BGA packages are prone to HIP, also known as HoP defects, during the reflow process, and it is an undeniable fact that the larger the size and the more balls a BGA package has, the more likely HIP is to occur. This double-ball soldering defect, Head in Pillow , is a particularly troublesome engineering problem because neither 2D X-ray inspection nor electrical testing (FVT or ICT), and even sending the product to burn-in (B/I), can detect HIP issues with 100% certainty.
In this blog, Workingbear has explored the causes and analysis of soldering cracks and electronic component detachment in several articles. It has consistently mentioned that most of the fractures occurring when electronic components detach happen at the Intermetallic Compound (IMC) layer. This indicates that the most vulnerable part of the overall structure, once the component is soldered to the PCB (Printed Circuit Board), is this IMC layer, hence the fractures occurring at this point.
However, nearly all articles also point out that it is essential to form this IMC layer between the solder paste and the component leads, as well as between the solder paste and the metal of the PCB, for effective soldering, ensuring solder joint strength.
A user asked: “How is the thickness of the stencil determined? What considerations are there for the shape of the stencil apertures?”
The thickness of the stencil is mainly determined by the amount of solder paste needed for component soldering. Thinner stencils usually mean less solder paste deposited during printing. The amount of solder paste required for each solder joint depends on the components on the printed circuit board (PCB), especially those sensitive to solder volume, such as BGAs with the smallest ball pitch and fine-pitch components. Additionally, smaller components demand higher precision in solder paste printed. Ultimately, the decision also depends on the quality of solder joints after soldering.
Selective soldering machines are a compromise solution in modern PCB Assembly (PCBA)processes. Despite the advancements in surface mount technology (SMT), electronic components still rely on traditional Through-Hole Technology (THT) due to cost or material constraints.
Even though most components on PCBs can be assembled using SMT, some electronic parts still require THT processes. While techniques like Past-In-Hole (PIH) or Pin-In-Paste (PIP) allow through-hole components to be used in SMT assembly or use selective mask wave soldering process, limitations persist due to component technology. As a result, selective soldering machines have emerged.
Workingbear initially thought that wave soldering machines should have been relegated to museums by now! However, despite the decades-long development of Surface Mount Technology (SMT), many PCBs still undergo wave soldering processes. But nowadays, most wave soldering processes employ Selective Mask Wave Soldering, rather than the old method of immersing entire boards into solder baths.
In Selective Mask Wave Soldering, the original wave soldering machine is still used, but the PCB is placed in a wave soldering carrier. Only the pins or solder joints of components requiring wave soldering (typically through-hole components) are exposed to the solder, while other components are shielded with a mask of carrier. It’s a bit like wearing a lifebuoy in a swimming pool – the parts covered by the lifebuoy won’t get wet. Similarly, in the wave soldering process, the areas covered by the carrier won’t get soldered, eliminating the risk of re-melting solder or components falling off.
