BGA/CSP Soldering: More Consistency Needed

As electronic assemblies pushed to higher circuit density, array packages arrived achieving higher circuit density compared to perimeter leaded packages such as the QFP. One concern at that time (>10 years ago) was that the BGA solder joints were not visually inspectable. This concern however was overcome by the promise of much higher yields for array packages. At that time, fine-pitch QFPs did suffer from solder assembly yields from solder bridging and other process problems. Consortiums were formed, papers were written and degrees were given. The conclusion was that with ordinary process optimization, the yields would be so high that the array packages would produce more consistent, high yield solder joints.

As of this writing in 2010, based on a reading of industry publications (ref 1, 2, 3),. BGA solder issues are quite common and the lack of visual inspection is a great hindrance to diagnoses. In fact, there is now an instrument (ERSAscope) which was developed specifically to address BGA solder problems. Meanwhile the fine-pitch QFPs show very high yields thanks to improvements in solder paste and processes. Although one might think this is just a yield issue, the difficulty in test and inspection raises reliability concerns.

When RoHS was introduced, pad cratering surprised the industry as the higher temperature PCB laminates became more brittle and therefore prone to cratering. Recently, Head-and-Pillow seems to be a particular problem judging from references 1, 2 and 3 below. This is partly due to increase warpage of components at higher reflow temperatures although it could happen at lower tin/lead processing temperatures as well. Warpage of BGAs as not been focused on as much in the industry including by component suppliers. There are also other BGA solder issues that are beginning to appear. As the BGA/CSP packages become finer in pitch (0.8 mil and below) and the pad areas get smaller, any contamination or marginal solderability on the substrate becomes a problem. Other packages such as SOICs and QFPs use a larger pad area and are not as sensitive to these conditions. The prediction from this blog is that more BGA soldering studies will be performed, more papers will be written, more tutorials will be given.

References

1. Head-and-Pillow SMT Failure Modes, Intel, SMTA International

2. Telecommunications Case Studies Address Head-in-Pillow Defects and Mitigation through Assembly Process Modifications and Control, Acatel-Lucent, AEPX 2010

3. Awakening from Head-and-Pillow: A Novel Pre-Production Test Method for BGA non-wet Issues, Senju Metal, SMTA International

A Galactic Failure

In my blog entry of June 4th, I described increasing concern of wear-out of multilayer ceramic capacitors. This increasing concern is driven by demands for increasing perfomance (CV) and minizaturzation of of chip sizes. Another concern in electronic systems is the increasing susceptitibility of soft errors from radiation sources. The two major sources of radiation upset are cosmic radiation and semiconductor packaging materials. I find it interesting to think about the sea of (mostly) protons which with were created by exploding stars millions of years ago and then traveling millions of miles, finding their way to the solar system, to earth, to North America, to a specific city, to a specific street, to a particular electronic system, to a single IC and then cause a high reliability system to fail. These particles were well on their way before any electronic systems were even thought about. Although the discovery of radiation effects on electronics was known in the 1990s, it has become an increasing concern is the last several years as IC geometies go below 90nm and Vdd voltages are at 1.5v and below. The secondary neutrons are more likely to cause a soft upset when the critical charge, Qc, gets smaller. Also, the natural radiation found in semiconductor packaging materials which previously did not result in a major problem, now must be calculated in the total system reliability. Most of the electronic systems in use now are consumer electronics where a device upset every 6 months is of no concern. However high reliability systems such as in medical electronics must carefully consider soft error rates. All high reliability system manufacturers should have some expertise in this area. IC suppliers must be asked to provide data on soft errors and then the data must be interpreted by a knowldegable engineer. The design of the IC can have an important effect on SER and the suppliers should have either tested or modeled small geometry devices and provided data to the system designer. This data should include effects from both cosmic radiation and semiconductor packaging materials such as molding compounds, lead solder and BPSG passivation.

Reference: SER - History, Trends and Challenges, Cypress Semiconductor, 2004

Note: Some of the early incidents of radiations effects were, in retrospect, somewhat humorous. For example a semiconductor fab facility was shut down for months due to radioactive phosphoric acid derived from radioactive bat droppings from Jordan Mountain which is near an old uranium mine. An apparantly insignificant decision cause a major problem (the "butterfly effect")