Latent Short Circuit Failure in High-Rel PCBs due to Cleanliness of PCB Processes and Base Materials
August 11, 2015 | Stan HeltzelEstimated reading time: 2 minutes
Latent short circuit failures have been observed during testing of PCBs for power distribution of spacecraft of the European Space Agency. Root cause analysis indicates that foreign fibers may have contaminated the PCB laminate. These fibers can provide a pathway for electromigration if they bridge the clearance between nets of different potential in the presence of humidity attracted by the hygroscopic laminate resin. PCB manufacturers report poor yield caused by contamination embedded in laminate. Inspections show that fiber contamination is present on prepreg and etched innerlayers. Further fiber contamination may be attracted in the manufacturing environment due to static charging. The requirements for cleanliness that are specified for final PCBs are orders of magnitude more stringent than
those specified for base materials. This paper describes inspections performed on base materials, manufacturing processes and final PCBs. It describes test methods that detect reduced insulation caused by contamination and electromigration. Moreover, a proposal is presented specifying tightened requirements for a new class of base materials for the manufacture of high-reliability PCBs.
I. Introduction
Latent short circuit failures have been observed in PCBs during testing of power distribution units of spacecraft for the European Space Agency (ESA). Root cause analysis has been conducted under review of non-conformance review boards (NRB). Printed circuit board assemblies failed after prolonged functional testing in ambient laboratory environment or after thermal vacuum cycles. Due to the large amount of damage caused by the electrical overstress, it was not possible to obtain direct evidence of the failure. However, a working hypothesis has been developed indicating that fiber contamination may have caused a latent short circuit. This hypothesis was further substantiated by reports on contamination issues in base materials and by a test method that demonstrated the breach of insulation due to fiber contamination.
At the time of the observed failures at equipment level, PCB manufacturers reported poor cleanliness levels of base laminate materials, causing poor yield. It is not possible to screen in an efficient manner for contamination in copper-clad laminate, since visual inspection requires stripping of the copper. Several inspection methods show the lack of cleanliness of base materials, which is specified in IPC4101[8]. This paper identifies a major gap between the requirements specified on base materials and the requirements on manufactured PCBs and presents a proposal for a new class of cleaner base materials for the manufacture of high-rel PCBs.
Editor's Note: This article originally appeared in the August 2015 issue of The PCB Magazine.
Suggested Items
Atotech to Participate at KPCA Show 2024
09/03/2024 | AtotechMKS’ Atotech will participate in this year’s KPCA Show 2024 in Incheon, held at Songdo Convensia from September 4-6, 2024.
Victory Announces Breakthrough in PCB Technology with New Product Launch
08/29/2024 | openPRShenzhen Victory Electronics Technology Co., Ltd., a leader in the printed circuit board (PCB) manufacturing industry, is proud to announce the successful development of a groundbreaking new product.
Connect the Dots: Designing for Reality—Electroless Copper
08/28/2024 | Matt Stevenson -- Column: Connect the DotsRoll up your sleeves because it's time to get messy. In a recent episode of I-Connect007’s On the Line with… podcast, we discussed electroless copper deposition. This process deposits a copper layer into the through-holes and vias of what will eventually be a PCB. Electroless copper deposition feels like a black box to many people. It sort of looks like a black box, too. The boards go in one side, come out the other, and emerge differently. So, let's crack open that black box and look inside.
Maximizing ROI Through Better Wet Process Control
08/20/2024 | I-Connect007 Editorial Team“When things get out of control, the variation in your wet process begins,” says Mike Carano,. “Just because they look like good boards and may even pass electrical test, it does not necessarily mean you have good boards. Once the chemistry is headed toward the right or left side of the process control parameter cliff, the plating is compromised. If the copper is thinner than it should be, when the customer puts it into service, the board may fail after 500 cycles vs. the requisite 1,000 or 2,000 cycles. The root cause issue is that you plated 7/10ths of a mil of copper instead of one-mil of copper because you were not controlling your process. The fact that you passed your own electric test becomes inconsequential.”
The Chemical Connection: Controlling an Alkaline Etch Bath With Low Copper Loading
08/07/2024 | Don Ball -- Column: The Chemical ConnectionThis month, I wanted to continue my discussion on how much process control is actually needed for high-density circuit production. However, in the last month, I have been called on to help with a specific process control problem: how to control an alkaline etch bath when you’re not etching much copper. This problem has been cropping up more frequently as smaller shops (and a few larger ones) concentrate on low-volume, high-value plated through-hole panels such as prototypes, resistor cores, etc.