Product Introduction

A lot of process engineers at module factories will tell you the same thing. On the junction box side, you only solder the busbar and the copper strip, so even a cold solder joint won't break down or burn out the diode.

Sounds reasonable at first, right? But they're missing one important chain reaction. Let me walk through it.

First, what exactly is a cold solder joint in a junction box?

A junction box cold solder joint (virtual soldering) is a defect where the internal electrical connection isn't soldered solidly and has poor contact. The joint isn't fully open. The contact area just shrinks, so local contact resistance goes up sharply. When current passes through this high-resistance point, it generates a lot of Joule heat, and the local temperature climbs fast.

Technical Parameters

Cold solder joints in a junction box usually show up at one of four spots: the diode lead to copper conductor connection, the busbar to copper conductor connection, the lead-out wire solder point, and the solder points between internal copper conductors, tin blocks and other conductive modules. These four fall into two groups: diode branch cold solder (spot one) and main loop cold solder (spots two, three, four).

Technical Advantages

• Diode branch cold solder: Under stable operation with no shading and no hotspot, there's no effective working current in the branch, so the joint doesn't heat up and the diode stays normal. But once part of the cells get shaded (bird droppings, fallen leaves, snow) or you get microcracks and performance decay, the bypass diode turns on and stays working for a long time. Large current runs through the high-resistance cold joint, the joint heats violently, and the diode temperature shoots up. Once it passes the diode junction temperature limit, the bypass diode suffers thermal breakdown, and the junction box can even burn.

• Main loop cold solder: The module works at full current the whole time it's generating, so the cold joint keeps heating. The space inside the junction box is tight, and that heat bakes the bypass diode directly, dragging down its reverse leakage resistance and overcurrent capability. On top of that, if shading and hotspots happen at the same time, the diode gets forward-biased. A diode already softened by heat now has to carry the whole loop's bypass current, which easily triggers thermal runaway and ends in a total burnout and short.

Product Application

Once the diode is broken down and burnt, it can go a step further and set off DC arcing, with temperatures reaching up to 3000°C. That finally torches the junction box and can start a module fire.

So during PV plant operation, checking for junction box cold solder joints on time really matters. You can scan the junction box area with an infrared thermal camera. A cold joint or an abnormally hot diode will show up as an abnormal high-temperature spot on the thermal image. Repair or replace it right away and you stop the fault from spreading, keeping the module running stable.

Ooitech's View

What this really shows is that junction box reliability is decided long before the plant goes live, right on the production line where those diode and busbar joints get soldered. Weak soldering here doesn't fail today, it fails years later under the first bad hotspot, which is why controlling solder quality on module equipment matters as much as any cell tech. On our own turnkey module lines we treat junction box and bussing soldering as a hard checkpoint, not an afterthought. If you want to see how proper module assembly and soldering is done, the YouTube channel www.youtube.com/ooitech has plenty of real factory footage worth a look.