Intel delves into PCB moisture tracking

Researchers from processor giant Intel are looking for ways to improve the monitoring of moisture in PCBs. Moisture in PCBs have become a critical variable to both performance and failure mechanisms.

Reliability has been the subject of most studies on moisture effects in PCBs, and the moisture referred to in these studies comes from the environment, i.e., air. However, Intel has a different approach. Richard Kunze, a senior staff engineer and technical lead in the Enterprise System Engineering organisation within the Data Centre Group at Intel Corp., explained that the group's primary focus is on moisture and temperature effects on the electrical performance, in particular on the insertion loss, of high speed signals, such as you would find in a typical computing device, and most particularly in servers found in data centres.

Commenting on high speed performance degradation, Kunze declared: "Moisture in PCBs directly affects various signal propagation characteristics, and we focus on the key one of insertion loss. We recognise that moisture will be absorbed by a PCB, and the questions then become how much under normal operating conditions and how to quantify its impact on high speed signalling performance."

Kunze added that power dissipation of the devices attached to the PCB raises its temperature and this elevated temperature also leads to increased loss and affects the moisture content in the PCB. In an upcoming DesignCon paper, he said Intel will provide a simple examination of both of these effects for a variety of different PCB constructions and materials and try to understand performance impact under full operating conditions.

Moving to the importance of characterizing moisture sensitivity and controlling moisture content in PCBs, Kunze noted that it can be difficult to remove moisture –through baking- from a typical PCB that has solid copper planes throughout its stack up. Prior to assembly, he continued, bare boards are typically stored in vacuum sealed bags along with some desiccant material. After assembly, boards are subject to their operating environment which can range widely in temperature and humidity. "One question we want to understand is the moisture and temperature state of the PCB while in operation under the normal range of environment conditions," he declared.

Kunze explained that many moisture studies conducted to date assess the amount of moisture in a PCB through an IPC-approved method, i.e. by weighing the PCB before and after exposure to water. Others have observed changes in capacitance to discern the amount of moisture in a PCB. "Our approach infers moisture content through its effect on insertion loss."

Three years ago at DesignCon, Intel indeed introduced a novel method of determining the insertion loss by taking time-domain measurements at only two points, both at the same end of the line. They called it "Single-Ended TDR to Differential Insertion Loss," or SET2DIL. This insertion loss test method was developed by Jeff Loyer, signal integrity lead for Intel's Enterprise Server Division, and Kunze himself and is now an IPC-approved test method for determining differential insertion loss.

Asked about the methods designers can use to avoid moisture in PCBs, Kunze said it is unclear what role designers can play, at least from a practical standpoint, on limiting moisture diffusion. He commented: "Moisture absorption into the boards is certainly impeded by solid copper planes, and also to some extent through the glass fabric in the dielectric layers, but board designers have to place openings for components to achieve the board's function and so it's not clear that much relief can be found in design in terms of keeping moisture out of the board."

However, Kunze continued, for electrical performance, designers need to take into account the effects of moisture and elevated temperature to insure that high speed signals are not degraded beyond acceptable levels. "We do believe now that moisture may not be as serious –but still important to take into account- a concern for performance while the board is operating at elevated temperatures.

Several well-established tools for simulating moisture diffusion in structures, PCBs included, now exist. Kunze, who is located at Intel's DuPont, Washington campus, said their future research will use these simulation tools to assess moisture content in operating PCBs.

- Anne-Francoise Pele
??EE Times