{"id":3897,"date":"2026-05-11T06:06:38","date_gmt":"2026-05-11T06:06:38","guid":{"rendered":"https:\/\/blogs.lcsc.com\/blog\/?p=3897"},"modified":"2026-05-11T06:07:01","modified_gmt":"2026-05-11T06:07:01","slug":"pcb-assembly-process-smt-soldering","status":"publish","type":"post","link":"https:\/\/blogs.lcsc.com\/blog\/pcb-assembly-process-smt-soldering\/","title":{"rendered":"PCB Assembly Process: SMT, THT &#038; Soldering"},"content":{"rendered":"<h2>Key<b><span data-font-family=\"Arial\"> Takeaways<\/span><\/b><\/h2>\n<ul>\n<li><b><span data-font-family=\"Arial\"><a href=\"https:\/\/www.lcsc.com\/pcba?spm=wm.sy.dhl.pcb___wm.ssy.ssl.lg&amp;lcsc_vid=QQddA1RXQgAKXwYATgIIBVdVEgdXAVNXQgVYV1xVElkxVlNRT1VZXlRWQFdbUzsOAxUeFF5JWBYZEEoKFBINSQcJGk4%3D\">PCB assembly<\/a> involves two primary component mounting technologies: <\/span><\/b><span data-font-family=\"Arial\">SMT (surface-mount) and through-hole (THT), each with distinct process flows.<\/span><\/li>\n<li><b><span data-font-family=\"Arial\">Reflow soldering is the standard process for SMT components; <\/span><\/b><span data-font-family=\"Arial\">wave soldering is the traditional process for through-hole components and selective SMT on the bottom side.<\/span><\/li>\n<li><span data-font-family=\"Arial\">The choice between SMT, through-hole, or mixed assembly affects board cost, cycle time, mechanical robustness, and component availability.<\/span><\/li>\n<li><span data-font-family=\"Arial\">Design for Manufacturing and Assembly (DFMA) principles applied during PCB layout directly reduce assembly defects, rework costs, and time-to-market.<\/span><\/li>\n<li><span data-font-family=\"Arial\">PCB surface finish (HASL, ENIG, OSP, ENEPIG) determines solderability, shelf life, and compatibility with fine-pitch components.<\/span><\/li>\n<li><b><span data-font-family=\"Arial\">Moisture Sensitivity Level (MSL) governs how long ICs can remain outside their moisture barrier bag before baking is required. <\/span><\/b><span data-font-family=\"Arial\">MSL 1 is unlimited; MSL 3 requires use within 168 hours of opening.<\/span><\/li>\n<\/ul>\n<h2><b><span data-font-family=\"Arial\">1. The PCB Assembly Process: An End-to-End Overview<\/span><\/b><\/h2>\n<p><span data-font-family=\"Arial\">A fully assembled PCB \u2014 called a <a href=\"https:\/\/blogs.lcsc.com\/blog\/from-pcb-prototype-to-pcba-how-lcsc-ensures-quality-and-efficiency\/\">PCBA<\/a> (Printed Circuit Board Assembly) \u2014 starts as a bare board and goes through a series of controlled manufacturing steps before it becomes a functional electronic module. Here is the complete process sequence for a standard <a href=\"https:\/\/blogs.lcsc.com\/blog\/high-reliability-smt-and-testing-techniques-for-automotive-pcbas\/\">SMT<\/a> assembly run:<\/span><\/p>\n<h4><b><span data-font-family=\"Arial\">Step 1: Incoming Inspection and Kitting<\/span><\/b><\/h4>\n<p><span data-font-family=\"Arial\">Before assembly begins, the bare PCBs and all components are inspected. The PCB is checked for warpage, surface finish quality, and dimensional accuracy. Components are verified against the Bill of Materials (BOM) \u2014 part numbers, package codes, polarity, and quantity. This kitting stage identifies substitutions, shortages, or counterfeit components before they reach the line.<\/span><\/p>\n<h4><b><span data-font-family=\"Arial\">Step 2: <a href=\"https:\/\/blogs.lcsc.com\/blog\/choosing-the-right-solder-mask-in-pcba-manufacturing\/\">Solder Paste<\/a> Application<\/span><\/b><\/h4>\n<p><span data-font-family=\"Arial\">A stainless steel or nickel-alloy stencil \u2014 laser-cut to match the PCB\u2019s pad geometry \u2014 is aligned over the board. A squeegee draws solder paste (a mixture of tin-silver-copper alloy particles suspended in flux) across the stencil, depositing a precise volume of paste onto each SMD pad. Aperture size in the stencil is typically 1:1 with the pad, or slightly reduced for fine-pitch components to prevent bridging.