Usr6 Posted May 8, 2017 Report Posted May 8, 2017 Basic Structure of a USB The first task is to remove the USB logic board from its enclosure. Oftentimes there is a seam that can be pried open with a plastic spudger tool. The board will likely be held in place by a few plastic latches or with adhesive. Once we have removed the logic board from its enclosure we can examine the board for any obvious signs of damage. Indicators of damage could range from melted components, scorch marks, bad solder joints, or cracks in the logic board. While one could attempt to repair observed damages, we will instead transplant the NAND storage chip to a functioning same model device. For this post, I tore down two USBs I had close at hand. One older unmarked 2 GB USB and a newer 8 GB SanDisk Cruzer. Both devices are pictured below. Both devices are made up of the same basic anatomy. The primary components of a USB that we will concern ourselves with are the USB connector (1), the USB controller (2), and the NAND storage chip (3). The SanDisk has its USB connector integrated with the logic board as opposed to the soldered on USB connector more commonly seen with most USBs. If the USB connector is damaged there will likely be obvious damage to the gold-plated pads within the connector or the solder joints connecting it to the logic board. A typical USB has four gold pads each corresponding to a specific signal: power, data -, data +, and ground. The gold-plated tabs should be straight, flat, and free of any residue. The solder joints where the connector meets the logic board should be holding the connector firmly in place. There should also be continuity between the gold pads and the solder joint where that electrical signal meets the board. If there is any apparent damage or the connector is not secured to the logic board, reflowing the joints may solve the issue. Failing that, one could use hot air (i.e., a hot air rework station) to remove the defective USB connector and replace it with a functioning one. The USB controller typically comes in a TQFP (Thin Quad Flat Package1) package with leads on all four sides of the chip. Discussing how to diagnose and fix issues with the controller is out of scope here. The NAND chip(s) houses all of the data on the USB. These chips are fairly durable and in most scenarios are not damaged. These chips are often one of two packages: TSOP (Thin Small Outline Package2) or BGA (Ball Grid Array3). In the photos above, both chips have a TSOP-48 NAND memory chip. The number 48 represents the total number of leads on the chip where two sides contain 24 leads each. These chips are easier to work with than their BGA counterparts where the leads are underneath the chip rather than on the side of it. Some USBs have more than one NAND chip. In that case, both NANDs would need to be swapped. In our scenario, we will discuss the steps necessary to transplant the NAND chip from a non-functioning USB to a same-model counterpart. Let’s get started. Recovering Data from a USB With the NAND chip(s) identified, let’s discuss how to remove the chip from the board. There are a number of different methods we could employ, but the end goal remains the same: melt the solder joints holding the chip in place long enough to remove it from the board. The most popular method would be to use a hot air or IR rework station. Other methods, like using a low temperature solder, exist and are worth exploring to determine the best tool for the job. Each come with their own pros and cons. When heating an electronic component to high temperatures there is always a change it may be damaged in the process. In addition to that, care needs to be taken to avoid overheating other components on the board. This is especially true when using a hot air gun. With that said, let’s continue our discussion on the assumption we have elected to use a hot air rework station. Depending on the composition of the solder used it will likely melt (reflow) around 190°C. When solder reflows, it takes on an observable shiny characteristic. Applying flux to the leads will facilitate the reflow process. The exact temperature, air flow speed, and nozzle to use is setup dependent. Practice on test devices to get a feel for the appropriate setting. Aim to reflow and remove the chip from the board after 10-20 seconds of sustained heating. If the reflow station has a preheater, the board can be heated up to near reflow temperatures to decrease the amount of time high heat needs to be applied. Preheating also allows for the board to be more evenly heated rather than heating a localized area which may stress and damage the board. With the appropriate temperature determined, apply hot air a few centimeters above the leads, taking turns to hit each side. It is critical that when using tweezers, or some other tool, to lift the chip up (once the leads have reflowed) to not apply much pressure. If there is resistance stop and do not continue pulling up on the chip. Continue applying heat. Ignoring this advice can result in tearing off pads which will certainly result in numerous headaches. Low-temperature solder is much safer with the chip but takes more time and leaves a mess on the board. The process involves applying low-temperature solder liberally on the existing solder joints and create a horizontal stream of low temp solder spanning across all of the leads on each side of the NAND. Heating this mixture of primarily low-temp solder with just a soldering iron keeps it reflowed for ~10 seconds, long enough to reflow both sides and easily remove the chip. Whichever method is preferred, remove the NAND chips from both the original and donor boards. Make sure to inspect the original chip’s leads for solder bridges (this is more likely to occur with the low-temp solder method). Solder bridges occur when one or more leads are connected with solder causing a short between those leads. Any such shorts must be removed prior to swapping the NAND onto the donor board. With the original NAND chip inspected, we can now swap it onto the functioning donor board. As a brief aside, if you have a chip programmer, and it supports your NAND, you can read directly from the chip without needing to perform this last step. Before swapping the NAND onto the donor board, inspect the board to make sure all pads are intact and there are not any solder bridges. It is also recommended to either even out or, preferably, remove solder on each pad with desoldering wick so the chip lays flat and in contact with all pads when placed on the board. Let’s discuss two different options for soldering the NAND onto the donor board. We can either use the hot air rework (or IR) station or manually solder the chip with a soldering iron. Manually soldering the chip is safer for the chip as you are not applying heat directly to the chip itself. This method is more time-consuming. Hot air is just the inverse of the process employed to remove the chip. Once complete, inspect the leads one more time to ensure there are no inadvertent electrical connections between leads. In addition to this, use a multimeter and check for continuity between each lead and the pad it connectors to to ensure all are making sound electrical connections. If all went well, reattempt acquisition of the device. Ideally it should now be recognized by your machine and allow you to image it. If that is not the case, reinspect the leads and rule out inadvertent electrical connections. Verify that the host and donor are the same make and model with similar board design. Know that this technique is potentially destructive. Therefore, ensure you practice this in test scenarios before applying it to casework. Sursa: https://dpmforensics.com/2017/05/07/extracting-data-from-damaged-usbs/ 2 Quote
