Recently, several customers called the EMPF Helpline with issues of intermittent Ball Grid Array (BGA) connections...

Recently, several customers called the EMPF Helpline with issues of intermittent Ball Grid Array (BGA) connections.  When pressure was applied to center of the BGA, performance improved.  Several different issues could 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, first review the board finish and ball alloy.  If the assembly is a mix of tin lead (SnPb) and lead (Pb) free components, it is possible that an incorrect solder paste reflow profile was used.  While SnPb solder pastes can be used with SAC (Tin-Silver-Copper) BGAs, the reflow profile should not be the standard SnPb profile.  SAC BGAs need a hotter profile in order to collapse the SAC solder balls. If the profile is too cool, the Pb in SnPb solder pastes will not fully mix with the SAC balls and good intermetallics will not form.  This would cause the formation of inferior solder joints between the balls and the lands. 

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

Even when a standard lead free reflow profile is used, certain types of BGAs may require non-standard lead free reflow profiles.  Super BGAs have metal backs which can interfere with the interior balls reaching reflow temperature and forming good solder joints to the 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 when the center of the BGA experiences applied finger pressure.  Tweaking the reflow profile and / or preheating the board may help the interior solder balls to reflow properly.   

The board design may also interfere with proper reflow of the solder balls.  A large ground plane near the BGA may conduct enough heat away 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 for the manufacturing engineer. 

Confirmation of incomplete solder ball collapse, large voids, shorts, and poor solder intermetallics requires destructive analysis, such as cross sectional analysis and dye and pry test.  In addition, destructive failure analysis is required to investigate less obvious failure mechanisms such as black pad, micro-cracks (Figure 2-1), and poor adhesion of pads to substrates. 

• Dye and Pry Test: The board and BGA are immersed in red dye and a vacuum is applied.  The vacuum allows the dye to penetrate micro-cracks.  After allowing the dye to dry, the BGA is pried off the board and is examined optically (Figure 2-2).  The presence of red dye on surfaces indicates that cracks were present prior to removing the BGA (Figure 2-3).  The way the solder joint fractures also indicates whether or not 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 is examined with a metallographic microscope and scanning electron microscope (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 the pads.  Cross sections can also confirm minimal voiding in the solder balls.   Cracks (Figure 2-1), 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 EMPF offers a full range of lab services that support BGA failure investigations.  These services include XRF, X-ray examinations, solderability tests, cross sectioning, SEM/EDS, and dye and pry tests.  The EMPF also offers board design reviews for manufacturing issues, such as excessive cooling and manufacturing process development support.  More information about these services can be found on the EMPF web site, http://www.empf.org or by calling the EMPF technical staff at (610) 362-1320.