirty or very slightly under etched, increase the rinse time in order to make the cavity perfect. The operator has the choice of Nitric, Sulfuric, or No Rinse for Nitric packages, Nitric, Sulfuric,MIxed or No Rinse for Mixed acid packages, or Sulfuric, or No Rinse for Sulfuric packages. As the Elite Etch has active pressure monitoring, there is little likelyhood of damage to the system during the purge process due to clogging of the system. 7