{"id":1649,"date":"2026-11-13T09:00:00","date_gmt":"2026-11-13T01:00:00","guid":{"rendered":"https:\/\/www.jingpengpcb.com\/?p=1649"},"modified":"2026-10-08T20:16:38","modified_gmt":"2026-10-08T12:16:38","slug":"selective-soldering-process","status":"publish","type":"post","link":"https:\/\/www.jingpengpcb.com\/en\/blog\/selective-soldering-process\/","title":{"rendered":"What Is the Selective Soldering Process? Flux, Preheating, and Soldering Steps"},"content":{"rendered":"<style>\n.jp-tw{overflow-x:auto;margin:26px 0;-webkit-overflow-scrolling:touch;}\n.jp-tw table{border-collapse:collapse;width:100%;min-width:560px;font-size:15px;line-height:1.65;background:#fff;}\n.jp-tw th,.jp-tw td{border:1px solid #dde3ea;padding:11px 14px;text-align:left;vertical-align:top;}\n.jp-tw thead th{background:#eef2f7;color:#1b2733;font-weight:700;white-space:nowrap;border-bottom:2px solid #c9d3de;}\n.jp-tw tbody tr:nth-child(even){background:#fafbfc;}\n.jp-tw tbody tr:hover{background:#f1f5fa;}\n.jp-tw tbody td:first-child{font-weight:600;color:#1b2733;}\n@media(max-width:767px){.jp-tw table{font-size:14px;min-width:520px;}.jp-tw th,.jp-tw td{padding:9px 11px;}}\n<\/style>\n<p>In the field of modern electronics manufacturing, as products continue to evolve and advance, soldering processes are also constantly evolving. Compared to traditional wave soldering, selective soldering represents an innovation in soldering methods and has garnered widespread attention in PCB (Printed Circuit Board) manufacturing. This article will delve into the process and significance of selective soldering in PCB manufacturing.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" width=\"716\" height=\"550\" class=\"alignnone size-large wp-image-7981\" src=\"https:\/\/www.jingpengpcb.com\/wp-content\/uploads\/2023\/09\/flexible-pcb.webp\" alt=\"Flexible Printed Circuit Boards\" srcset=\"https:\/\/www.jingpengpcb.com\/wp-content\/uploads\/2023\/09\/flexible-pcb.webp 716w, https:\/\/www.jingpengpcb.com\/wp-content\/uploads\/2023\/09\/flexible-pcb-300x230.webp 300w\" sizes=\"auto, (max-width: 716px) 100vw, 716px\" \/><\/p>\n<h2>Advanced: How to Choose Among the Three Technical Approaches for Through-Hole Soldering<\/h2>\n<p>To understand the value of selective soldering, we must first view it within the broader context of through-hole soldering. Currently, there are three main types of through-hole soldering methods used in mass production:<\/p>\n<div class=\"jp-tw\">\n<table>\n<thead>\n<tr>\n<th>Method<\/th>\n<th>Instructions<\/th>\n<th>Advantages<\/th>\n<th>Limitations<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Wave Soldering<\/td>\n<td>Run the entire board through the solder bath<\/td>\n<td>Highest efficiency, lowest cost<\/td>\n<td>Since the entire board is heated, a fixture is needed to protect the surface-mount components.<\/td>\n<\/tr>\n<tr>\n<td>Selective Soldering<\/td>\n<td>The nozzle directs the tin wave only at the specified solder joints.<\/td>\n<td>Localized heating, no fixtures required<\/td>\n<td>Welding is performed one spot at a time in sequence, resulting in a longer cycle time<\/td>\n<\/tr>\n<tr>\n<td>Manual Welding<\/td>\n<td>Manual, piece-by-piece welding<\/td>\n<td>Flexible, with zero equipment investment<\/td>\n<td>Poor consistency; operator-dependent<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>The key advantage of selective soldering lies in<strong>\"Only the areas that need to be soldered should come into contact with the solder wave.\"<\/strong>. This results in three interrelated benefits: no need to manufacture masking jigs, no secondary heating of already mounted components, and a significant reduction in flux and solder consumption. For scenarios involving double-sided boards with a small number of remaining through-hole components, it is virtually the only viable automated solution.<\/p>\n<ul>\n<li><strong>There are few through-hole components, and there are already surface-mount components on the board.