Electronic devices continue to shrink in size with advancement in chip fabrication technology. Packaging technology has responded with improved formats to present these chips for assembly to PCBs. Despite (or possibly because of) reductions of dimensions in chip manufacturing technology, they are still increasing in functionality year by year, even beating Moore’s law in some cases. Credit to improvements in chip fabrication technology for this. But inevitably this leads to even more reliance on Ball Grid Array (BGA) and other specialised Surface Mounted packaging solutions.
One benefit of accommodating BGA devices in your design is a significant improvement in reliability and performance. Production assembly, if you get it right, is actually more straightforward than for many other leadless component styles.
BGA devices are used widely in modern products, from mobile phones and a variety of industrial and consumer applications to transport and Satellite applications. To give you a better idea of BGA soldering, it would first be beneficial to explain a little more about what a Ball Grid Array is.
What is a BGA?
A Ball Grid Array is descended from the pin grid array (PGA): a historic VLSI through-hole packaging technology, featuring a regular array of metal pins underneath the package on a not very dimensionally challenging grid.
A PGA was either mounted on a printed circuit board using traditional through-hole technology or inserted into a socket.
With the BGA, the pins previously bonded to underside base metalisations are replaced by specially formulated solder balls pre-placed on those same base metalisations. These solder balls melt during reflow, and they bond the connections on the underside of the BGA device directly to the PCB pads. BGA internal die connections are routed to the solder balls through internal metal traces within the package. The chip devices inside are usually wire bonded to the relevant traces as normal with most packaging technology. The winning advantage is that BGA packages offer more I/O connections than other flat package styles (Quad flatpacks for example). This is because the entire ‘2D’ underside area of the package is available for connections, not just the four perimeter lengths.
How a BGA is Soldered to a PCB in volume production.
Within the PCB production assembly process, a BGA is normally soldered onto the circuit board by heat applied through a reflow tunnel or by immersion in a Vapour Phase Reflow tank. BGA packages come in various sizes with solder ball dimensions to fit the application. In reflow soldering, the solder balls melt when the PCB assembly reaches the required temperature. Surface tension helps the melted solder to pull the BGA package in optimum alignment with its corresponding PCB pad footprint. Apart from the solder alloy, the solder ball compound sometimes contains some other ingredients (possibly fine metal particles) which prevent the ball collapsing completely, and thereby maintain a gap between the PCB and the component for cleaning and inspection. Whether the device has collapsing, or non-collapsing solder balls, the process of assembly and inspection is the same.
Basic principles of process development for a new assembly.
Every prototype and volume production run will need consideration in order to get the process parameters right. There is no ‘one size fits all’ formula for a process window of ‘temperature vs time’ with BGAs. It is important to establish the ‘process window’ of time and temperature in order to ensure the underside of the BGA achieves the necessary temperature for guaranteed solder ball reflow, every time. Here, training and experience are necessary and can be aided by “Certified Interconnect Designer” training, or more advanced courses. These help the original designers and the production engineers who implement those designs, to work together on a common framework for more repeatable results.
There are usually common features that make it easier for an experienced production engineer to estimate what settings to use. But factors such as large windings or heatsinks on the board nearby can affect the heat-up rate of BGAs. Inspection is therefore an essential tool. Because of the format of the device, inspection is inherently difficult as the joints are not visible with traditional inspection equipment. More advanced techniques are needed (see below).
Common criteria which must be achieved for good BGA soldering.
These are factors which if understood and considered all the way through the design to shipment cycle, help provide consistent products with low defect rates;
- Sufficient heat must be applied to ensure all balls melt sufficiently for a strong bond.
- You must choose, very precisely, the temperature and time required in the reflow tunnel, so that the solder flows and the ball collapses properly as intended. Process windows are often fairly wide, but you need to specify correctly to take advantage of this by being in the middle of the window, not at the edges where failure begins to show.
- Because the solder joints are hidden underneath the BGA, you need specialised optical techniques to inspect the joints (e.g. Ersascope). Attention during design of the PCB is necessary to allow sufficient access to do this. Even so, you cannot completely visually inspect the joints.
- Further inspection is achievable by a combination of electrical test and X-Ray examination, for 100% cover.
- It is recommended that you use X-Ray inspection, at least on an SQA basis, to verify ongoing process compliance. X-Ray can show where full solder ball melt and collapse has not happened across the whole device, in a way that no other technique can. 100% X-Ray inspection is usually used for first-offs and to establish process parameters for the batch. You may wish to continue this at 100% for critical applications and ensure each and every device has soldered correctly.
- Finally, you should use JTAG electrical testing to electrically test all joints for confidence of good soldering before you add any further value to the assembly. A good design will specify chips that are Boundary-Scan compatible and thereby enable this JTAG function on the product. Good design practice will make sure that this facility can be accessed through tracks on the PCB layout.
How a BGA Is soldered by hand to a PCB and removed for repairs.
Apart from machine reflow soldering, you can achieve soldering of a BGA by hand. First you need to apply flux on the bottom of the BGA package. Then with extreme caution, put the BGA package solder balls in good alignment with the board image. This needs care and understanding, and possibly jigs to assist, depending on the task. All you need to do then, is apply heat through a Hot Air Gun or focussed IR heat source and watch for movement indicating collapse of the solder balls across the component. There is some skill needed for this, and training is available.
It is also possible to remove a defective BGA from a board in a similar manner. Workstations such as those made by PDR are required for best accuracy and swiftest completion with minimum time and less risk of heat damage to other parts of the board. Practice and training are particularly important if you intend to do this with products intended for sale. A process development engineer should give a view on the timings and temperatures required in each case to avoid damage.
Contact “Advanced Rework Technology Ltd” for training and advice.
Advanced Rework Technology Ltd (ART) offers you well-designed training courses to teach the skills required to perform to this level. We also offer bespoke courses for specific challenges you face in your production (please enquire), and industry certified training for inspecting the acceptability of all PCBs produced.
Whether you come to ART’s own facilities in Witham, or we bring the equipment to your facility, we can teach you all you require. To contact us, please call 01245 237083 and talk to one of our team, or visit our website and use our contact page.