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70 SMT Magazine • September 2017 the stencil is peeled away, leaving hardened re- placement mask in its path. Solder mask repair pens are labeled as simple- to-use PCB repair tools which can replace sol- der mask. The pens can be simply drawn across the areas requiring solder mask. The liquid ooz- es out of the soft-tipped pen and then can be air or heat-cured. The pen tips themselves are large with respect to modern BGA pads site pitch, making it a challenge to precisely dispense the mask material. Due to the porous nature of the dispensing tip, it tends to pick up debris from the board (including flux residue, remnant mask and cleaning agents). The soaked up de- bris then can re-contaminate other areas of the board. The skill level for someone making sol- der mask repairs using this technique will be at the advanced level; however, the repair quality will tend to be low. The most common method used for IPC Class 2 and 3, if allowed, solder mask repairs is via the use of liquid replacement solder mask. In this method, a skilled technician selective- ly brushes/dabs a small amount of solder mask onto the damaged areas underneath a BGA (first removing the BGA) being careful to not get the material onto the component or solderable ar- eas. Depending on the type of mask used, this mask can then either be heat or UV-cured. On the fine-pitched devices of today the exacting placement of the replacement mask requires a great deal of dexterity, requiring the use of a mi- croscope. As a time-saving enhancement to the above technique, a stencil can be designed and fab- ricated using the GERBER files of the board. This will help selectively cover solder mask un- derneath a specific location underneath the BGA. The stencil is peeled off its carrier back- ing, aligned, then placed on to the PCB surface. Replacement mask is squeegeed with a micro squeegee across the surface of the stencil and then cured. After curing, it is removed in an ionizing environment as not to cause any latent static charge damage to the components in the area. This method, while time saving for multi- ple boards requiring the exact same area of re- pair or for a very complex large area of repair, requires a repair technician with an advanced skill level. The last approach to repairing solder mask is using a semi-permanent stay-in-place BGA sten- cil. This stencil, while having the benefit of be- ing a simple way to place a BGA, also provides isolation between pads and prevents shorting between the IO. This will fix the standoff height between the base of the BGA and the PCB to control collapse height, while serving as a mask band-aid. Once in place the stencil acts like a solder mask repair stencil (Figure 7). This allows even the beginning repair technicians to repair damaged solder mask underneath the BGA. Pad Damage Another common trouble spot is the po- tential for pad damage while the BGA is being reworked. Pad damage is usually the re- sult of improper site preparation which can be a result of numerous processing problems. Items on this list are contaminants on the solder wick, improper tip temperature, too much pressure of the soldering iron on the solder wick materi- al, improper solder tip selection or expired flux material. Improper temperature profiles found in the BGA removal process would also be a contributing factor to pad lifting. Finally, there may be cases where the original adherence of the pad to the laminate was insufficient due to problems in fabricating the PCB. The proper procedure for the preparation of BGA pad sites after a BGA has been removed is outlined in IPC 7711 procedure 4.1.3 and re- TOP 5 BGA REWORK CHALLENGES TO OVERCOME Figure 7: Stay-in-place stencil for solder mask repair.