<\/strong>: Selective soldering is the most appropriate;<\/li>\n<li><strong>Many through-hole components; simple board layout<\/strong>: Wave soldering is more cost-effective;<\/li>\n<li><strong>Samples and Repairs<\/strong>: Manual welding is sufficient.<\/li>\n<\/ul>\n<ol>\n<li>Count and map the distribution of through-hole solder joints;<\/li>\n<li>Check whether there are any soldered surface-mount components on the board;<\/li>\n<li>Select a welding method based on cycle time requirements and budget.<\/li>\n<\/ol>\n<p><img loading=\"lazy\" decoding=\"async\" width=\"1200\" height=\"800\" class=\"alignnone size-large wp-image-205\" src=\"https:\/\/www.jingpengpcb.com\/wp-content\/uploads\/2022\/11\/smt-liucheng.jpg\" alt=\"SMT Assembly Process\" srcset=\"https:\/\/www.jingpengpcb.com\/wp-content\/uploads\/2022\/11\/smt-liucheng.jpg 1200w, https:\/\/www.jingpengpcb.com\/wp-content\/uploads\/2022\/11\/smt-liucheng-300x200.jpg 300w, https:\/\/www.jingpengpcb.com\/wp-content\/uploads\/2022\/11\/smt-liucheng-1024x683.jpg 1024w, https:\/\/www.jingpengpcb.com\/wp-content\/uploads\/2022\/11\/smt-liucheng-768x512.jpg 768w\" sizes=\"auto, (max-width: 1200px) 100vw, 1200px\" \/><\/p>\n<p>For the surface-mount process, see<a href=\"\/en\/smt-factory\/\">SMT Assembly<\/a>; For assembly capabilities, see<a href=\"\/en\/pcba-capabilities\/\">PCBA Assembly Capabilities<\/a>\u3002<\/p>\n<h2><strong>Selective Soldering Process Flow<\/strong><\/h2>\n<p>The essence of the selective soldering process lies in its name: \u201cselective.\u201d Unlike wave soldering, which coats the entire board, selective soldering exposes only specific areas to the solder wave, resulting in a more precise and controllable soldering process. The following are the main steps in the selective soldering process:<\/p>\n<h3><strong>1. Flux Application<\/strong><\/h3>\n<p>In selective soldering, the flux application process is critical. Flux serves multiple purposes during the soldering process, including preventing bridging (short circuits between two conductors), preventing PCB oxidation, and ensuring solder joint reliability. Flux application is typically performed by an X\/Y robot that carries the PCB and evenly applies flux to the areas to be soldered through a flux nozzle. This step can be carried out in various ways, including single-nozzle spraying, micro-hole jetting, and synchronized multi-point or pattern spraying.<\/p>\n<h2>Advanced: What Exactly Does Flux Do in Soldering?<\/h2>\n<p>The original text summarizes the functions of flux into three points: preventing bridging, preventing oxidation, and ensuring reliability. To elaborate, flux performs four functions during a single soldering process:<\/p>\n<div class=\"jp-tw\">\n<table>\n<thead>\n<tr>\n<th>Function<\/th>\n<th>Mechanism<\/th>\n<th>Behavior After Failure<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Removing the Oxidation Layer<\/td>\n<td>The active ingredient reacts with copper and tin oxides<\/td>\n<td>Solder Does Not Wet the Surface; Cold Solder Joints<\/td>\n<\/tr>\n<tr>\n<td>Prevent Reoxidation<\/td>\n<td>Molten flux covers the metal surface, isolating it from the air.<\/td>\n<td>Dark solder joints, poor wetting<\/td>\n<\/tr>\n<tr>\n<td>Reduce surface tension<\/td>\n<td>Improving the Flowability of Molten Tin<\/td>\n<td>Poor hole filling, tip formation<\/td>\n<\/tr>\n<tr>\n<td>Heat Transfer Medium<\/td>\n<td>Liquid Flux Enhances Heat Transfer<\/td>\n<td>Local Cold Welding<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>Fluxes can be classified into several categories based on their activity and residue levels; when selecting a type, it is important to ensure it meets the product\u2019s cleaning performance and reliability requirements:<\/p>\n<ul>\n<li><strong>Rosin-based (R \/ RMA \/ RA)<\/strong>: Reactivity increases in order; good weldability; cleaning may be required depending on residual reactivity;<\/li>\n<li><strong>No-Clean Type<\/strong>: Low solids content and minimal residue, making it suitable for products with test probes or those that cannot be cleaned;<\/li>\n<li><strong>Water-soluble<\/strong>: Highly reactive; can be washed off with water, but must be thoroughly rinsed;<\/li>\n<li><strong>Halogen-free<\/strong>: Meets halogen content limits; commonly used in export products.