This article addresses common soldering defects in printed circuit modules produced using surface-mount technology (SMT), such as solder balls, standing solder (the \u201cManhattan phenomenon\u201d), bridging, and other soldering defects in printed circuit modules produced using surface-mount technology (SMT). It provides an in-depth analysis of the mechanisms behind their formation and, drawing on practical production experience, summarizes targeted process optimization strategies and solutions. This paper serves as a practical reference for soldering quality control during the SMT production process, helping to improve the reliability and yield of printed circuit modules.<\/p>\n
Surface-mount technology (SMT), as a core technology in modern electronics manufacturing, is widely used in various types of electronic equipment due to its advantages of reducing product size and weight while increasing assembly density and reliability. In particular, it meets the stringent requirements of future strategic weapons in terms of intercontinental range, mobile launch capabilities, and safety and reliability.<\/p>\n
SMT technology integrates multiple processes, including solder paste materials, stencil fabrication, screen printing, component placement, and soldering. Parameter fluctuations or operational deviations in any of these processes can lead to the failure of the printed circuit board assembly (PCBA), thereby affecting the performance of the entire device.<\/p>\n
The quality of component solder joints is critical to the reliability of printed circuit assemblies (PCAs) and finished products, and is influenced by multiple factors, including solder paste performance, substrate quality, solderability, screen printing accuracy, and soldering processes. Long-term production experience has shown that optimizing process parameters throughout the entire production cycle and strengthening quality control are key to improving SMT production quality.<\/p>\n
This article analyzes the mechanisms behind typical welding defects and proposes practical solutions, providing a reference for industry production.<\/p>\n
Solder balls are the most common defect in SMT production. They occur when solder fails to fully coalesce into a solder joint and instead adheres to the PCB surface or between pins in the form of small spheres. This not only affects the appearance but also poses a risk of short circuits. The internationally recognized standard stipulates that a maximum of 5 solder balls with a diameter of \u22640.1 mm is permitted within a 600 mm\u00b2 area; any number exceeding this limit is deemed non-conforming. The causes of solder balls are complex, and the root cause must be precisely identified by examining the production process.<\/p>\n
Solder balls in wave soldering primarily form around PCB through-holes and at the wave soldering contact surface. The core causes can be divided into two categories: First, the quality of the PCB through-holes and the influence of moisture. The high temperatures of wave soldering (240\u2013260\u00b0C) cause residual moisture in the through-holes and substrate to vaporize. If the copper plating on the through-holes is too thin or contains pinholes, high-pressure steam can force out the molten solder paste, forming solder balls; Second, improper process parameters\u2014such as excessive flux application or insufficient preheating\u2014prevent solvents from fully evaporating, causing them to vaporize and splatter the solder at high temperatures, resulting in solder balls.<\/p>\n
Targeted Solutions (Proven to reduce defect rates by 80% or more):<\/p>\n
Strictly control the quality of PCB through-holes: copper plating thickness \u2265 25 \u03bcm, no pinholes or gaps; conduct random sampling inspections prior to production; maintain storage humidity between 40% and 60%; if necessary, pre-bake at 120\u00b0C for 2 hours to remove moisture.<\/p>\n
Optimize flux application: Give priority to spray or foam application methods; when using the foam method, ensure the bubbles are fine and uniform. Adjust the application pressure and speed according to the PCB specifications to ensure an application rate of 0.05\u20130.1 mg\/cm\u00b2.<\/p>\n
Optimize preheating parameters: Based on the PCB thickness and component package type, ensure that the temperature on the top surface of the PCB is \u2265100\u00b0C, with a preheating time of 60\u201390 seconds, to prevent thermal shock and solder splatter.<\/p>\n
Solder balls in reflow soldering are often hidden at both ends of rectangular surface-mount components or between fine-pitch pins, making them difficult to detect. The core mechanism is that after the solder paste melts, it does not wet the pads and leads adequately; the liquid solder contracts and overflows, forming solder balls upon cooling. Poor wettability is the root cause.<\/p>\n
a) Improper temperature profile: The preheating rate was too fast (>4\u00b0C\/s), the plateau temperature (150\u2013180\u00b0C) was not maintained for at least 60 seconds, the solvent did not fully evaporate, and solder vaporized and splattered during reflux. It is recommended to preheat at a rate of 1\u20134\u00b0C\/s, maintain the plateau temperature for 60\u201390 seconds, and set the reflow peak temperature to 220 \u00b1 5\u00b0C (maintained for 10\u201320 seconds).<\/p>\n
b) Insufficient stencil accuracy: Deviations in aperture dimensions and soft materials (such as pure copper stencils) can cause blurred or overlapping solder paste printing, a problem that is particularly pronounced with fine-pitch components. For fine-pitch components, prioritize the use of 0.15\u20130.2 mm stainless steel stencils to ensure that the apertures align with the pads.<\/p>\n
c) Improper use of solder paste: If the interval between component placement and reflow exceeds 4 hours, the solder particles will oxidize and the flux will lose its effectiveness. Select solder paste with a working life of at least 4 hours; complete soldering within 2\u20134 hours after printing; and store opened solder paste in a sealed container in the refrigerator.<\/p>\n
