BGA Repair in SMT Assemblies
All surface mount components, including BGA, require uniform heating of components and boards and a mechanism for accurate component placement. The pick-up tool can be a vacuum cup, tweezers or some other mechanical tool, but the vacuum pick-up system with vision capability is necessary for accurate BGA placement.
Preheat is important for all surface mount components; however, for BGA repair, it becomes critical because it is important to provide the thermal energy underneath the package to melt all the hidden balls simultaneously. Preheating the board is highly recommended to expedite component removal and to prevent thermal damage to the board.
Some preheat units also serve as a post-cooler after rework by blowing cool instead of hot air under the board. Faster solder joint cooling decreases the grain size, improving fatigue resistance. For preheat, stand-alone infrared (IR) lamp and hot plate preheat units can be used, but they may not be as effective as hot air systems for preheating the boards uniformly during rework.
Batch convection ovens for preheat before rework also is an alternative. However, they are inconvenient to use and not as effective because the boards generally cool off before rework is completed. So, if the rework machine does not have a built-in preheat capability, a portable stand-alone preheat unit with adjustable board holder is well worth the investment. Such a system is especially helpful during desoldering of through-hole devices connected to power or ground planes or metal cores to minimize the potential for thermal damage.
Laser, hot air and focused IR systems along with systems that simulate a convection oven profile commonly are used. No matter what type of rework equipment is used, the goal is to achieve adequate and uniform temperatures across the entire part.
Hot air rework systems have evolved from their origins in surface mount and also are used for reworking components other than BGAs. In some hot air rework systems, a thermocouple is mounted to the nozzle to monitor exhaust gas temperature. When using such systems, a correlation is developed between the exhaust gas temperature and the solder temperature under the package. When the thermocouple senses the temperature of the exhaust gas above a preprogrammed temperature (205º to 210ºC), it triggers the vacuum nozzle to remove the component from the board. Such a feature prevents picking up the component prematurely or waiting too long after the solder joints have melted. Unfortunately such features do not always work. There are other hot air systems that simulate convection oven profiles and heat the solder joints primarily from the package’s top. The main drawback of hot air systems is that they tend to reflow solder joints of adjacent components as well and require sufficient interpackage spacing to accommodate mini-stencils and hot air nozzles for each component to be removed. This not only requires added spacing for rework but also adds to the rework cost because custom nozzle and mini-stencils are needed for each size part.
In hot air rework, nozzle design is critical to obtain an effective rework profile. Generally, corner leads heat more quickly than center leads. Consequently, the corner leads reach the solder’s melting point before the center leads on each side of the package.
The temperature lag between the corner and center leads is greater for larger packages — as much as 20ºC for poorly designed nozzles. An excessive differential in temperature between the center and corner leads actually means that more heat than necessary is being supplied to the component body. It is better to have a uniform temperature throughout to prevent overheating the component body.
The main advantage of laser rework systems over hot air is that they allow tighter design for manufacture guidelines for rework spacing because they do not use custom nozzles and mini-stencils. The laser heats only the package to be removed for uniform heating without impacting adjacent components.
Regardless of the rework machine used, it is key to develop the rework profile for reliable component removal and replacement. To ensure that components or boards are not damaged, it is important to develop a unique profile for each component to be removed or replaced by any given equipment.
Some systems heat all four sides of a component more uniformly than others. It is important to control the heating rate in rework. The peak temperature of leads (or balls) during the rework procedure should be greater than the solder’s melting point in the solder joint. If the peak temperature is too high, however, the package or printed board may be damaged by excessive heat application. For eutectic tin/lead solder, this peak temperature should be in the range of 200º to 230ºC. Different equipment have different rework profiles for the type of component to be removed.
Additionally, keep in mind that there are six or seven (if BGA is to be reballed) steps in BGA repair. These include component removal, site preparation, reballing (if component is functional), flux or paste dispensing or printing, component placement, reflow, and final cleaning. Except for the reballing step, the BGA rework process flow is similar to the repair of any active surface mount component. A typical BGA rework process flow is as follows:
· Preheat the boards before rework.
· Dispense flux underneath the component.
· Heat the part to 210º to 220ºC and pick up the part by vacuum tip.
· Remove solder from board and package and clean surface.
· Apply paste or flux on balls or the substrate.
· Place new (or reballed) components.
· Preheat board again.
· Reflow to 210º to 220ºC
· Clean (skip this step if using no-clean flux or paste).
Because of the different ball compositions and attachment methods to the package, there are some major differences in the repair of plastic BGA (PBGA), ceramic BGA (CBGA) and tape BGA (TBGA) packages.
PBGA balls are made of eutectic solder with a melting point of 183ºC (or 179ºC if 2 percent silver is added). Because flux does not reach all balls evenly during the removal process, the PBGA balls break unevenly and the package is hard to salvage. Reballing the PBGA package is difficult. However, solder preforms (balls) in solvent or water-soluble carriers are available in a grid array matrix of balls to match the PBGA’s footprint.
Solder ball composition in CBGA packages is high-temperature 90 Pb/10 Sn (melting point of 302ºC), but the CBGA balls are joined to the package and the board with eutectic solder (melting point of 183ºC). During rework, the CBGA balls do not melt, but the eutectic solder between the ball and the package and between the ball and the board does melt. The balls may stay with the package or the board. In TBGA, however, the high-temperature balls are welded into the package by partially melting the high-temperature balls. Therefore, the balls stay with the package in TBGA.
Also, in CBGA and TBGA removal, the high-temperature solder in the balls mixes with the eutectic pad metallization on the package and the board, and raises the solder’s melting point on the lands of the board. Because the joint between the package and the ball goes through one more reflow cycle than the joint between the ball and the board, the melting point of the latter is slightly lower than the former. This explains why the balls stay with the package most of the time during removal, eliminating the need for reballing.
Both PBGA and TBGA packages are susceptible to cracking during rework (the “popcorn” effect). However, CBGAs are not susceptible to cracking because they are hermetic (do not absorb moisture).
All surface mount components, including BGA, require uniform heating of components and boards and a mechanism for accurate component placement.