<\/li>\n<\/ul>\n<p>Selective soldering is typically used<strong>Low-solids, no-rinse<\/strong>Flux is applied only to the area to be soldered, in small quantities, and is often not rinsed off afterward. Controlling the amount used is critical:<strong>Applying too much can leave excess residue and increase the risk of splashing, while applying too little results in insufficient oxidation capacity.<\/strong>\u3002<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" width=\"1238\" height=\"250\" class=\"alignnone size-large wp-image-534\" src=\"https:\/\/www.jingpengpcb.com\/wp-content\/uploads\/2022\/11\/smt-featured-pictures.jpg\" alt=\"SMT\" srcset=\"https:\/\/www.jingpengpcb.com\/wp-content\/uploads\/2022\/11\/smt-featured-pictures.jpg 1238w, https:\/\/www.jingpengpcb.com\/wp-content\/uploads\/2022\/11\/smt-featured-pictures-300x61.jpg 300w, https:\/\/www.jingpengpcb.com\/wp-content\/uploads\/2022\/11\/smt-featured-pictures-1024x207.jpg 1024w, https:\/\/www.jingpengpcb.com\/wp-content\/uploads\/2022\/11\/smt-featured-pictures-768x155.jpg 768w\" sizes=\"auto, (max-width: 1238px) 100vw, 1238px\" \/><\/p>\n<p>For solder paste and flux, see<a href=\"\/en\/blog\/solder-paste-for-smt-processing\/\">Solder Paste for SMT Manufacturing<\/a>; For welding defects, see<a href=\"\/en\/blog\/reasons-for-poor-soldering-of-circuit-boards\/\">Analysis of the Causes of Soldering Defects on Printed Circuit Boards<\/a>\u3002<\/p>\n<h3><strong>2. PCB Preheating<\/strong><\/h3>\n<p>The purpose of preheating a PCB is to remove solvents from the flux and ensure that the flux has the correct viscosity before entering the solder wave. Unlike traditional soldering, the main purpose of preheating is not to reduce thermal stress, but rather to create ideal conditions for soldering.<\/p>\n<h2>Advanced: How to Set Preheating Parameters<\/h2>\n<p>The original text points out that the purpose of preheating is not to reduce thermal stress, but to bring the flux to the appropriate viscosity. This statement is accurate and explains why the criterion for determining preheating is<strong>Baking sheet temperature, not time<\/strong>\uff1a<\/p>\n<div class=\"jp-tw\">\n<table>\n<thead>\n<tr>\n<th>Parameters<\/th>\n<th>Common Range<\/th>\n<th>Excessive Consequences<\/th>\n<th>Consequences of Being Too Low<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Top Surface Preheating Temperature<\/td>\n<td>90\u2013110 \u00b0C<\/td>\n<td>Premature Failure of Flux<\/td>\n<td>Solvent residue, splatter<\/td>\n<\/tr>\n<tr>\n<td>Base Preheating Temperature<\/td>\n<td>100\u2013130 \u00b0C<\/td>\n<td>Heat Damage to Panels and Components<\/td>\n<td>There is a significant temperature difference upon entering Xibo<\/td>\n<\/tr>\n<tr>\n<td>Preheating Time<\/td>\n<td>30\u201390 seconds<\/td>\n<td>Decline in Production Capacity<\/td>\n<td>The solvent has not fully evaporated<\/td>\n<\/tr>\n<tr>\n<td>Heating Rate<\/td>\n<td>\u2264 2\u20133 \u00b0C\/s<\/td>\n<td>Thermal Shock Cracking of Components<\/td>\n<td>\u2014<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>The most typical sign of insufficient warm-up is<strong>Flux splatters when it comes into contact with the tin bath<\/strong>. If the solvent has not fully evaporated when it comes into contact with molten tin at high temperatures, the sudden expansion of the solvent can cause tin beads to be flung onto the board, which not only causes contamination but may also result in short circuits caused by the tin beads. Therefore, if you notice an unusual increase in tin beads, you should first check whether the preheating temperature is too low.