<\/span><\/p>\n<p><span data-font-family=\"Arial\">Critical parameter: solder paste volume consistency determines joint quality. Too little paste causes insufficient joints and cold solder; too much causes bridging between adjacent pads. Automated Solder Paste Inspection (SPI) systems verify paste volume and alignment after every stencil print.<\/span><\/p>\n<h4><b><span data-font-family=\"Arial\">Step 3: SMT Component Placement<\/span><\/b><\/h4>\n<p><span data-font-family=\"Arial\">The paste-printed board moves to a pick-and-place machine, which uses vacuum nozzles to pick components from tape-and-reel, tray, or tube feeders and place them precisely onto the pasted pads. Modern pick-and-place machines achieve placement accuracies of \u00b125 \u03bcm at speeds of 30,000 to 100,000 components per hour.<\/span><\/p>\n<h4><b><span data-font-family=\"Arial\">Step 4: Reflow Soldering<\/span><\/b><\/h4>\n<p><span data-font-family=\"Arial\">The populated board passes through a reflow oven \u2014 a conveyor furnace with multiple temperature zones. The thermal profile brings the board through four stages: preheat (activates flux, drives off volatiles), soak (equalises temperature across the board), reflow (melts the solder paste, forming solder joints), and cooling (solidifies the joints with the correct microstructure).<\/span><\/p>\n<p><span data-font-family=\"Arial\">Standard lead-free solder (SAC305: 96.5% Sn, 3% Ag, 0.5% Cu) has a melting point of approximately 217\u00b0C. A typical lead-free reflow profile peaks at 245\u2013260\u00b0C with the time above liquidus (TAL) controlled to 30\u201390 seconds.<\/span><\/p>\n<table>\n<tbody>\n<tr>\n<td colspan=\"1\" rowspan=\"1\" width=\"146\"><b><span data-font-family=\"Arial\">Reflow Zone<\/span><\/b><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"133\"><b><span data-font-family=\"Arial\">Temperature Range<\/span><\/b><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"117\"><b><span data-font-family=\"Arial\">Duration<\/span><\/b><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"226\"><b><span data-font-family=\"Arial\">Purpose<\/span><\/b><\/td>\n<\/tr>\n<tr>\n<td colspan=\"1\" rowspan=\"1\" width=\"146\"><b><span data-font-family=\"Arial\">Preheat<\/span><\/b><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"133\"><span data-font-family=\"Arial\">25\u00b0C \u2192 150\u00b0C<\/span><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"117\"><span data-font-family=\"Arial\">60\u2013120 s<\/span><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"226\"><span data-font-family=\"Arial\">Activates flux; evaporates solvents; prevents thermal shock<\/span><\/td>\n<\/tr>\n<tr>\n<td colspan=\"1\" rowspan=\"1\" width=\"146\"><b><span data-font-family=\"Arial\">Soak<\/span><\/b><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"133\"><span data-font-family=\"Arial\">150\u00b0C \u2192 180\u00b0C<\/span><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"117\"><span data-font-family=\"Arial\">60\u2013120 s<\/span><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"226\"><span data-font-family=\"Arial\">Equalises temperature across the board; flux activates and cleans pad surfaces<\/span><\/td>\n<\/tr>\n<tr>\n<td colspan=\"1\" rowspan=\"1\" width=\"146\"><b><span data-font-family=\"Arial\">Reflow<\/span><\/b><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"133\"><span data-font-family=\"Arial\">217\u00b0C \u2192 245\u2013260\u00b0C peak<\/span><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"117\"><span data-font-family=\"Arial\">30\u201390 s above liquidus<\/span><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"226\"><span data-font-family=\"Arial\">Solder melts, wets pads and component leads, forms metallurgical joint<\/span><\/td>\n<\/tr>\n<tr>\n<td colspan=\"1\" rowspan=\"1\" width=\"146\"><b><span data-font-family=\"Arial\">Cooling<\/span><\/b><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"133\"><span data-font-family=\"Arial\">260\u00b0C \u2192 &lt; 100\u00b0C<\/span><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"117\"><span data-font-family=\"Arial\">Controlled