Last summer, my team launched a line of waterproof poolside Bluetooth speakers. We tested them in our lab: submerged in 1m of water for 30 minutes, they worked perfectly. But when customers started using them by the pool, complaints rolled in: “The speaker cuts out when it’s rainy,” “It stops working after a week of humid weather.”
We sent a team to a customer’s backyard to investigate. When we opened the faulty speaker, we found the culprit: the FPC (Flexible Printed Circuit) inside had white, fuzzy corrosion on its surface. “Humidity seeped through the speaker’s seal and got to the FPC,” our engineer, Lena, said, pointing to the corroded traces. “The surface insulation wasn’t strong enough—moisture caused a short circuit, and the speaker died.”
That moment taught us a hard lesson: FPCs in humid environments (pools, bathrooms, outdoor gear, medical devices) need more than “basic” insulation. Humidity doesn’t just “get wet”—it lingers, seeps into tiny gaps, and breaks down insulation over time. A weak surface insulation can turn a $100 device into a useless brick in weeks.
In this article, we’ll break down why humidity ruins FPC insulation, share how we fixed our pool speaker problem, and outline the most effective tips to keep FPCs from leaking in wet, humid spaces.
FPCs rely on a thin layer of insulation (usually the PI film base or an extra coating) to keep copper traces from shorting out. Humidity destroys this insulation in three sneaky ways:
FPCs have small gaps: between traces, around components, or in the edges of the PI film. Humid air condenses into tiny water droplets that seep into these gaps. Over time, the water builds up and creates a “bridge” between two traces—causing a short circuit.
“We tested our pool speaker FPC in 90% humidity (simulating a rainy day),” Lena said. “After 48 hours, we found water droplets between two power traces. The insulation didn’t block them, and the traces shorted out.”
PI film is naturally resistant to moisture, but constant humidity weakens it. The water reacts with the film’s surface, making it porous (like a sponge). Once the PI film is porous, moisture seeps through to the copper traces, causing corrosion and leakage.
“We had a batch of FPCs for bathroom scales that failed after 3 months,” said Raj, a quality engineer at a consumer tech firm. “The PI film looked fine, but under a microscope, we saw tiny holes—humidity had eaten through it. The traces corroded, and the scale gave wrong readings.”
Humidity near pools, oceans, or even sweaty gym gear isn’t “pure water”—it has salt, chlorine, or oils. These chemicals speed up insulation breakdown. Saltwater, for example, is conductive—even a tiny amount on the FPC surface can cause leakage, no “short circuit” needed.
Our pool speaker had this issue: “The customer’s pool had chlorine, which stuck to the FPC,” Lena said. “The chlorine mixed with humidity and ate through the insulation 3x faster than pure water. The speaker failed in a week instead of a month.”
Fixing FPC insulation in humid environments isn’t just about “adding more coating”—it’s about choosing the right materials and processes. Below are the tips we used to save our pool speaker, plus advice from Raj and other engineers.
PI film alone isn’t enough for humid spaces—add a conformal coating: a thin, flexible layer that covers the entire FPC surface (traces, components, edges) to block moisture.
-
Silicone-Based Coating: Best for most humid environments (pools, bathrooms). It’s flexible (so it bends with the FPC), water-resistant, and cheap. Look for a coating with a thickness of 50–100μm (thicker = better protection).
-
Parylene Coating: For extreme humidity (ocean gear, medical devices). It’s ultra-thin (10–20μm) but dense—blocks even saltwater and chemicals. It’s more expensive, but worth it for critical devices.
We added a 80μm silicone conformal coating to the FPC, covering every trace and edge. “We tested it in 95% humidity with chlorine for 2 weeks—no moisture seepage, no short circuits,” Lena said. “Customers stopped complaining.”
FPC edges (where the PI film is cut) are porous and let moisture in. Seal them with a small amount of epoxy or silicone to block gaps.
-
Use a syringe to apply a thin line of medical-grade silicone (for skin-contact devices) or epoxy (for industrial gear) along the FPC’s cut edges.
-
Let it cure for 24 hours at room temperature—this creates a waterproof “seal” that stops moisture from seeping in.
