Recently, a customer called the EMPF Helpline after observing intermittent opens on their Thin and Fine Pitch Ball Grid Arrays (TFBGA)...

Recently, a customer called the EMPF Helpline after observing intermittent opens on their Thin and Fine Pitch Ball Grid Arrays (TFBGA). Their Printed Wire Assembly (PWA) is a lead free design and the TFBGAs were soldered with SAC305 (Sn 96.5%, Ag 3.0%, Cu 0.5%) solder paste.

X-ray inspection of the identified failure locations on the TFBGA indicated that there may have been non-wetting between the paste and the solder balls. This type of non-wetting is seen in Figure 2-1 and is referred to as “head and pillow”. Confirmation of “head and pillow” failures requires destructive analysis. The EMPF micro-sectioned the failed TFBGAs to examine the possible failure location (Figure 2-2).

Optical microscopy indicated that the solder paste never wetted the solder balls. Further analysis using high magnification Scanning Electron Microscopy (SEM) imaging (Figure 2-3), clearly shows a gap between the solder ball and solder paste. The pads were observed to be well adhered to the substrate with minimal voiding in the solder balls. The failed ball joints did not show cracks or separations at the solder to pad interface or at the component to solder interfaces. Energy Dispersive Spectroscopy (EDS) analysis of the bulk solder confirmed that lead free, Sn based solder was present in both the ball and paste regions.

SEM/EDS analysis at the component to solder interface (Figure 2-4) detected the presence of Sn and Ni intermetallic compounds (IMC). These compounds indicate that sufficient heat was applied to obtain the necessary melting and dissolution required for good solder joint formation. In addition, SEM/EDS analysis at the solder to pad interface detected the presence of Sn and Cu, another indication of an IMC formation which confirms good wetting of solder to the pads and components. Due to the lack of phosphorous and cracks in the intermetallics, the EMPF concluded the failure mechanism was not related to “black pad”. The destructive failure analysis confirms that the intermittent open failures are the classic “head and pillow” case of non-wetting between solder balls and solder paste. This is usually the result of a non-optimized reflow profile allowing the solder paste to finish reflowing and the flux to completely volatize before reflow occurs at the solder ball. The observation that only certain solder joints displayed the “head and pillow” appearance may be due to these solder joints being located near more thermally conductive traces.

The EMPF recommends reviewing the reflow profile in order to solve “head and pillow” failure modes. In addition, the EMPF recommends checking the paste printing variables (printing speed and pressure) to assure that the proper amount of paste is deposited on the PCB. This can be affected by the contact between the stencil and the PCB, the stencil thickness, and the aperture design.

Finally, the EMPF recommends reviewing the board designs for excessive heat transfer that may prevent the proper reflow of the solder balls. A large ground plane located near the TFBGA may conduct enough heat away to interfere with proper reflow of the solder balls. Preheating the board and modifying the reflow profile may be necessary for the solder balls to reflow properly. A board designed for extreme conductive cooling may become a soldering challenge for the manufacturing engineer.

The EMPF offers a full range of lab services that support BGA and TFBGA failure investigations. These services include X-ray examinations, elemental analysis using X-ray fluorescence (XRF), solderability tests, micro-sectioning, and SEM/EDS. The EMPF also offers manufacturing process development support such as board design reviews for excessive cooling. 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.