<\/p>\n<ul>\n<li><strong>Thick Plates and Components with High Thermal Capacity<\/strong>: Requires a longer preheating time or a higher preheating temperature;<\/li>\n<li><strong>Temperature differences across the plate surface must be controlled<\/strong>: Excessive temperature differences can lead to inconsistent weld quality in different areas;<\/li>\n<li><strong>Actual Measurements, Not Settings<\/strong>: Use a furnace temperature tester or a temperature measurement plate to verify the actual surface temperature.<\/li>\n<\/ul>\n<ol>\n<li>Preliminary preheating parameters are set based on the board thickness and the thermal capacity of the components;<\/li>\n<li>Measured board temperature curve;<\/li>\n<li>Observe the splashing and wetting patterns, and make minor adjustments until the process stabilizes.<\/li>\n<\/ol>\n<p><img loading=\"lazy\" decoding=\"async\" width=\"515\" height=\"389\" class=\"alignnone size-large wp-image-6789\" src=\"https:\/\/www.jingpengpcb.com\/wp-content\/uploads\/2023\/08\/Faulty-soldering.png\" alt=\"Cold solder joint\" srcset=\"https:\/\/www.jingpengpcb.com\/wp-content\/uploads\/2023\/08\/Faulty-soldering.png 515w, https:\/\/www.jingpengpcb.com\/wp-content\/uploads\/2023\/08\/Faulty-soldering-300x227.png 300w\" sizes=\"auto, (max-width: 515px) 100vw, 515px\" \/><\/p>\n<p>For welding recommendations, see<a href=\"\/en\/blog\/12-suggestions-for-smt-processing-and-welding\/\">12 Tips for SMT Soldering<\/a>; For testing methods, see<a href=\"\/en\/pcba-testing\/\">PCBA Testing<\/a>\u3002<\/p>\n<h3><strong>3. Welding Processes<\/strong><\/h3>\n<p>Selective soldering processes are generally classified into two types: drag soldering and dip soldering.<\/p>\n<h4><strong>Drag Welding Process<\/strong><\/h4>\n<p>The drag soldering process is performed on a solder wave using a single small soldering tip and is suitable for very tight spaces on a PCB. The PCB moves across the solder wave at the nozzle at varying speeds and angles to achieve optimal soldering quality. The advantage of drag soldering lies in the combined motion of the solder bath and the PCB, which improves heat transfer efficiency. However, please note that the drag soldering process takes longer.<\/p>\n<h4><strong>Dip-Soldering Process<\/strong><\/h4>\n<p>The dip-soldering process typically involves multiple solder nozzles, each corresponding to a specific solder joint on the PCB. Although it is less flexible than the wave-soldering process, it offers high throughput and relatively low equipment costs. Depending on the size of the PCB, single-board or multi-board parallel conveyance is possible, and all solder joints undergo flux application, preheating, and soldering simultaneously in a parallel manner.<\/p>\n<h2>Advanced: Criteria for Selecting Drag Soldering and Immersion Soldering<\/h2>\n<p>The original text describes two methods: drag welding and immersion welding. Once the differences between the two are quantified, the selection criteria will be clearer:<\/p>\n<div class=\"jp-tw\">\n<table>\n<thead>\n<tr>\n<th>Comparison Items<\/th>\n<th>Drag Welding<\/th>\n<th>Dip Soldering<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Soldering Tip<\/td>\n<td>Single small welding nozzle<\/td>\n<td>Each of the multiple soldering tips corresponds to a single solder joint.<\/td>\n<\/tr>\n<tr>\n<td>Types of Exercise<\/td>\n<td>The PCB moves along a path on the tin bath<\/td>\n<td>PCB remains stationary; multiple nozzles are immersed simultaneously<\/td>\n<\/tr>\n<tr>\n<td>Beat<\/td>\n<td>Spot welding, slower<\/td>\n<td>Parallel welding, faster<\/td>\n<\/tr>\n<tr>\n<td>Flexibility<\/td>\n<td>Gao: Changing the model requires only modifying the trajectory<\/td>\n<td>Low; a nozzle assembly replacement is required when changing models.