BGA Rework - The Process of Reflow

BGA work on Reflow

One of the foremost essential methods for BGA work on is that the process of reflow. The reflow method happens once the previous device has been removed and also the website prepped. The replacement device is replaced with exploitation either flux or pastes attachment. The reflow method in BGA work on ought to emulate the producing method as closely as attainable. Given the thermal mass of the board in and around the BGA the profile ought to match that of the replacement solder balls (if the device has been reballed and can be used a the replacement device) or match as closely as attainable the profile of the device vendor's knowledge sheet of the solder paste written.

Their square measure many "rules of thumb" once zeroing within the method of reflow profile. it's sensible to find out concerning the thermal characteristics of the PCB once making an attempt to dial in a very reflow method profile. one in all the simplest ways that to "learn" concerning the thermal characteristics of the PCB once there's just one PCB and there's not an identification board accessible, is to use what's learned throughout the removal method to assist "dial in" a reflow method. repeatedly a BGA work on technician can use a regular profile so as to get rid of the device, tweaking or adjusting the profile supported the results achieved. If there's the supply of a solder sample that permits the BGA technician to enter thermocouples into the solder balls (a corner, and one or a pair of alternative places looking on the scale of the package), into the die, round the BGA, close to alternative elements etc. bushed conjunction with learning concerning the thermal characteristics of the device and board throughout the reflow method. the right technique for embedding these thermocouples is high-temperature epoxy for his or her attachment to the device or internal to the PCB. Another "rule of thumb" for the reflow profile is ensuring that for nonleaded profiles the solder joint, as seen by the temperature in each corner and alternative balls, is higher than liquidus for an amount of 60-90 seconds. Tin-lead solder ought to be higher than liquidus for an amount of 30-45 seconds.

Not solely ought to the profile be correct and be confirmed through temperature measurements however the elements in and around the BGA being reflowed have to be compelled to be protected. this is often particularly tried once employing a hot air supply and for the device that is heat-sensitive together with however not restricted to ceramic capacitors, plastic connectors, batteries, and MELFs. additionally, devices with underfill or elements with TRV or glue around them ought to be watched and guarded as these materials can become soft and probably run all of the board creating an outsized mess. Protection from the warmth sources, particularly once employing a hot air reflow supply comes in many various flavors. one in all the foremost common-used however least-effective forms of protection used is that the Kapton™ tape found in several areas of the SMT method. This has been shown in many studies on this subject to be LEASE effective sort of heat shielding material. alternative more practical sources embody a soluble gel or a ceramic-based nonwoven material. regardless of the sort of heat shielding material accustomed defend neighboring devices throughout reflow, their use is very important to protective devices from excessive heat that injury the devices.

In order to run a whole profile, the PCB ought to be adequately supported. this is often very true of there square measure "imbalanced" copper section of the board or in cases wherever terribly skinny.032" thick boards square measure being reflowed. while not adequate board support, there could also be board distortion which can injury inner layers or cause the board to be badly ill-shapen creating placement of elements troublesome or have a reliableness downside with relevancy the solder joints. There square measure a spread of board networks on the market with most higher finish work on systems providing a versatile board mounting and support system style.

Not solely is adequate board support needed however correct bottom aspect heating of the boards can facilitate guarantee lowest variations in temperature across the board and a lesser propensity for board warpage. fashionable BGA work on systems square measure equipped with refined bottom aspect heaters. Advancements for creating positive the method of reflow is optimized embody multizone bottom aspect heaters. These heaters enable the user to possess the work on space at the next temperature than the rest of the PCB thereby reducing the chance of board warpage throughout the method of reflow.

A typical nonleaded, hot air supply thermal profile is seen below. First, all-time low aspect heating begins to heat up the board with one temperature (typically a hundred and sixty or 170C) being within the work on location and another, generally a hundred and fifty C, being at alternative areas of the board. throughout the time this temperature is applied to an all-time low aspect of the board the nozzle temperature begins to climb throughout the "ramp" amount of the reflow profile. Too quick a ramp could injury neighboring elements or the laminate. Then a "soak" section, that lowers the ramp rate and starts to activate the flux, begins. once this section, the liquidus temperature, somewhere between 205-220C is reached. This begins the reflow zone. In this, the most temperature is reached and also the work on location "sees" a temperature that is higher than the liquidus temperature for a minimum of sixty and as long as ninety seconds. The reflow profile ends with a settle down zone. The settle down zone can't be thus extreme thus on cool the board wherever the negative gradient could cause breakableness within the solder joint at the top of the method of reflow.