ramp<\/span><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"226\"><span data-font-family=\"Arial\">Solidifies joints with correct grain structure; too fast causes brittle joints<\/span><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<h4><b><span data-font-family=\"Arial\">Step 5: Automated Optical Inspection (AOI)<\/span><\/b><\/h4>\n<p><span data-font-family=\"Arial\">After reflow, every board passes under an AOI system \u2014 a camera array that compares the assembled board against a reference image. AOI detects missing components, wrong polarity, lifted leads, solder bridges, insufficient solder, and tombstoning (a defect where a small passive stands on end due to unequal reflow on both pads).<\/span><\/p>\n<h4><b><span data-font-family=\"Arial\">Step 6: Through-Hole Insertion (if applicable)<\/span><\/b><\/h4>\n<p><span data-font-family=\"Arial\">For mixed-technology boards, through-hole parts are inserted after SMT reflow. Axial and radial leaded components, connectors, and large electrolytics are inserted by hand or automated insertion machines, then soldered by wave or selective soldering.<\/span><\/p>\n<h4><b><span data-font-family=\"Arial\">Step 7: Wave or Selective Soldering<\/span><\/b><\/h4>\n<p><span data-font-family=\"Arial\">Through-hole components are soldered by passing the underside of the board over a wave of molten solder. The board is first fluxed (spray or foam flux), preheated, then passed over the solder wave. Selective soldering uses a small localised solder nozzle to solder specific through-hole areas on mixed boards without exposing SMT components to the wave.<\/span><\/p>\n<h4><b><span data-font-family=\"Arial\">Step 8: Electrical Testing<\/span><\/b><\/h4>\n<p><span data-font-family=\"Arial\">Functional test (FCT) powers up the assembled board and exercises its functionality using test fixtures or boundary scan (JTAG). In-circuit test (ICT) uses a bed-of-nails fixture to verify component values, solder joint continuity, and shorts at the component level. Flying probe testing is a flexible, fixtureless alternative for low-volume or prototype assemblies.<\/span><\/p>\n<h2><b><span data-font-family=\"Arial\">2. SMT vs. Through-Hole: Choosing the Right Assembly Method<\/span><\/b><\/h2>\n<table>\n<tbody>\n<tr>\n<td colspan=\"1\" rowspan=\"1\" width=\"160\"><b><span data-font-family=\"Arial\">Factor<\/span><\/b><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"232\"><b><span data-font-family=\"Arial\">SMT (Surface-Mount)<\/span><\/b><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"232\"><b><span data-font-family=\"Arial\">Through-Hole (THT)<\/span><\/b><\/td>\n<\/tr>\n<tr>\n<td colspan=\"1\" rowspan=\"1\" width=\"160\"><b><span data-font-family=\"Arial\">Component size<\/span><\/b><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"232\"><span data-font-family=\"Arial\">Much smaller; 0201, 0402, QFN, BGA<\/span><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"232\"><span data-font-family=\"Arial\">Larger leads and bodies<\/span><\/td>\n<\/tr>\n<tr>\n<td colspan=\"1\" rowspan=\"1\" width=\"160\"><b><span data-font-family=\"Arial\">Assembly speed<\/span><\/b><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"232\"><span data-font-family=\"Arial\">High \u2014 fully automated pick-and-place<\/span><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"232\"><span data-font-family=\"Arial\">Slower \u2014 manual or semi-automated insertion<\/span><\/td>\n<\/tr>\n<tr>\n<td colspan=\"1\" rowspan=\"1\" width=\"160\"><b><span data-font-family=\"Arial\">Mechanical strength<\/span><\/b><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"232\"><span data-font-family=\"Arial\">Moderate \u2014 solder joints on surface only<\/span><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"232\"><span data-font-family=\"Arial\">High \u2014 leads clinched through board<\/span><\/td>\n<\/tr>\n<tr>\n<td colspan=\"1\" rowspan=\"1\" width=\"160\"><b><span data-font-family=\"Arial\">Vibration resistance<\/span><\/b><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"232\"><span