Package on Package Structures Increase PCB Assembly Complexity
The history of circuits and PCB assembly is mostly one of a race to miniaturize circuitry as efficiently as possible to meet consumer demand for ever smaller and more portable devices. Unfortunately, the smaller and more complex the circuitry is, the more complex and fault-intolerant its assembly becomes.
Let's look briefly at the challenges brought by attempts to make PCB's smaller and more complex simultaneously:
From Through-Holes to BGAs
First we had traditional through-hole circuitry, with leads being directly threaded through pre-made holes in the printed circuit board. This was simple enough to be done by hand, but extremely inefficient in terms of optimizing space usage.
This was followed by Ball Grid Array (BGA) style circuit boards, which used small circular balls of solder to allow the flow of electricity between parts of the circuit. This brought with it a number of advantages to PCB assembly:
Higher-density circuits: As through-hole circuits were more densely-populated, soldering them accurately without crossover or short-circuits became nearly impossible. BGA circuit boards allow for far more connections, although they require machine soldering for accuracy.
Heat conduction: BGA circuits allow heat to pass much more easily from the integrated circuit outwards, reducing overheating problems.
Lower inductance: Because each solder ball in an BGA circuit is generally only a handful of millimeters large, problems from interference within the circuit are greatly minimized.
This was the standard way of making circuit boards and microprocessors for most of the modern computing age, but as complex mobile devices like smartphones became more common, new PCB assembly methods were needed that could be further miniaturized while providing ever-increasing amounts of computing power.
The Package-on-Package Structure
The concept behind package-on-package (PoP) circuit design is simple: stack multiple BGA circuits on top of each other to further reduce space. This has become the standard method for designing circuits for small mobile devices.
This stacking was accomplished by separating the memory and logic circuits and placing each of them on their own BGA, before integrating them together into the final printed circuit with a common interface bridging them. Besides saving space, this method of decoupling thememory from the logic brings several benefits of its own:
The memory and logic assemblies can be tested discretely and, if one is found to be faulty, only that part of the circuit has to be discarded. Other circuit-stacking methods require discarding the entire assembly if one element fails testing.
There are a wider array of options for memory integration, since virtually any capacity memory circuit can be paired with any other logic circuit. This makes it easier to produce a variety of integrated processors at different performance and price points.
The memory itself is only added during final assembly, so memory can be sourced from any supplier or even be swappable by the end user.
The Importance of Quality Control
While PoP circuit assemblies are currently the best solution for combining miniaturization, power, and ease-of-manufacturing, "ease" is a relative term. The high complexity of these boards makes computer-controlled manufacturing with tight tolerances mandatory. Failure in QC on either side of the PoP circuit packaging, the final product will fail.
Also, because memory can be ordered "off the shelf," this adds to the need to ensure your supply chain is reliable. If a custom circuit fails because of substandard OEM memory, the circuit has still failed.
In any process involving PCB assembly, quality and attention to detail are key for any business looking to distinguish itself in PCB manufacturing. Whenever you're investigating new manufacturing options, you need to ensure your QC is able to guarantee constant, quality production.