<\/td>\n<\/tr>\n<tr>\n<td>Equipment Costs<\/td>\n<td>Higher<\/td>\n<td>Relatively low<\/td>\n<\/tr>\n<tr>\n<td>Suitable for<\/td>\n<td>Dense solder joints, small spacing, and a wide variety of types<\/td>\n<td>Consistent solder joints, high-volume production, single product type<\/td>\n<\/tr>\n<tr>\n<td>Heat Transfer<\/td>\n<td>Okay, relative motion between the molten tin and the board<\/td>\n<td>Generally<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>The key to making this judgment lies in<strong>Spatial Distribution of Solder Joints<\/strong>. If the spacing between solder joints is very small and their arrangement is irregular, the multiple nozzles used in dip soldering cannot be positioned simultaneously, so drag soldering must be used; if the solder joints are arranged in a regular pattern and the batch size is large, the parallel processing advantage of dip soldering is very evident.<\/p>\n<ul>\n<li><strong>The spacing between the weld spots is less than the outer diameter of the nozzle.<\/strong>: Only drag welding can be used;<\/li>\n<li><strong>There are a large number of solder joints on the circuit board.<\/strong>: The cycle time for drag soldering will become a bottleneck; immersion soldering should be evaluated;<\/li>\n<li><strong>Wide variety, small batches<\/strong>: Drag welding involves lower changeover costs and is therefore more suitable.<\/li>\n<\/ul>\n<ol>\n<li>Count the number of solder joints and measure their spacing;<\/li>\n<li>Confirm the product types and changeover frequency;<\/li>\n<li>Determine the approach based on production requirements and evaluate the equipment investment.<\/li>\n<\/ol>\n<p><img loading=\"lazy\" decoding=\"async\" width=\"667\" height=\"376\" class=\"alignnone size-large wp-image-10145\" src=\"https:\/\/www.jingpengpcb.com\/wp-content\/uploads\/2023\/08\/reasons-for-poor-soldering-of-circuit-boards.jpg\" alt=\"Poor Soldering on the Circuit Board\" srcset=\"https:\/\/www.jingpengpcb.com\/wp-content\/uploads\/2023\/08\/reasons-for-poor-soldering-of-circuit-boards.jpg 667w, https:\/\/www.jingpengpcb.com\/wp-content\/uploads\/2023\/08\/reasons-for-poor-soldering-of-circuit-boards-300x169.jpg 300w\" sizes=\"auto, (max-width: 667px) 100vw, 667px\" \/><\/p>\n<p>See \"Components and Procurement\"<a href=\"\/en\/components-sourcing\/\">Component Supply<\/a>; For more information on selective soldering, see<a href=\"\/en\/faq\/selective-soldering\/\">Guide to Selective Soldering Technology<\/a>\u3002<\/p>\n<h2><strong>The Importance of Selective Soldering<\/strong><\/h2>\n<p>The introduction of selective soldering has opened up new possibilities for PCB manufacturing. Its precision and controllability have made soldering on highly complex PCBs much easier. This is particularly important for modern electronic products, which are filled with a large number of small, high-density<a href=\"\/en\/components-sourcing\/\">Components<\/a>This is crucial for PCBs.<\/p>\n<p>In addition, selective soldering helps improve soldering quality and reliability by reducing unnecessary defects, such as bridging and cold solder joints. It also lowers production costs by making more efficient use of flux and energy.<\/p>\n<h2>Advanced: Flux Application Methods and Nozzle Maintenance<\/h2>\n<p>The original text mentions three coating methods: single-nozzle spray, micro-orifice jet, and synchronous multi-point\/pattern spray. The differences between them primarily lie in precision and cycle time:<\/p>\n<div class=\"jp-tw\">\n<table>\n<thead>\n<tr>\n<th>Method<\/th>\n<th>Principle<\/th>\n<th>Accuracy<\/th>\n<th>Beat<\/th>\n<th>Applicable<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Single-nozzle spray<\/td>\n<td>A single nozzle sprays along a path<\/td>\n<td>\u4e2d<\/td>\n<td>slower<\/td>\n<td>The solder joints are scattered and irregular in shape<\/td>\n<\/tr>\n<tr>\n<td>Microporous Jet<\/td>\n<td>Micro-hole Nozzles: On-Demand Spraying<\/td>\n<td>\u9ad8<\/td>\n<td>\u5feb<\/td>\n<td>Fine pitch; usage must be precisely controlled<\/td>\n<\/tr>\n<tr>\n<td>Synchronized Multi-Point\/Pattern Spraying<\/td>\n<td>Synchronized Spraying by Multiple Nozzles According to a Pattern<\/td>\n<td>Upper-middle<\/td>\n<td>Fastest<\/td>\n<td>Large quantities arranged according to specific rules<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>Regardless of the method used, the nozzle is a component that requires special attention during maintenance.