data-font-family=\"Arial\">Lower without underfill<\/span><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"232\"><span data-font-family=\"Arial\">Excellent \u2014 ideal for automotive, industrial<\/span><\/td>\n<\/tr>\n<tr>\n<td colspan=\"1\" rowspan=\"1\" width=\"160\"><b><span data-font-family=\"Arial\">Best for<\/span><\/b><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"232\"><span data-font-family=\"Arial\">High-volume consumer, IoT, mobile, computing<\/span><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"232\"><span data-font-family=\"Arial\">Connectors, power components, harsh environments<\/span><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p><span data-font-family=\"Arial\">Use through-hole when: high-current power connectors and terminal blocks require mechanical pull-out strength; components subject to repeated mating cycles; large electrolytics and inductors where the mass would stress SMT pads; or environments with high vibration, shock, or thermal cycling that would fatigue SMT solder joints.<\/span><\/p>\n<h2><b><span data-font-family=\"Arial\">3. PCB Surface Finishes: Solderability, Shelf Life, and Application<\/span><\/b><\/h2>\n<table>\n<tbody>\n<tr>\n<td colspan=\"1\" rowspan=\"1\" width=\"133\"><b><span data-font-family=\"Arial\">Surface Finish<\/span><\/b><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"106\"><b><span data-font-family=\"Arial\">Process<\/span><\/b><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"104\"><b><span data-font-family=\"Arial\">Shelf Life<\/span><\/b><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"104\"><b><span data-font-family=\"Arial\">Fine-Pitch<\/span><\/b><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"176\"><b><span data-font-family=\"Arial\">Best Application<\/span><\/b><\/td>\n<\/tr>\n<tr>\n<td colspan=\"1\" rowspan=\"1\" width=\"133\"><b><span data-font-family=\"Arial\">HASL<\/span><\/b><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"106\"><span data-font-family=\"Arial\">Board dipped in molten solder; air-levelled<\/span><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"104\"><span data-font-family=\"Arial\">12 months<\/span><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"104\"><span data-font-family=\"Arial\">No \u2014 uneven<\/span><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"176\"><span data-font-family=\"Arial\">General purpose; low-cost boards<\/span><\/td>\n<\/tr>\n<tr>\n<td colspan=\"1\" rowspan=\"1\" width=\"133\"><b><span data-font-family=\"Arial\">ENIG<\/span><\/b><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"106\"><span data-font-family=\"Arial\">Electroless Ni then Immersion Au<\/span><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"104\"><span data-font-family=\"Arial\">12 months<\/span><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"104\"><span data-font-family=\"Arial\">Yes \u2014 flat<\/span><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"176\"><span data-font-family=\"Arial\">Fine-pitch QFN, BGA, wire bonding<\/span><\/td>\n<\/tr>\n<tr>\n<td colspan=\"1\" rowspan=\"1\" width=\"133\"><b><span data-font-family=\"Arial\">OSP<\/span><\/b><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"106\"><span data-font-family=\"Arial\">Organic coating on bare copper<\/span><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"104\"><span data-font-family=\"Arial\">6 months<\/span><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"104\"><span data-font-family=\"Arial\">Yes<\/span><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"176\"><span data-font-family=\"Arial\">High-volume SMT; cost-sensitive<\/span><\/td>\n<\/tr>\n<tr>\n<td colspan=\"1\" rowspan=\"1\" width=\"133\"><b><span data-font-family=\"Arial\">ENEPIG<\/span><\/b><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"106\"><span data-font-family=\"Arial\">Ni, Palladium, then Au<\/span><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"104\"><span data-font-family=\"Arial\">12 