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Features Articles

Sue MuchaThe flow of migrants from south of the border is choking supply-chain routes.

As we enter the middle of 2019, it appears to be shaping up as a good year for many in EMS. Anecdotally, I’m hearing the best reports in years on firm backlog from the providers I regularly talk with. I think this is part function of a good economy, but also due to the material constraints the market has experienced for several years. OEMs recognize that having orders in place can be critical to parts availability. Hopefully, this is actual demand and not the double-booking phenomenon seen in previous constrained markets.

The materials market appears to have good news as well. While a few commodities are showing longer lead-times or allocation, the bulk of component lead-times and prices appears to be stabilizing. A few have even decreased lead-times. This is good news for EMS because component availability has impacted companies’ ability to grow. This is a good time to check customer forecasts, since if orders have been inflated, stabilizing material availability may translate to a pushout
in demand.

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Greg PapandrewIt’s time for an industry program to train board buyers.

A printed circuit board is unique to every different application or customer, has over one hundred separate required manufacturing processes, and may come from down the street or halfway around the world. In other words, PCB purchasing is a complicated business. The traditional way of board buying can lead to costly mistakes and may expose companies to financial liability.

I am on a mission to fix that.

PCB buying has changed a lot since I started as a salesman in this industry more than 25 years ago. Back then, purchasing departments were larger. Buying was broken down into specific commodities, with buyers assigned to manage only one or, at most, a few of them. Buyers had the time and available resources to be well-versed in their assigned commodities. Many buying teams resided in the very facilities that designed the boards’ products and used the parts.

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Clive AshmoreWhen it comes to components, the devil’s in the details.

The interdependency of all elements of stencil printing to achieve a high-yield result can be overwhelming. When you consider just the things we’ve addressed in this column – from tooling to warped boards to stencil tension to solder paste types and everything in between – it’s clear controlling the printing process is a balancing act that takes a fair level of expertise (or at least a robust self-learning system!). Nothing underscored this reality more than a recent experience with a customer.

When printing a relatively high-mix board that contained a range of components from 01005s to large QFNs, the customer was experiencing different results from the front and rear print strokes. One direction printed relatively well. But the reverse direction left smearing on the stencil and, according to SPI results, non-optimal paste transfer efficiency. The assembler tried troubleshooting the issue, but nothing seemed to work, other than applying different print pressures on the front and rear strokes in order to get the same output from the process. This, of course, is not normal for solder paste printing.

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Akber RoyThe basics of bare board laminate selection.

Selecting the right materials for a printed circuit board assembly is essential to ensuring the robust reliability of the product and circuitry throughout the entire life of the equipment. In other words, it’s not just how you build it, but what you construct it with. A wide range of substrate materials is available, with different specific properties, and these properties are tied to the expected performance and environment for the circuit being built. Will the PCB assembly be a high-power circuit? Harsh environment capable? High-density?

Engineers specify the material for PCBs with great care and detail. TABLE 11 lists laminates by their industrial names and material types, and specifies material characteristics related to each type. However, to reach that level, it is necessary to understand the various characteristics such as thermal decomposition temperature (Td), glass transition temperature (Tg), dielectric constant (Dk) and dissipation factor (Df) related to PCB materials. Although such tables are important, it is easier to start with a higher-level perspective of selecting materials depending broadly on their actual use, based on a comparison of PCB materials.

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Bob WillisTry this test to determine paste problems.

This month we look at solder paste slump during preheating. It is important to know how much, if any, of the paste slumps like butter on a hot day during reflow. If solder paste does slump, it can lead to shorts, solder balls or solder beads, or cause variations in solder joint volume on selected joints.

Variation in joint volume occurs when one joint acquires more solder from an adjacent joint during reflow due to the paste being linked. Testing of solder paste is well covered in IPC specifications, and equipment is available to test paste and record the results.

Alternatively, if you think you have an issue, a simple shop floor test is to use the existing profile but change the temperatures of the final reflow zones. Setting the final zones to final preheat temperatures will slow the degree of slump. Normally the maximum slump is seen earlier during reflow. Typically, as solder paste is changed or the metal particle size is reduced, slumping can be seen more often.

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David BernardIs the reflow profile the problem? X-ray can help.

Looking through some recent x-ray images of what I would call “good bad” boards (at least, that is what they are for me, as they showcase “good” examples of how certain “bad” types of failure look under x-ray inspection), I came across a number of different issues that are different from “traditional” BGA/QFN problems mentioned in this space before. To wit, I noted some images showed where solder paste had not reflowed under the devices, and there was the presence of foreign object(s), such as discrete components, trapped under the package.

FIGURES 1 and 2 show how unreflowed solder paste typically looks under QFN joints in an x-ray image. In the magnified view (Figure 2), individual grains of the solder paste are seen clearly, instead of appearing as a typical single smooth continuous joint. The cause of this is probably not an insufficient reflow profile. Rather, it is more likely the board has not been reflowed at all. As it may be desired, or necessary, to x-ray inspect (representative) boards after placement but before reflow as part of a quality control process, it is worth noting this characteristic shape of the solder under the components is different from what would be expected post-reflow.

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