<strong>Flux can crystallize and dry out at the nozzle opening, gradually altering the spray pattern and flow rate.<\/strong>...which manifests as insufficient coating or misalignment. Therefore, a schedule for regular cleaning and replacement should be established, and the spray pattern should be checked at the start of each shift.<\/p>\n<ul>\n<li><strong>Clean the nozzle daily<\/strong>: Use a compatible cleaning agent as specified by the manufacturer;<\/li>\n<li><strong>Regularly calibrate the spray volume<\/strong>: Verify using the weighing method or by testing with spray paper;<\/li>\n<li><strong>Flux should be stored in a sealed container away from light.<\/strong>: Solvent evaporation can alter viscosity.<\/li>\n<\/ul>\n<h2><strong>Conclusion<\/strong><\/h2>\n<p>The selective soldering process has opened up new prospects for PCB manufacturing and provided more possibilities for the development of electronic products. Through precise soldering control and process optimization, it not only improves product quality but also reduces manufacturing costs. As a result, the selective soldering process has become an indispensable part of PCB manufacturing.<\/p>\n<h2>Advanced: Common Defects in Selective Soldering and Process Windows<\/h2>\n<p>By matching the most common defects found on-site with their process causes, you can avoid unnecessary detours during troubleshooting:<\/p>\n<div class=\"jp-tw\">\n<table>\n<thead>\n<tr>\n<th>Defects<\/th>\n<th>Main Causes<\/th>\n<th>Adjust Direction<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Bridge Connection<\/td>\n<td>Insufficient flux, narrow pin spacing, and excessive soldering speed<\/td>\n<td>Add flux, reduce the soldering speed, and adjust the angle<\/td>\n<\/tr>\n<tr>\n<td>Poor Solder Joint \/ Lack of Wetting<\/td>\n<td>Insufficient flux activity, inadequate preheating, and oxidized leads<\/td>\n<td>Increase the preheat temperature and replace the flux<\/td>\n<\/tr>\n<tr>\n<td>Insufficient Solder \/ Poor Through-Hole Soldering<\/td>\n<td>Insufficient solder paste height, short soldering time, and high aspect ratio<\/td>\n<td>Increase the wave height and extend the tinning time<\/td>\n<\/tr>\n<tr>\n<td>Solder balls<\/td>\n<td>Insufficient preheating causes solvent splatter<\/td>\n<td>Increase the preheating temperature or extend the preheating time<\/td>\n<\/tr>\n<tr>\n<td>Taper<\/td>\n<td>Evacuation proceeded too quickly; the tin temperature was too low<\/td>\n<td>Adjust the evacuation speed and temperature<\/td>\n<\/tr>\n<tr>\n<td>Lifted Solder Pads<\/td>\n<td>Localized overheating, excessive tinning time<\/td>\n<td>Reduce processing time and lower the tin temperature<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>Among them<strong>Poor through-hole weld quality (insufficient pore fill rate) is a criterion specific to through-hole welding.<\/strong>. The industry typically requires that the solder fill height in through-holes reach at least 75% of the board thickness (IPC Class 2), with even higher requirements for high-reliability products. The main factors affecting the fill rate are the ratio of the hole diameter to the pin diameter, the contact time in the solder bath, and the preheat temperature.<\/p>\n<ul>\n<li><strong>Moderate pore size ratio<\/strong>: Filling is best when the aperture is 0.2\u20130.4 mm larger than the pin;<\/li>\n<li><strong>The tinning time must be sufficient.<\/strong>: Typically 1\u20133 seconds; for thick plates, use the upper limit;<\/li>\n<li><strong>Tin-Zinc Alloy Specifications<\/strong>: Approximately 260\u2013270 \u00b0C for lead-free, and approximately 240\u2013250 \u00b0C for leaded.<\/li>\n<\/ul>\n<h2>Frequently Asked Questions<\/h2>\n<h3>Q1: Is selective soldering suitable for all PCBs?