LED SMT Reflow Soldering Defect X-ray Inspection Evaluation.

LED SMT Reflow Soldering Defect X-ray Inspection Evaluation is critical because it detects hidden solder joint defects, such as voids, cracks, insufficient solder, or misaligned components, making sure they are invisible to optical inspection. By ensuring proper solder connections, it improves LED reliability, prevents premature failures, and maintains consistent performance in lighting and display applications. This non-destructive inspection method enhances manufacturing quality, reduces costly rework, and helps meet industry standards for electronic assemblies. In this chapter, we want to dive deep and give a brief explanation for any common LED SMT soldering defect.

First defect is percentage of soldering voids. These voids often form in the solder layer of LED chips. This is caused by the expansion of air or flux compounds trapped in the solder during the heating process in the reflow oven. The reliability of solder joints depends not only on the solder alloy but also on the metal plating of the LED device and PCB. Additionally, the time and temperature profile of the reflow soldering process significantly impacts the performance of lead-free solder joints, as it affects the solder joint’s wetting properties and microstructure. Compared to tin-lead solder, lead-free solder is more prone to brittle failure at the joint due to thermal stress and fatigue cracks caused by temperature cycling. Nectec’s X-ray inspection machines, such as the NX-EF, can use non-destructive testing methods to test the void ratio in solder joints after SMT reflow soldering, eliminate defective products, ensure that the heat from the lamp beads is perfectly conducted to the aluminum substrate, and thus ensure that the service life of the lamp meets the design requirements. 

One of the causes of soldering voids is an excessively high void ratio. In the environment of thermal shock testing, thermal expansion and contraction of bubbles can cause solder cracking, thereby reducing the reliability of the LED chip. This directly leads to issues such as increased thermal resistance and reduced thermal conductivity due to the high void ratio. For LED chips with larger pads, the high void ratio plays a primary role in affecting heat dissipation. The higher the void ratio, the greater the thermal resistance, and the poorer the heat dissipation performance.

Second defect is solder ball. Electronic circuit boards have high component density and small spacing, which may cause solder balls to fall off during use. The reason behind this is because they are small, unintended spheres of solder that can form due to excessive solder paste, improper reflow profiles, or contamination. These stray solder balls pose significant risks to chip components, as they can cause short circuits by bridging adjacent conductive traces or pins, particularly in high-density PCB designs. Additionally, they may lead to electrical leakage, signal interference, or even component failure if they migrate during operation. In fine-pitch or miniaturized assemblies, such as LED or IC packages, solder balls can also create mechanical stress or interfere with proper heat dissipation, further compromising reliability. Their presence often indicates poor process control, necessitating corrective measures in stencil design, solder paste application, or reflow parameters to prevent long-term performance issues. On the other hand, the safety concern of workers is needed.

Solder balls can pose serious risks to workers during PCB handling and assembly. If these tiny metal spheres scatter across the workspace, they may accidentally be inhaled or come into contact with skin, potentially causing respiratory irritation or allergic reactions due to lead or flux residues. Additionally, solder balls on the floor create a slipping hazard, while those trapped in equipment could lead to electrical shorts or sparks, increasing the risk of burns or fires. In high-volume production, repeated exposure to solder splatter may also raise long-term workplace safety concerns.

Third defect is broken down into false soldering, cold soldering, empty soldering, and virtual soldering issues. For false soldering, it happens when the solder appears to form a connection but lacks proper metallurgical bonding due to contamination, such as oxidation and flux residue, or insufficient heat. As a result, the solder joint may look acceptable but fails under stress or electrical testing; For cold soldering, it happens when the solder does not fully melt during reflow, resulting in a dull, grainy, or cracked joint. This is caused by insufficient reflow temperature, uneven heating, or premature cooling, leading to weak mechanical and electrical connections; For empty soldering, it happens when there is a missing or incomplete solder joint where the solder fails to properly wet the pad or component lead. This can occur due to poor solder paste application, misalignment, or pad contamination, leaving gaps in the connection; Lastly for virtual soldering, it happens when an intermittent connection, such as sometimes working, sometimes failing. This is usually due to micro-cracks, poor wetting, or mechanical stress.

It often passes initial tests but fails under vibration or thermal cycling. What makes each of them distinguishable is that false soldering involves apparent bonding without true adhesion, while cold soldering results from inadequate melting, creating brittle joints. Empty soldering means missing solder material, whereas virtual soldering is an unstable connection that fails intermittently. Cold soldering and false soldering are often process-related, like heat or contamination issues, while empty soldering stems from paste deposition or placement errors. Virtual soldering is particularly dangerous because it may go undetected until field failure.

Fourth defect can also be broken down into cold solder joints, bridging, and tombstoning issues. We will discuss this part using real-life case studies. To begin, one of Nectec’s previous customers requested that Nectec test the void ratio, specifically to observe the soldering results of the solder paste after reflow soldering. We inspected the LED packaging using real-time X-ray imaging and discovered a significant number of soldering voids, with the void ratio of the heat dissipation pads exceeding 30% in all cases. Compared to lead-containing solder, lead-free solder is more prone to brittle joint failure caused by thermal flow and fatigue cracks resulting from temperature cycling. Excessively high solder void ratios reduce the reliability of the LED chips, leading to thermal expansion and contraction of bubbles during thermal shock testing, which causes solder cracking. 

Coincidentally, another customer of Nected contacted us via email one day to report that their product had a high incidence of dead pixels, with a probability exceeding 38%, and requested that we provide them with an analysis report. After ruling out a series of issues such as the driver, heat sink weight, heat sink surface area, thermal adhesive, thermal conductivity of the aluminum substrate, and whether the circuit was short-circuited, we were still unable to identify the cause of the dead pixels. After analyzing the situation, we discovered that the solder layer from the reflow soldering process had not been tested. We then conducted a non-destructive X-ray inspection and found that the solder layer behind the burned-out LEDs had voids, with the void area generally accounting for over 25% of the pad area. Another instance involved a lighting client of Nectec who reported that their LED heat dissipation issues remained unresolved. After conducting X-ray inspections, we discovered that the weld porosity ratio in their products was as high as 40%, with all tested samples exhibiting porosity ratios exceeding 10%. Among the samples, 37% had void ratios between 20% and 30%, 42% had void ratios between 30% and 40%, and 12% had void ratios above 40%, which was quite alarming. After implementing our improvement plan, their products maintained a void ratio below 10% during reflow soldering, and the customer was very satisfied.