Raj’s team sealed their scale FPC edges with epoxy: “Before, 30% of scales failed in 3 months. After sealing, failure rate dropped to 2%. The PI film no longer gets porous—moisture can’t get in.”
Not all PI film is the same—look for PI film with an extra moisture barrier layer (usually a thin layer of aluminum or ceramic) to block water.
The barrier layer acts like a “shield” on top of the PI film, preventing humidity from reacting with the film’s surface. It’s especially useful for devices that stay in humidity 24/7 (like underwater sensors).
We switched to PI film with a 5μm ceramic barrier layer. “Even if the conformal coating gets a tiny scratch, the barrier layer stops moisture,” Lena said. “The FPC is double-protected.”
Moisture needs a “bridge” between two traces to cause leakage. If you space traces farther apart, it’s harder for water to create that bridge.
-
For humid environments, space copper traces at least 0.2mm apart (instead of the standard 0.1mm).
-
If you need to fit more traces, use a multi-layer FPC—put traces on separate layers (with insulation between them) instead of crowding them on one layer.
“We used to space scale FPC traces 0.1mm apart—moisture caused leaks often. We increased it to 0.2mm, and leaks dropped by 70%. It’s a simple fix that makes a big difference.”
The only way to know if your FPC insulation works is to test it in real-world humidity. Don’t just rely on lab tests—simulate the environment your device will live in.
-
Humidity Chamber Test: Put the FPC in a chamber set to 90–95% humidity and 40°C (simulating a hot, humid day) for 7–14 days. Check for leakage with a multimeter.
-
Salt Spray Test: For ocean/pool gear, spray the FPC with a saltwater solution (5% salt) and let it sit in 80% humidity for a week. This simulates saltwater humidity.
We now test every FPC batch in a humidity chamber for 14 days. “We catch insulation issues before they reach customers,” Lena said. “It adds a week to production, but it’s worth it to avoid returns.”
Let’s look at how a fitness tracker company used these tips to fix their humidity problem. Their waterproof tracker kept failing in sweaty gym bags—moisture seeped into the FPC, causing the screen to flicker or die.
-
Insulation Failure: The FPC used only PI film (no conformal coating) and had 0.1mm trace spacing.
-
Moisture Seepage: Sweat and gym bag humidity seeped through the FPC edges, corroding traces.
-
Failure Rate: 15% of trackers failed within a month of use.
-
Add Conformal Coating: A 60μm silicone coating on the entire FPC.
-
Seal Edges: Epoxy seal on all cut edges of the FPC.
-
Widen Trace Spacing: From 0.1mm to 0.2mm.
-
Switch to Barrier PI Film: PI film with a ceramic moisture barrier.
-
Failure Rate: Dropped from 15% to 0.8% in 3 months.
-
Customer Feedback: No more “flickering screen” complaints.
-
Durability: Trackers now work for 6+ months in sweaty gym bags.
“The FPC insulation wasn’t ‘bad’—it just wasn’t built for sweat,” said the company’s engineering lead. “These tips turned a faulty product into our top seller.”
Our pool speaker failure taught us that FPCs in humid environments need intentional insulation design. PI film alone isn’t enough—you need conformal coatings, sealed edges, barrier layers, and smart trace spacing. These aren’t “extra” steps—they’re the difference between a device that lasts weeks and one that lasts years.
As FPCs are used in more humid-friendly devices (waterproof phones, underwater cameras, medical implants), insulation will only grow in importance. The teams that get it right won’t just avoid returns—they’ll build products customers trust, even in the wettest, stickiest, most humid places.
Next time you design an FPC for a humid environment, ask: “Can this block moisture for 6 months? A year?” If not, add a conformal coating, seal the edges, and test it. Your device (and your customers) will thank you—even when it’s raining, sweating, or splashing in the pool.
Founded in 2009, our company has deep roots in the production of various circuit boards. We are dedicated to laying a solid electronic foundation and providing key support for the development of diverse industries.
Whether you are engaged in electronic manufacturing, smart device R&D, or any other field with circuit board needs, feel free to reach out to us via email at sales06@kbefpc.com. We look forward to addressing your inquiries, customizing solutions, and sincerely invite partners from all sectors to consult and collaborate, exploring new possibilities in the industry together.
English
Wechat
+86 13670210335
+86 13670210335
E-mail