<\/h3>\n<p>No. Selective soldering is typically used for PCBs that require highly precise component placement, particularly surface-mount boards such as BGAs and ICs.<\/p>\n<h3>Q2: Does selective soldering increase manufacturing costs?<\/h3>\n<p>Yes, selective soldering may slightly increase manufacturing costs, but it improves the quality and reliability of mounted components, thereby reducing the risk of subsequent repairs and failures, so it is generally worth the investment.<\/p>\n<h3>Q3: Is selective soldering suitable for small-batch production?<\/h3>\n<p>Yes, selective soldering is suitable for production runs of all sizes, including small batches. It ensures that every PCB has high-quality solder joints.<\/p>\n<h3>Q4: Does selective soldering require special equipment?<\/h3>\n<p>Yes, selective soldering typically requires specialized equipment, such as flux sprayers, soldering machines, and preheating ovens. This equipment can be selected and configured according to production needs.<\/p>\n<h3>Q5: Does selective soldering increase the manufacturing cycle time?<\/h3>\n<p>Selective soldering may slightly increase the manufacturing cycle time because it involves additional steps. However, this delay can usually be minimized through advance planning and proper scheduling.<\/p>\n<p>In any case, the selective soldering process is essential for ensuring high-quality PCB connections. If you would like to learn more about selective soldering, please feel free to consult our professional engineers.<\/p>\n<h3>Q6: Can selective soldering completely replace wave soldering?<\/h3>\n<p>It cannot be a complete replacement. Wave soldering offers clear efficiency advantages and lower unit costs in high-volume production scenarios with dense through-hole components on the board; selective soldering, on the other hand, offers the benefits of localized soldering and the elimination of fixtures. Most factories operate both processes in parallel: simple boards are processed using wave soldering, while small quantities of through-hole components on double-sided assemblies are processed using selective soldering.<\/p>\n<h3>Q7: Why doesn\u2019t selective soldering require a fixture?<\/h3>\n<p>Since the solder wave is confined to a small area around the nozzle and does not come into contact with the rest of the board, the already-mounted components are naturally not reheated. In wave soldering, however, the entire board passes through the solder wave, so fixtures must be used to shield areas that do not require soldering. Eliminating the need for fixtures not only reduces costs but also shortens changeover time.<\/p>\n<h3>Q8: Is nitrogen protection necessary?<\/h3>\n<p>It depends on the product. A nitrogen atmosphere can reduce tin dross formation and improve wettability, which is particularly beneficial for lead-free processes and high-reliability products; however, it increases operating costs. If no-clean flux is used and the soldering yield has stabilized, it is generally not necessary to install a nitrogen system.<\/p>\n<h3>Q9: How do you determine the speed and angle for drag welding?<\/h3>\n<p>Typically, start with the manufacturer\u2019s recommended settings and then fine-tune them based on the appearance of the solder joints. Excessive speed can cause insufficient solder and bridging, while too slow a speed can lead to localized overheating; the angle affects the flow and release of the molten solder, and an inclination of 4\u00b0\u201310\u00b0 is generally used. The final determination is based on through-hole yield and visual inspection; if necessary, a cross-section should be examined for confirmation.<\/p>\n<h3>Q10: How do you estimate the cycle time for selective soldering?<\/h3>\n<p>Roughly equal to \u201cnumber of solder joints \u00d7 time per joint + travel time + time to load and unload the board\u201d . The time per solder joint in drag soldering is typically 1\u20133 seconds; if a board has hundreds of solder joints, the cycle time may reach several minutes. Therefore, cycle time calculations should be performed using actual board designs during the selection phase to avoid production capacity falling short of expectations.