Low-Temperature Solder May Cause Serious Power Loss in Busbarless Solar Modules

New research shows SmartWire Solar Modules can suffer abnormal performance degradation in hot climates.

Background: Why Busbarless Technology Matters

Modern solar modules continue to evolve to achieve higher power output, lower silver consumption, and improved long-term reliability. One important innovation is busbarless technology, which replaces traditional thick busbars with many fine conductive wires.

One of the most widely adopted busbarless solutions is SmartWire, developed by Meyer Burger. Instead of soldering flat ribbons onto busbars, SmartWire uses thin copper wires embedded in a polymer film to collect electrical current from the Solar Cell.

While this design improves efficiency and reduces shading losses, recent research suggests it may introduce hidden reliability risks, especially in hot environments.

Research Findings: Power Loss After High-Temperature Testing

A research team from Sandia National Laboratories (USA) investigated SmartWire mini-modules using accelerated aging tests.

They found that any sample exposed to high-temperature test sequences experienced about 9% power loss. This loss mainly came from a reduction in the fill factor (FF), rather than changes in voltage or current.

This indicates increased electrical resistance instead of direct damage to the solar cells themselves. These test samples are referred to as Type A samples in the study.

Comparing Commercial SmartWire Modules

To determine whether the same issue exists in real-world products, the researchers examined full-size commercial SmartWire modules produced in different years:

• Type B: manufactured in 2020
• Type C: manufactured in 2022
• Type D: manufactured in 2018

Material differences were also identified:

• Type A, B, and C modules used tin–bismuth (Sn-Bi) low-temperature solder
• Type D modules used indium–tin (In-Sn) solder

The researchers prepared cross-sections of these modules and analyzed them using electron microscopy.

Key Discovery: Strong Wire Connection, Weak Gridline Contact

Microscopic analysis revealed a critical difference in connection quality:

• Between the copper wire and the solder, a solid metal alloy layer had formed, indicating a strong metallurgical bond.
• Between the silver gridlines on the solar cell and the solder, no alloy layer was observed.

Instead, only physical contact was present, along with micro-cracks and small gaps.

In simple terms:

• Copper wire and solder form a strong, durable connection.
• Solder and silver gridlines form a weak connection.

This weak interface plays a key role in later performance degradation.

Climate-Based Aging Tests Reveal the Risk

The research team designed four accelerated aging test sequences to simulate different climate conditions:

• Sequence I: winter climate
• Sequence II: spring climate
• Sequence III: tropical climate
• Sequence IV: desert or extreme heat climate

The Type A samples were divided into three groups:

• Group A: tested with Sequences I and II
• Group B: tested with Sequences III and IV
• Group C: tested with all four sequences

The results were clear: Group A showed only minor degradation, while Group B and Group C both experienced about 9% power loss. This confirmed that high temperature is the main trigger for the problem.

Failure Evidence: What Happens Inside the Module

After aging tests, electroluminescence (EL) imaging revealed multiple defects, including:

• Delamination of encapsulation materials
• Disconnected or broken gridlines
• Failed wire-to-cell connections

Further microscopic analysis of damaged areas showed voids and separation between the solder and the silver gridlines.

Why High Temperature Causes the Failure

Unlike traditional soldered ribbon technology, SmartWire relies on a mechanical pressure mechanism to maintain electrical contact.

During lamination and cooling:

• The polymer film shrinks
• This shrinkage presses the solder-coated wires onto the silver gridlines

However, under high-temperature conditions:

• The polymer film expands
• Pressure on the wires decreases
• The soldered wires gradually separate from the gridlines

As a result, contact resistance increases, the fill factor drops, and overall power output declines.

Simulation Confirms the Explanation

To validate this mechanism, the researchers performed finite-element simulations of:

• The cooling process after lamination
• The high-temperature aging process

The simulations showed strong compressive forces during cooling and an opposite trend during high-temperature exposure. These results support the conclusion that thermal expansion and contraction of the polymer film directly affect electrical contact quality.