months<\/span><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"104\"><span data-font-family=\"Arial\">Yes \u2014 premium<\/span><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"176\"><span data-font-family=\"Arial\">Wire bonding pads; gold stud bumping<\/span><\/td>\n<\/tr>\n<tr>\n<td colspan=\"1\" rowspan=\"1\" width=\"133\"><b><span data-font-family=\"Arial\">Immersion Silver<\/span><\/b><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"106\"><span data-font-family=\"Arial\">Silver deposited on copper<\/span><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"104\"><span data-font-family=\"Arial\">6\u201312 months<\/span><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"104\"><span data-font-family=\"Arial\">Yes<\/span><\/td>\n<td colspan=\"1\" rowspan=\"1\" width=\"176\"><span data-font-family=\"Arial\">RF boards; press-fit connectors<\/span><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p><span data-font-family=\"Arial\">ENIG vs. HASL: ENIG is the default choice for any board with QFN, BGA, or other fine-pitch components, because its flat, consistent surface enables reliable solder paste deposition and joint formation. HASL\u2019s uneven surface causes paste volume variation on small pads. However, ENIG costs 20\u201340% more than HASL and requires careful handling to prevent \u2018black pad\u2019 \u2014 a corrosion failure mode at the nickel-gold interface caused by hypercorrosion during gold deposition.<\/span><\/p>\n<h2><b><span data-font-family=\"Arial\">4. Moisture Sensitivity Level (MSL)<\/span><\/b><\/h2>\n<p><span data-font-family=\"Arial\">ICs and other plastic-packaged components absorb atmospheric moisture over time. During reflow, this moisture vaporises explosively and can crack the package. MSL ratings (IPC\/JEDEC J-STD-020) define how long a component can remain outside its moisture barrier bag before baking is required. MSL 1 components are unlimited; MSL 3 must be used within 168 hours of opening.<\/span><\/p>\n<h2><b><span data-font-family=\"Arial\">5. Design for Manufacturing and Assembly (DFMA) \u2014 Rules That Save Money<\/span><\/b><\/h2>\n<ul>\n<li><b><span data-font-family=\"Arial\">Fiducial marks: <\/span><\/b><span data-font-family=\"Arial\">Place at least 3 fiducial marks (bare copper circles, typically 1 mm diameter) on the PCB for pick-and-place machine vision alignment. Global fiducials go at board corners; local fiducials go near fine-pitch ICs.<\/span><\/li>\n<li><b><span data-font-family=\"Arial\">Component clearance: <\/span><\/b><span data-font-family=\"Arial\">Maintain a minimum 0.2 mm clearance between adjacent SMD component bodies. Components placed too close interfere with nozzle access and create solder bridging risk.<\/span><\/li>\n<li><b><span data-font-family=\"Arial\">Uniform pad design: <\/span><\/b><span data-font-family=\"Arial\">Use IPC-7351 standard land patterns. Non-standard pad sizes cause paste volume errors and unreliable joints.<\/span><\/li>\n<li><b><span data-font-family=\"Arial\">Consistent component orientation: <\/span><\/b><span data-font-family=\"Arial\">Orient all polarised components (electrolytic capacitors, diodes, ICs) consistently. This dramatically reduces placement errors and inspection time.<\/span><\/li>\n<li><b><span data-font-family=\"Arial\">Thermal relief vias: <\/span><\/b><span data-font-family=\"Arial\">Essential on large copper pads. Without them, the copper plane acts as a heat sink and prevents the pad from reaching reflow temperature.<\/span><\/li>\n<li><b><span data-font-family=\"Arial\">Panel design: <\/span><\/b><span data-font-family=\"Arial\">For production volumes, arrange multiple PCBs in a panel (array) with V-score or tab-routed break lines, fiducials, and tooling holes in the panel frame. This enables efficient automated assembly.<\/span><\/li>\n<\/ul>\n<h2><b><span data-font-family=\"Arial\">6. Component Sourcing for PCB Assembly<\/span><\/b><\/h2>\n<h3><b><span data-font-family=\"Arial\">The BOM Hierarchy: Primary, Second Source, and Approved Equivalents<\/span><\/b><\/h3>\n<ul>\n<li><b><span data-font-family=\"Arial\">Primary part: <\/span><\/b><span data-font-family=\"Arial\">Your first-choice component, fully characterised and qualified to your design.