Tutorial: Mixed technology problems in electronics manufacture given short shrift!
Author : Gordon McAlpine | Production Manager | Dynamic EMS
01 September 2021
Dynamic EMS_Tutorial_Mixed technology problems in electronics manufacturing
Despite a long history of electronics manufacturing in the UK, every new design throws up its own challenges. This applies to older technologies as much as it does to the latest ones. Fortunately, most EMS companies have production experts on hand to ensure that designs can be manufactured efficiently. In this case note, Gordon McAlpine, Production Manager at EMS provider, Dynamic EMS explains how the following example challenged his team recently…
This tutorial was originally featured in EPDT's H2 2021 Electronics Outsourcing supplement, included in the September 2021 issue of EPDT magazine [read the digital issue]. And sign up to receive your own copy each month.
Some components, such as connectors, combine elements of surface-mount and through-hole technologies that can introduce problems when manufacturing. These connectors use both production methods to ensure the best combination of electrical connectivity and robustness. As the two technologies are fabricated differently, offsets can occur and cause failures.
Copper features on the PCB are produced by a photomechanical process, which uses artwork that is aligned by tooling holes over a photo sensitive film on the PCB. How the artwork alignment and the board alignment combine dictates the direction of the tolerance. The pattern alignment can vary from board manufacturer to manufacturer, and even batch to batch. Drill alignment uses the same tooling holes as the photomechanical process, but the drilling process is completely different. Drilling aligns the drill location to the tooling holes, much like the alignment to fiducials that are used for component placement. The boards are then stacked for drilling and each hole has its own location tolerance.
The problem with mixing two different methods of manufacture comes with tolerance build up. The component placement alignment tolerances can compound and foul the connector leads as they enter the hole, causing the connector to jump out of alignment. Placement machines use the fiducials as an alignment reference, but placement accuracy can vary by up to 75um, depending on the machine. The situation isn’t helped by the fact that most connectors have a poor centre of gravity, and uneven surfaces that require special nozzles and often need tapes fitted for placement.
Dynamic EMS_Case Note Tutorial_Mixed technology problems in electronics manufacture given short shrift
Shorts can be caused in a variety of ways – as little as a 1° rotation of the connector body can cause leads to short. Poor insertion alignment can also result in considerable gap reduction, due to rotated leads across the pads, which creates the correct conditions for a short circuit. Finally, full tolerance mismatches, with the drilled hole drifting one way and copper features the other, makes shorts inevitable. Some of these problems are more common than others and there can be a difference in severity from batch to batch.
This challenge was faced by Dynamic EMS recently, when the company had to manufacture a design that used an HDMI connector. To get around the immediate problem, the company had to align the component to the drilled hole tolerance drift by adjusting the placement machine to accommodate the drift direction. In the longer term, increasing the hole size was found to be a better solution.
However, that wasn’t the end of the story. A further batch of boards showed that the hole to copper alignment was excellent, but the board still produced random shorts. Further investigation found that some boards had nodules in the left-hand side of the hole that misaligned the HDMI connector body to one side and produced a slight rotation shorting the pins. The issue was finally resolved by offsetting the component placement to the right.
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