Mixed leaded and Pb-free PCBs may require nonstandard profiles.

Intermittent BGA connections are a commonly noted problem. When pressure is applied to the center of the BGA, performance improves. Several issues produce this symptom: solder paste reflow profile, printed circuit board design, solderability, microcracks, black pad, or poor adhesion of pads to substrates.

To identify the cause of the failure, review the board finish and ball alloy. If the assembly is a mix of SnPb and Pb-free components, it is possible that an incorrect solder paste reflow profile was used. While SnPb solder pastes can be used with SAC BGAs, the standard SnPb reflow profile should not be used. SAC BGAs need a hotter profile in order to collapse the SAC solder balls. If the profile is too cool, the lead in SnPb pastes will not fully mix with the SAC balls and good intermetallics will not form. This would cause inferior solder joints to form between the balls and lands.

In a completely Pb-free assembly (such as this case, where the Pb-free BGAs were soldered with SAC paste), it is important to verify the solder and reflow profile. If the EMS firm assembles Pb-free and SnPb assemblies at the same facility, it is possible that an incorrect solder or profile was used. Nondestructive testing such as x-ray fluorescence (XRF) can confirm use of Pb-free solder while x-ray inspection can be used to look for shorts and proper reflow of solder balls. Void calculations of the solder balls can confirm the void percentages meet IPC standards (25%).

Certain types of BGAs may require nonstandard Pb-free reflow profiles. Super BGAs have metal backs that can impede interior balls from reaching reflow temperature and forming good solder joints to bond pads. Even large plastic BGAs may not experience enough heat for the interior solder balls to reflow properly. Poor interior solder joints may produce symptoms of an intermittent open that improve under finger pressure to the BGA center. Tweaking the reflow profile or preheating the board may help the interior solder balls to reflow.

The board design may also interfere with solder ball reflow. A large ground plane near the BGA may conduct enough heat from the BGA to prevent proper reflow. Boards designed for conductive cooling may have a similar issue. Preheating the board and adjusting the reflow profile will reduce this problem, but a board designed for extreme conductive cooling may become a BGA soldering challenge.

Confirming incomplete solder ball collapse, large voids, shorts and poor solder intermetallics requires destructive analysis, such as cross-sectional analysis and dye and pry test. Destructive failure analysis is required to investigate less obvious failure mechanisms such as black pad, micro-cracks (Figure 1) and poor pad-to-substrate adhesion.


Dye and pry test.  The board and BGA are immersed in red dye and a vacuum is applied. The vacuum permits the dye to penetrate micro-cracks. After the dye dries, the BGA is pried from the board and is examined optically (Figure 2). The presence of red dye on surfaces indicates cracks were present prior to removing the BGA (Figure 3). The way the solder joint fractures also indicates whether there was strong wetting to the pads and good solder reflow. A clean failure between the BGA land and substrate could indicate thermal shock from reflow, wave soldering or rework.


Cross-sectional analysis. A solder ball row is cut and polished and the cross-section examined with a metallographic microscope and SEM. Solder and land composition are investigated with electron dispersive spectroscopy (EDS). The presence of expected intermetallic compounds confirms good wetting of the solder to pads. Cross-sections can also confirm minimal voiding in the solder balls. Cracks (Figure 1), and the presence of large amounts of phosphorous in the intermetallics between the land and ball, may indicate black pad failure mechanisms. Finally, the pads are examined for good adhesion to the substrate.

The American Competitiveness Institute (aciusa.org) is a scientific research corporation dedicated to the advancement of electronics manufacturing processes and materials for the Department of Defense and industry. This column appears monthly.