BGA Reworking: The Reflow Process

BGA Rework Reflow

One of the most critical processes for BGA rework is the reflow process. The redistribution process occurs after the old device has been removed and the site has been prepared. The replacement device is replaced using a flux or paste fitting. The reflow process in BGA rework must emulate the manufacturing process as closely as possible. Given the thermal mass of the board in and around the BGA, the profile should either match that of the replacement solder balls (if the device has been refunded and will be used as a replacement device) or should closely match the profile. from the device provider. printed solder paste data sheet.

There are several “rules of thumb” when zeroing in the reflow profile process. It’s good to learn about PCB thermal characteristics when trying to mark up a reflow process profile. One of the best ways to “learn” the thermal characteristics of the PCB when there is only one PCB and no profiling board is available, is to use what is learned during the stripping process to help “mark up” a reflow process. . Often times, a BGA rework technician will use a standard profile to remove the device, tweaking or adjusting the profile based on the results obtained. If a solder sample is available that allows the BGA technician to embed thermocouples in the solder balls (one corner and 1-2 other places depending on package size), in the die, around the BGA, near other components etc all together with learning the thermal characteristics of the device and plate during the reflow process. The proper method to embed these thermocouples is high temperature epoxy for attachment to the device or to the interior of the printed circuit board. Another “rule of thumb” for reflow profiling is to ensure that for lead-free profiles, the solder joint, as seen by the temperature at the corners and other balls, is above the liquid for a period of 60 to 90 seconds. Tin-lead solder should be above liquidus for a period of 30 to 45 seconds.

Not only must the profile be correct and confirmed by temperature measurements, but the components in and around the BGA being reflowed must be protected. This is especially tested when using a hot air source and for device that is sensitive to heat, including but not limited to ceramic capacitors, plastic connectors, batteries, and MELF. Also, devices with insufficient padding or components with TRVs or glue around them should be watched and protected, as these materials will soften and potentially damage the entire board. Protection against heat sources, especially when using a hot air reflow source, comes in many different flavors. One of the most widely used but least effective types of shielding is Kapton™ tape found in many areas of the SMT process. It has been shown in various studies on this subject to be an effective type of heat shielding material. Other more effective sources include a water soluble gel or a ceramic based non-woven material. Whatever type of heat shield material is used to protect neighboring devices during reflow, its use is important to protect devices from damaging excessive heat.

To run a full profile, the printed circuit board must be adequately supported. This is especially true in case there is an “unbalanced” copper section of the plate or in cases where plates of very thin thickness are reflowed. be severely deformed, making it difficult to place components, or have a reliability issue regarding solder joints. There are a variety of plate support systems on the market and most high end rework systems offer a flexible plate mounting and support system design.

Not only is proper board support required, but proper heating of the underside of the boards will help ensure minimal temperature differences across the board and a reduced propensity for board warping. Modern BGA rework systems are equipped with sophisticated bottom side heaters. Advancements to ensure the reflow process is optimized include multi-zone bottom side heaters. These heaters allow the user to have the rework area at a higher temperature than the rest of the PCB, reducing the chance of the board warping during the reflow process.

A typical thermal profile of a lead-free hot air source is shown below. First, underside heating begins to heat the plate with one temperature (typically 160 or 170 C) at the rework location and another, usually 150 C, in other areas of the plate. During the time this temperature is applied to the underside of the board, the nozzle temperature begins to rise during the “ramp” period of the reflow profile. Too fast a ramp can damage neighboring components or the laminate. A “soak” phase then begins, which slows the ramp rate and begins to activate the flow. After this phase, the liquidus temperature is reached, somewhere between 205-220C. This begins the reflux zone. In this, the maximum temperature is reached and the rework location “sees” a temperature that is above the liquidus temperature for a minimum of 60 and up to 90 seconds. The reflow profile ends with a cooling zone. The cooling zone cannot be so extreme as to cool the board where the negative temperature gradient can cause brittleness in the solder joint at the end of the reflow process.

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