Red Phosphorus and Electronic Failures

Posted by Craig Hillman on Dec 3, 2019 11:48:03 AM

Red Phosphorus_A connector that shows evidence of corrosion from red phosphorus

Picture: A connector that shows evidence of corrosion from red phosphorus.

Electronics manufacturing is a complicated business. Consumer demand, manufacturing supply chains, materials, and regulations are constantly changing. Every company wants to be the first to market with the newest, coolest, most profitable technology. But sometimes the rush to get to market leaves safety and reliability in the dust. Design review and thorough supply chain assessment help mitigate electronic failure risks before they happen. Unfortunately, we get to see the same mistakes being made over and over again.  

In this article, we examine how twice in ten years the use of red phosphorus impacted the safety and reliability of electronics products costing millions of dollars in recalls and incalculable damage to consumer confidence and brand reputations.

Background

In the late 1990’s, due to increased consumer pressure and changing government legislation, electronics manufacturers began to design and produce more environmentally friendly products. Companies throughout the electronics supply chain began to introduce green materials into their manufacturing process. This resulted in the industry actively seeking green alternatives to electronic packaging materials that would still ensure adequate performance, cost, reliability, and safety.

A listing of part manufacturers and part numbers that are encapsulted with epoxy compounds containing red phosphorus flame retardants

Picture: A listing of part manufacturers and part numbers that are encapsulated with epoxy compounds containing red phosphorus flame retardants. 

One area that was addressed in the quest for more environmentally responsible materials was flammability in electronics. To mitigate the risk of unintended thermal events that could result in fire, flame-retardants are incorporated into materials such as plastic encapsulants and printed wiring board laminates used to package and construct electronic products. At that time, brominated flame-retardants (BFR) dominated the electronics and electrical equipment market. But they were widely considered to pose significant health and environmental risks.

Phase 1: Enter Red Phosphorus

Phosphorus-based flame-retardants were considered an appropriate replacement to BFRs. Outside the electronics industry, the use of phosphorus and phosphorus-based compounds as flame-retardants was quite common. Red phosphorus has many advantages, not the least of which is its low cost, a big driver in the electronics industry.  But red phosphorus has well known limitations as well.  Exposed to ambient humidity at high temperatures, red phosphorus can form corrosive phosphorus acids. It can react and corrode both silver and copper – materials commonly found in electronics. Typically, to address this issue, coatings and stabilizers were used to mitigate the reaction of the red phosphorus.

Red Phosphorus - Benchmarking of Halogen replacements Table

Picture: Benchmarking of halogen replacements.

While these methods greatly limited the activity of the red phosphorus, there were concerns about the long-term stability of red phosphorus-based fire retardants and their use in microelectronic applications, particularly semiconductors and integrated circuits. It was known that electronic components encapsulated in epoxy resins with red phosphorus flame-retardants were subjected to degradation of the insulation and corrosion of the metallic leads due to the deterioration of the red phosphorus flame retardant and the eventual formation of phosphine and corrosive oxidation products.

Not long after red phosphorus was introduced to electronics, the industry started reporting electrical shorts, corrosion, thermal events, and ultimately costly product recalls all linked to red phosphorus flame retardants.  As a result, by 2002 most red phosphorus-based molding compounds used in electronics manufacturing were pulled from the market.

Phase 2: Red Phosphorus and Hydrolysis

Then, surprisingly, in 2014, we, at ANSYS-DfR Solutions, began to receive reports from manufacturers of corrosion, shorts, thermal events in a variety of electrical and electronic components, this time in connectors, sockets, and power cords. These products were manufactured between 2011 and 2012. An initial analysis indicated that all these issues or events seem to be linked to metal migration.

Our experience and initial review let us to believe that there was more to this story than just metal migration. Diving deeper, the first thing we uncovered was that all the companies that came to us with these issues, and there were many, had issues that seemed to be related to the same carrier material, polybutylene terephthalate or PBT. PBT is a thermoplastic engineering polymer that is used as an insulator in the electronics industry. Our analysis concluded that these failures were due to a breakdown of the PBT. PBT is known to be susceptible to hydrolysis. But what was causing the hydrolysis?

Red Phosphorus_A connector (on the roght) showing evidence of sweating and breakdown from hydrolysis

Picture: A connector (on the right) showing evidence of sweating and breakdown from hydrolysis.

As we’ve already learned, red phosphorus needs to be coated properly or it can react with moisture around it and form phosphoric acid. Acids cause corrosion, especially in electrical or electronic insulators. This can lead to the cracking, further moisture intrusion, and dissolution and migration of the metal. Once we have migration of metal, we can get electrical shorts, current flow, and dual heating especially in power cords that pull in a lot of current.

Red Phosphorus_Cross section of encapsulant with red phosphors particles

Picture: Cross-section of encapsulant with red phosphorus particles.

In this case, red phosphorus was causing the hydrolysis that was breaking down the PBT. There was also evidence that humidity played some critical role as well. Reported observations of hydrolysis were primarily occurring in high humidity environments.

So once again, red phosphorous was the culprit. Same material, different failure mechanism.

Why is this lesson important?

We are all familiar with the famous quote, “those who do not learn history are doomed to repeat it.” In this case we learned, on two separate occasions, that red phosphorus flame retardants cause failures in electronics. So why do we continue to use it as part of the electronics manufacturing process?

The electronics supply chain is long and very complex. In many cases, insufficient communication between component manufacturers, suppliers, and OEMs means important information may not be shared across the supply chain. Manufacturers rely on their supply chain to validate that components and materials have met specifications. But testing to spec does not predict future failure. So how do OEMs know what they don’t know?  A careful supply chain assessment with the understanding of use case scenarios components and materials is essential.

Red Phosphorus_Cross-section of an integrated circuit with red phosphors showing copper migration

Picture: Cross-section of an integrated circuit with red phosphorus showing copper migration.

In the case of red phosphorus, we believe that there is an intrinsic incompatibility with its use as a flame retardant in electronics. The electronics industry has gone through this very costly and time-consuming exercise two different times now. How do we make sure it doesn’t happen again, especially in new technology insertions?  Stop using red phosphorus. Period. Given the extent of the evidence, going forward, we believe that red phosphorus should not be used in any kind of electrical or electronic insulating application.

If you’re not sure whether your suppliers are using red phosphorus, you should consider performing a supplier assessment. Alternatively, a thorough design review can provide suggestions on what components are the best for your product ahead of time or if you’re considering making changes to your design.

 


Red Phosphorus_Particle Degradation

 

whitepaper:

RED PHOSPHORUS INDUCED FAILURES IN ENCAPSULATED CIRCUITS

 


Red Phosphorus_Cross section of encapsulant with red phosphors particles

 

webinar recording:

PRODUCT RELIABILITY ALERT: RED PHOSPHOROUS

 

Topics: electronic failures, red phosphorus

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