d) Inadequate process control: PCBs with incorrectly printed solder paste were not thoroughly cleaned, or component placement deviations led to uneven solder paste distribution. Strengthen operating procedures; ensure no residue remains after cleaning, and increase production inspections to correct anomalies.<\/p>\n
The \"Manhattan Effect\" refers to a phenomenon in which a rectangular surface-mount component is soldered in place at one end while the other end tilts upright, resulting in a failed electrical connection. The core cause is uneven heating at both ends of the component, which causes the solder paste to melt at different rates; the resulting difference in surface tension causes one end to lift. The specific causes and solutions are as follows:<\/p>\n
a) Defects in component placement design: One end of the component crosses the reflow soldering limit line (183\u00b0C liquidus) first, causing the solder paste to melt and generate surface tension, while the solder paste at the other end remains unmelted. Insufficient bonding strength causes the component to stand upright. During PCB design, ensure that both ends of the component are within the limit line simultaneously to guarantee that the solder paste melts synchronously.<\/p>\n
b) Insufficient preheating for vapor-phase brazing: The temperature in the saturated vapor zone was 217\u00b0C; inadequate preheating caused a thermal shock, causing the small component flanges to stand upright. Optimized preheating process: Maintain a constant temperature of 145\u2013150\u00b0C for 1\u20132 minutes, preheat for an additional minute in the equilibrium zone, and then slowly enter the saturated vapor zone; this reduces the defect rate to below 0.51 TP3T.<\/p>\n
c) Poor pad design: Asymmetrical pad dimensions and improper spacing result in uneven solder paste deposition and uneven heating. Design pads strictly in accordance with specifications to ensure symmetrical dimensions and proper alignment with the leads, and conduct a process review prior to production.<\/p>\n
Fine-pitch pin bridging (pin center-to-center distance \u2264 0.635 mm) refers to a short circuit caused by solder bridging between adjacent pins. This issue is difficult to repair and results in a high scrap rate. The primary causes include solder paste printing defects and improper process parameters. Therefore, stricter control measures must be implemented at key process steps:<\/p>\n
Quality issues in the SMT process (70%) are related to printing, with the stencil being the key influencing factor. Compared to copper stencils, stainless steel stencils have a lower coefficient of friction and better release properties, making them suitable for fine-pitch printing. Testing of QFP208 devices with a 0.5 mm pin pitch showed that the defect rate for missed printing with copper stencils was approximately 20%, while it decreased to 3% with stainless steel stencils.<\/p>\n
For fine-pitch components, a 0.15\u20130.2 mm stainless steel stencil must be used, along with a laser etching process, to ensure hole accuracy of \u00b10.01 mm and smooth edges, thereby preventing solder paste printing defects.<\/p>\n
Poor screen printing results (such as solder paste sagging) are the primary cause of bridging; therefore, the following parameters must be carefully controlled:<\/p>\n
a) Solder Paste and Environmental Control: Use spherical solder paste with a particle size of 45\u201375 \u03bcm (such as Aifa RMA390DH3), with a viscosity matched to the stencil; maintain the screen printing environment at a temperature of 20 \u00b1 3\u00b0C and a humidity range of 40%\u201360% to ensure stable viscosity.<\/p>\n
b) Stencil and Squeegee Optimization: Give priority to stainless steel stencils and use polyurethane squeegees with a Shore hardness of 70\u201380, set at an angle of 45\u00b0\u201360\u00b0, with a printing speed of 20\u201330 mm\/s and a pressure of 0.1\u20130.2 MPa, to prevent solder paste overflow or insufficient filling.<\/p>\n
c) Solder paste parameter control: Maintain a moderate modulus of elasticity and control the solvent content between 8% and 12% to prevent sagging, bubbles, and poor flow.<\/p>\n
d) Equipment Selection and Maintenance: Select high-precision screen printing machines (such as Topo Electromechanical or the DEK series), and regularly calibrate the squeegee, positioning accuracy, and table levelness to ensure stable equipment performance.<\/p>\n
Optimize the reflow soldering temperature profile by gradually increasing the temperature to prevent excessive solder flow; improve placement accuracy so that any offset does not exceed one-third of the pin width; strengthen quality control of components and PCBs by selecting components with good solderability and ensuring that pads are clean and free of oxidation.<\/p>\n
SMT soldering quality directly determines the reliability of printed circuit modules and the entire device. Solder balls, the Manhattan effect, and fine-pitch bridging are the primary quality bottlenecks; these issues result from the combined effects of multiple factors, requiring process optimization and enhanced control at their root causes.<\/p>\n
By optimizing the soldering temperature profile, strictly controlling the quality of PCBs and stencils, standardizing the use of solder paste, and enhancing the accuracy of screen printing and component placement, it is possible to effectively reduce various types of defects; at the same time, strictly adhering to operating procedures and improving staff competence are also key to quality control.<\/p>\n
The practical methods summarized in this article are highly targeted and actionable. We hope they will serve as a reference for SMT soldering engineers, help improve the industry\u2019s process standards, and drive the development of electronic devices toward higher reliability, smaller size, and higher density.<\/p>","protected":false},"excerpt":{"rendered":"
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