BGA Rework - The Process of Reflow

One of the maximum crucial tactics for BGA rework is the process of reflow. The reflow system takes place after the previous tool has been eliminated and the website online prepped. The substitute device is changed using both flux or paste attachment. The reflow technique in BGA rework must emulate the manufacturing process as closely as viable. Given the thermal mass of the board in and around the BGA the profile have to healthy that of the alternative solder balls (if the tool has been reballed and can be used a the substitute tool) or fit as carefully as viable the profile of the device seller's facts sheet of the solder paste revealed. There are numerous "guidelines of thumb" whilst zeroing within the procedure of reflow profile. It is ideal to find out about the thermal traits of the PCB whilst trying to dial in a reflow procedure profile. One of the exceptional methods to "analyze" approximately the thermal traits of the PCB while there is simplest one PCB and there is not a profiling board to be had, is to apply what's found out during the removal system to assist "dial in" a reflow procedure. Many instances a BGA transform technician will use a widespread profile to be able to do away with the tool, tweaking or adjusting the profile primarily based on the results achieved. If there is the supply of a solder sample which permits the BGA technician to embed thermocouples into the solder balls (a nook, and 1 or 2 other places depending on the dimensions of the package deal), into the die, across the BGA, close to other additives etc. All along side learning approximately the thermal traits of the device and board during the reflow procedure. The proper method for embedding these thermocouples is high-temperature epoxy for their attachment to the tool or internal to the PCB. Another "rule of thumb" for the reflow profile is making sure that for lead-loose profiles the solder joint, as seen with the aid of the temperature in both corner and other balls, is above liquidus for a duration of 60-ninety seconds. Tin-lead solder have to be above liquidus for a length of 30-forty five seconds. Not simplest need to the profile be correct and be showed through temperature measurements however the components in and across the BGA being reflowed need to be covered. This is specifically attempted whilst the use of a hot air supply and for the tool which is heat-touchy inclusive of however now not restrained to ceramic capacitors, plastic connectors, batteries, and MELFs. In addition devices with underfill or additives with TRV or glue round them must be watched and guarded as those substances will become smooth and potentially run all the board creating a huge mess. Protection from the warmth assets, mainly while the usage of a hot air reflow source comes in many exceptional flavors. One of the most not unusual-used but least-effective forms of safety used is the Kapton™ tape located in many areas of the SMT process. This has been proven in several research in this subject matter to be LEASE effective sort of heat protecting cloth. Other extra effective assets encompass a water-soluble gel or a ceramic-primarily based nonwoven fabric. Whatever the kind of warmness defensive material used to defend neighboring gadgets in the course of reflow, their use is critical to protecting devices from immoderate warmness which damage the gadgets.

In order to run a whole profile, the PCB have to be effectively supported. This is specially real of there are "imbalanced" copper phase of the board or in cases where very thin.032" thick forums are being reflowed. Without ok board support, there may be board warping which can also harm internal layers or motive the board to be badly deformed making placement of components difficult or have a reliability hassle with appreciate to the solder joints. There are a variety of board guide systems available on the market with most higher stop remodel structures imparting a bendy board mounting and guide gadget layout. Not simplest is ok board support required however proper bottom facet heating of the forums will help make sure minimal variations in temperature throughout the board and a lesser propensity for board warpage. Modern BGA transform systems are ready with sophisticated backside facet heaters. Advancements for ensuring the system of reflow is optimized include multizone bottom side warmers. These heaters permit the person to have the rework location at a better temperature than the remainder of the PCB thereby decreasing the likelihood of board warpage for the duration of the technique of reflow. A standard lead-loose, hot air supply thermal profile is seen beneath. First, the bottom aspect heating begins to warm up the board with one temperature (generally 160 or 170C) being inside the rework location and any other, normally 150 C, being at different regions of the board. During the time this temperature is carried out to the bottom facet of the board the nozzle temperature begins to climb for the duration of the "ramp" length of the reflow profile. Too rapid a ramp may damage neighboring components or the laminate. Then a "soak" phase, which lowers the ramp price and starts offevolved to spark off the flux, starts offevolved. After this segment, the liquidus temperature, somewhere among 205-220C is reached. This begins the reflow sector. In this, the most temperature is reached and the rework place "sees" a temperature which is above the liquidus temperature for no less than 60 and so long as 90 seconds. The reflow profile ends with a cool down sector. The cool down sector can not be so severe so that you can cool the board in which the poor temperature gradient may additionally reason brittleness in the solder joint at the give up of the procedure of reflow.