<\/p>\n<h3>Q11: Is cleaning required after welding?<\/h3>\n<p>In most cases, cleaning is not necessary. Selective soldering typically uses no-clean flux in very small quantities, and the resulting residue is insufficient to affect reliability. However, if the product operates in a high-humidity or high-voltage environment, or if subsequent processes such as conformal coating or dispensing are involved, it is still recommended to evaluate the need for cleaning. The decision on whether to clean should be determined during the process validation phase, rather than being skipped by default.<\/p>\n<h3>Q12: How can you verify that the selective soldering process is properly set up?<\/h3>\n<p>We recommend performing three steps: First, conduct a visual and AOI inspection of the appearance to ensure there are no bridging, solder balls, or insufficient solder. Second, perform a cross-section inspection to verify the through-hole fill rate meets the required percentage. Third, conduct electrical and functional tests to confirm reliable connections. Once all three steps have been successfully completed, finalize the parameters and incorporate them into the inspection checklist.<\/p>\n<p><strong>Additional Information<\/strong>\uff1a<a href=\"\/en\/faq\/selective-soldering\/\">Guide to Selective Soldering Technology<\/a> \u00b7 <a href=\"\/en\/blog\/pcb-selective-soldering-technology\/\">A Detailed Explanation of Selective Soldering Technology for PCBs<\/a><\/p>","protected":false},"excerpt":{"rendered":"<p>\u5728\u73b0\u4ee3\u7535\u5b50\u5236\u9020\u9886\u57df\uff0c\u968f\u7740\u4ea7\u54c1\u7684\u4e0d\u65ad\u8fed\u4ee3\u548c\u53d1\u5c55\uff0c\u710a\u63a5\u5de5\u827a\u4e5f\u5728\u4e0d\u65ad\u6f14\u8fdb\u3002\u4e0e\u4f20\u7edf\u7684\u6ce2\u5cf0\u710a\u76f8\u6bd4\uff0c\u9009\u62e9\u6027\u710a\u63a5\u662f\u4e00\u79cd\u710a\u63a5\u65b9 [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":7981,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"slim_seo":{"title":"What Is the Selective Soldering Process? Flux, Preheating, and Soldering Steps","description":"What is the selective soldering process like? Explain the role and key parameters of each step in the following order: flux application, PCB preheating, soldering, drag soldering, and dip soldering. Break it down step by step, from the underlying principles to the practical implementation, so that even beginners can understand it."},"_slim_seo_primary_term_category":0,"_slim_seo_primary_term_post_tag":0,"footnotes":""},"categories":[409,26],"tags":[],"class_list":["post-1649","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-smt-assembly","category-blog"],"_links":{"self":[{"href":"https:\/\/www.jingpengpcb.com\/en\/wp-json\/wp\/v2\/posts\/1649","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.jingpengpcb.com\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.jingpengpcb.com\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.jingpengpcb.com\/en\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.jingpengpcb.com\/en\/wp-json\/wp\/v2\/comments?post=1649"}],"version-history":[{"count":5,"href":"https:\/\/www.jingpengpcb.com\/en\/wp-json\/wp\/v2\/posts\/1649\/revisions"}],"predecessor-version":[{"id":20011,"href":"https:\/\/www.jingpengpcb.com\/en\/wp-json\/wp\/v2\/posts\/1649\/revisions\/20011"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.jingpengpcb.com\/en\/wp-json\/wp\/v2\/media\/7981"}],"wp:attachment":[{"href":"https:\/\/www.jingpengpcb.com\/en\/wp-json\/wp\/v2\/media?parent=1649"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.jingpengpcb.com\/en\/wp-json\/wp\/v2\/categories?post=1649"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.jingpengpcb.com\/en\/wp-json\/wp\/v2\/tags?post=1649"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}