<\/span><\/li>\n<li><b><span data-font-family=\"Arial\">Second source: <\/span><\/b><span data-font-family=\"Arial\">An alternative manufacturer\u2019s equivalent part with identical pin-out and electrical parameters. Always identify a second source before production \u2014 primary parts go on allocation.<\/span><\/li>\n<li><b><\/b><b><span data-font-family=\"Arial\">Approved equivalent: <\/span><\/b><span data-font-family=\"Arial\">A functionally equivalent part that may require a minor schematic or footprint change. Requires engineering sign-off before substitution.<\/span><\/li>\n<\/ul>\n<h3><b><span data-font-family=\"Arial\">What to Check Before Ordering Components<\/span><\/b><\/h3>\n<ul>\n<li><b><span data-font-family=\"Arial\">Package code match: <\/span><\/b><span data-font-family=\"Arial\">Verify the exact package (e.g., SOT-23-5 vs. SOT-23-3 are different footprints).<\/span><\/li>\n<li><b><span data-font-family=\"Arial\">Temperature range: <\/span><\/b><span data-font-family=\"Arial\">Industrial (\u221240\u00b0C to +85\u00b0C) vs. commercial (0\u00b0C to +70\u00b0C) ratings matter for reliability.<\/span><\/li>\n<li><b><span data-font-family=\"Arial\">MSL rating: <\/span><\/b><span data-font-family=\"Arial\">Check moisture sensitivity for ICs \u2014 MSL 3 or higher requires baking before assembly.<\/span><\/li>\n<li><b><span data-font-family=\"Arial\">RoHS compliance: <\/span><\/b><span data-font-family=\"Arial\">Verify lead-free compliance for markets that require it (EU, China, and most global markets).<\/span><\/li>\n<\/ul>\n<h2><b><span data-font-family=\"Arial\">Quick Selection Guide: PCB Assembly Process in 60 Seconds<\/span><\/b><\/h2>\n<ul>\n<li><span data-font-family=\"Arial\">High-volume SMT design (IoT, consumer electronics, computing) \u2192 Full SMT; pick-and-place + reflow oven; ENIG for fine-pitch, HASL for general use<\/span><\/li>\n<li><span data-font-family=\"Arial\">High-current connectors or components subject to repeated mating cycles \u2192 Through-hole for mechanical strength; wave or selective soldering<\/span><\/li>\n<li><span data-font-family=\"Arial\">Mixed SMT + through-hole (most industrial boards) \u2192 SMT top-side reflow first, then through-hole insertion + wave or selective soldering on bottom<\/span><\/li>\n<li><span data-font-family=\"Arial\">Fine-pitch QFN, BGA, or components with pads &lt; 0.5 mm pitch? \u2192 ENIG surface finish mandatory; stencil aperture reduce by 10\u201315% to prevent bridging<\/span><\/li>\n<li><span data-font-family=\"Arial\">Low-volume prototype or NPI \u2192 Flying probe test (no fixture NRE); OSP or HASL finish; manual through-hole assembly acceptable<\/span><\/li>\n<li><span data-font-family=\"Arial\">IPC Class 3 required (aerospace, medical, military) \u2192 100% AOI + X-ray inspection for BGAs; IPC\/WHMA-A-620 Class 3 workmanship standard; hi-pot testing at 2\u00d7 rated voltage<\/span><\/li>\n<li><span data-font-family=\"Arial\">BOM includes MSL 3+ ICs \u2192 Bake components per IPC\/JEDEC J-STD-033 before assembly; use within 168 hours of opening moisture barrier bag<\/span><\/li>\n<\/ul>\n<h2><b><span data-font-family=\"Arial\">Frequently Asked Questions<\/span><\/b><\/h2>\n<h4><b><span data-font-family=\"Arial\">What is the difference between PCB assembly and PCB manufacturing?<\/span><\/b><\/h4>\n<p><span data-font-family=\"Arial\">PCB manufacturing (fabrication) produces the bare board \u2014 drilling, laminating, plating, and applying solder mask. PCB assembly (PCBA) is the subsequent process of populating the bare board with components. Many contract manufacturers offer both services under one roof, simplifying supply chain management.<\/span><\/p>\n<h4><b><span data-font-family=\"Arial\">Can I mix SMT and through-hole components on the same board?