When BGAs move during reflow, intermittent shorts can result.

This month we look at ball grid array (BGA) opens and solder compression. Intermittent joints and shorts can be caused by package warpage at elevated temperatures. Hence the interest in lowering soldering temperatures commonly used for SAC alloys.

FIGURE 1 was part of an experiment to chart the movement of a BGA package during reflow soldering. Using our reflow simulation, we can see solder ball compression by the package laminate in the image. In many of our video experiments, we see package warpage causes solder shorts and open connections during second reflow. Intermittent open connections have been experienced on double-sided reflow and package rework of adjacent parts. This procedure has been helpful to demonstrate why and how this problem can exist, particularly with smaller packages.

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Precision mass-alignment of singulated substrates.

Discussion around smaller devices, complex designs and manufacturing challenges as a result of miniaturization: a never-ending story, isn’t it? Truth is, just when it appears the industry has hit a wall in terms of capability, we find a way forward. Yes, miniaturization is rolling on, and the industry continues to overcome perceived obstacles, this time enabling a higher accuracy approach to mass processing of singulated substrates.

Several years ago, the general thinking was components would keep getting smaller. The prevailing view was that by this time, the metric 03015 and the metric 0201 would be working their way into mainstream production. Although the processes to accommodate these small devices have long since been developed, it will likely be some time before they appear on a majority of BoMs. What is happening, though, is manufacturers are trying to eek out slightly more with standard 01005s by placing them closer together, creating a much narrower gap from the edge of one component to the edge of the next. (See “Screen Printing,” December 2020.) These narrow gap designs – which today see pitches of approximately 100µm with 75µm on the horizon – in combination with the other elements of miniaturization require much tighter alignment tolerances in the stencil printing to ensure solder paste hits the pad target.

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Strategies for vias and routing.

It seems every new design has at least one BGA component on the board. The 1.0mm pitch BGA has become vanilla. Even the 0.8mm pitch BGA is commonplace. These components are not limited to rigid PCBs; BGAs of all shapes and sizes are implemented in flex and rigid-flex designs as well.

The rules for BGAs are much the same whether the board is rigid or rigid-flex. Due to some of the material differences in a rigid-flex, however, extra care is recommended when it comes to the artwork and the trace routing in the BGA field.

Let’s start with pad and via design. For microvias, many suppliers recommend staying at or above 0.005" diameter vias for reliability reasons. Much experience tells us vias smaller than 0.005" tend to have a much lower mean time between failure (MTBF) than vias at or greater than 0.005". In more benign applications, smaller vias may be an option. If the product will experience temperature extremes, however, the conservative bet is to stay above 0.005" diameter microvias. Depending on the design and manufacturer, the associated pads may range from 0.010" to 0.012". Smaller pads risk a via sliding off the edge of the pad. If it does, the risk is the laser may cut through the dielectric and down to the next copper layer.

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The electronics industry should adopt data-driven planning methods.

For many companies, supply-chain management has become a major challenge as the pandemic has continued to disrupt all our lives. As lifestyles have become home-based, for work and leisure, demands have shifted from services to products: materials and tools for lockdown projects, gaming and video equipment, and extra work-from-home IT. There is a global shortage of shipping containers and ships to carry them. As a result, shipping costs have increased sharply. It could take a long time for container costs to return to pre-pandemic levels. Added to that, the spread of the virus has disrupted and depleted workforces, resulting in backlogs and delays.

On top of the misery came the recent blockage of the Suez Canal, adding several days of delay as the backlog was cleared. And, of course, there were domino effects at ports around the world, as cargo was unable to move into or out of the system. The problem has raised questions about the future of super-large container ships and strengthens the argument for using larger numbers of smaller vessels.

Far-flung supply chains, designed to enhance competitiveness and minimize costs, are now under threat and will likely need to change. The world is simply too impatient to wait for things to return to normal. Moreover, there are strong calls for a “new normal” that should, at the very least, strive for environmental sustainability.

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