<\/span><\/b><\/h4>\n<p><span data-font-family=\"Arial\">Yes, and most production boards do. The typical process is: SMT components on the top side are placed and reflowed first, then through-hole components are inserted and soldered by wave or selective soldering. Bottom-side SMT components that must survive wave soldering are bonded with adhesive before wave soldering to prevent them washing off the wave.<\/span><\/p>\n<h4><b><span data-font-family=\"Arial\">What causes tombstoning and how do I prevent it?<\/span><\/b><\/h4>\n<p><span data-font-family=\"Arial\">Tombstoning occurs when a small passive component (0402, 0201) stands vertically on one pad during reflow. It is caused by unequal solder paste volumes or unequal thermal mass on the two pads, causing one end to melt and wet before the other. Prevention: use symmetric pad designs per IPC-7351, ensure uniform paste deposition with SPI, and keep small passives away from large thermal mass components that create temperature gradients across the pad pair.<\/span><\/p>\n<h4><b><span data-font-family=\"Arial\">Why does my board need a specific IPC class?<\/span><\/b><\/h4>\n<p><span data-font-family=\"Arial\">IPC-A-610 classifies PCBAs into three classes based on the consequences of failure.<\/span><\/p>\n<div id=\"model-response-message-contentr_0615d3fe06fce93f\" class=\"markdown markdown-main-panel enable-updated-hr-color\" dir=\"ltr\" aria-live=\"polite\" aria-busy=\"false\">\n<p data-path-to-node=\"0\">Class 1 applies to general electronics like toys and consumer goods with relaxed workmanship standards.However, Class 2 is designated for dedicated service electronics, such as industrial instrumentation, requiring higher reliability. Finally, Class 3 represents high-performance electronics for aerospace, medical, and military sectors, which must adhere to the strictest standards.<\/p>\n<\/div>\n<h2><b><span data-font-family=\"Arial\">Conclusion: Understanding Assembly Makes You a Better Designer<\/span><\/b><\/h2>\n<p><span data-font-family=\"Arial\">PCB assembly is not a black box that happens after you send your Gerber files. It is a precision manufacturing process with dozens of variables, each of which you can influence through thoughtful design decisions. Knowing how solder paste is deposited, how reflow profiles affect joint quality, why surface finish matters for fine-pitch components, and how DFMA principles reduce defects gives you the vocabulary and insight to design boards that are not just electrically correct, but production-ready.<\/span><\/p>\n<h2><b><span data-font-family=\"Arial\">Find What You Need on <a href=\"http:\/\/lcsc.com\">LCSC<\/a><\/span><\/b><\/h2>\n<p><span data-font-family=\"Arial\">LCSC Electronics provides the component depth and datasheet transparency you need to build a bulletproof BOM for your next PCB assembly run \u2014 with MSL ratings, RoHS status, AEC-Q100 qualification filters, and real-time stock availability.<\/span><\/p>\n","protected":false},"excerpt":{"rendered":"<p>Key Takeaways PCB assembly involves two primary component mounting technologies: SMT (surface-mount) and through-hole (THT), each with distinct process flows. Reflow soldering is the standard process for SMT components; wave soldering is the traditional process for through-hole components and selective SMT on the bottom side. The choice between SMT, through-hole, or mixed assembly affects board [&hellip;]<\/p>\n","protected":false},"author":3,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"footnotes":""},"categories":[176,175],"tags":[181,155,160,293],"class_list":["post-3897","post","type-post","status-publish","format-standard","hentry","category-pcb-smt-basics","category-pcb-smt","tag-pcb","tag-pcba","tag-smt","tag-solder"],"blocksy_meta":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.8 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>PCB Assembly Process: SMT, THT &amp; Soldering Blog | LCSC Electronics<\/title>\n<meta name=\"description\" content=\"Flex vs. rigid PCBs, rigid-flex, 3D electronics, CNC RF enclosures &amp; S-parameters. 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