{"id":208,"date":"2026-07-17T09:00:00","date_gmt":"2026-07-17T09:00:00","guid":{"rendered":"https:\/\/rlxsolutions.com\/blog\/reflow-soldering-process\/"},"modified":"2026-07-17T09:00:00","modified_gmt":"2026-07-17T09:00:00","slug":"reflow-soldering-process","status":"publish","type":"post","link":"https:\/\/rlxsolutions.com\/blog\/reflow-soldering-process\/","title":{"rendered":"Reflow Soldering: Process, Profiles &#038; PCB Assembly Control"},"content":{"rendered":"<p>\n    Reflow soldering joins surface-mount components to a printed circuit board by<br \/>\n    printing solder paste, placing parts, and heating the assembly through a controlled<br \/>\n    thermal profile. <a href=\"https:\/\/rlxsolutions.com\/\">RLX Solutions<\/a>, a contract electronics manufacturer in Concord,<br \/>\n    Ontario, treats the profile as one part of a larger control system that starts with<br \/>\n    paste deposition and continues through post-reflow inspection.\n  <\/p>\n<p>\n    A reflow oven cannot correct every upstream error. Too little paste may leave an<br \/>\n    open joint. Excess paste can bridge adjacent leads. Uneven component or copper<br \/>\n    density can create large temperature differences across the circuit board.<br \/>\n    Moisture-sensitive parts can introduce another failure mechanism before the board<br \/>\n    even reaches peak temperature.\n  <\/p>\n<p>For that reason, reflow soldering control is better understood as a chain:<\/p>\n<ol>\n<li>Confirm the assembly requirements and materials.<\/li>\n<li>Print solder paste through the stencil.<\/li>\n<li>Inspect the deposited paste.<\/li>\n<li>Place surface-mount components.<\/li>\n<li>Run the board through the programmed thermal profile.<\/li>\n<li>Inspect visible solder joints.<\/li>\n<li>X-ray joints that cannot be assessed optically.<\/li>\n<li>Validate the first article before releasing the production lot.<\/li>\n<\/ol>\n<p>Each stage constrains the next.<\/p>\n<h2>Reflow soldering starts before the PCB reaches the oven<\/h2>\n<p>\n    The oven determines the board&#8217;s thermal history, but solder joint quality begins<br \/>\n    at stencil printing.\n  <\/p>\n<p>\n    Solder paste is a mixture of metal alloy particles and flux. During reflow, the<br \/>\n    flux activates as temperature rises, oxides are reduced, the solder alloy melts,<br \/>\n    and surface tension helps the molten solder wet compatible metallized surfaces.<br \/>\n    Cooling then solidifies the joint.\n  <\/p>\n<p>\n    The amount and position of deposited paste therefore matter before any thermal<br \/>\n    settings are considered.\n  <\/p>\n<p>\n    A stencil aperture that deposits too much paste can produce bridging, solder balls,<br \/>\n    excessive joint volume or component movement. Too little paste can produce<br \/>\n    insufficient fillets or opens. Paste that is offset from the pad adds another<br \/>\n    variable because the component and molten solder must self-align within the<br \/>\n    available geometry.\n  <\/p>\n<p>\n    Solder paste inspection (SPI) measures this upstream condition. In a controlled<br \/>\n    assembly process, paste volume, area and offset can be checked before components<br \/>\n    are placed and before defective printing becomes a reflow problem.\n  <\/p>\n<p>\n    Consider a hypothetical board with a 0.5 mm pitch QFN beside several 0603<br \/>\n    passives. If paste deposition on one QFN pad row shifts 0.10 mm while the passive<br \/>\n    locations remain centred, changing the oven profile does not address the source<br \/>\n    of the asymmetry. The print process needs correction first.\n  <\/p>\n<p>\n    That distinction matters because profile adjustment is sometimes used as a<br \/>\n    universal troubleshooting response. It is not one.\n  <\/p>\n<h2>The four thermal regions of a reflow profile do different jobs<\/h2>\n<p>\n    A reflow profile is the measured temperature of the assembly over time as it<br \/>\n    travels through the oven. The important temperature is the board and component<br \/>\n    temperature, not simply the air temperature displayed for an oven zone.\n  <\/p>\n<p>Most profiles can be understood through four functional regions.<\/p>\n<table>\n<thead>\n<tr>\n<th>Profile region<\/th>\n<th>Main purpose<\/th>\n<th>What poor control can cause<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Preheat<\/td>\n<td>Raise assembly temperature at a controlled rate<\/td>\n<td>Thermal stress, paste spatter, excessive flux loss<\/td>\n<\/tr>\n<tr>\n<td>Soak<\/td>\n<td>Reduce temperature differences and prepare the flux system<\/td>\n<td>Premature flux exhaustion or poor thermal equalization<\/td>\n<\/tr>\n<tr>\n<td>Reflow<\/td>\n<td>Bring the solder above liquidus and form the joint<\/td>\n<td>\n          Poor wetting, insufficient reflow, component damage or excessive<br \/>\n          intermetallic growth\n        <\/td>\n<\/tr>\n<tr>\n<td>Cooling<\/td>\n<td>Solidify the solder joint after reflow<\/td>\n<td>Uncontrolled joint structure or added thermal stress<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>\n    The exact profile cannot be selected from the solder alloy alone. Board thickness,<br \/>\n    copper distribution, component mass, package limits, solder paste chemistry,<br \/>\n    surface finish and oven capability all affect the result.\n  <\/p>\n<h3>Preheat controls the rate of thermal change<\/h3>\n<p>\n    The first problem is not simply reaching a high enough temperature. It is getting<br \/>\n    the assembly there without creating an excessive temperature gradient.\n  <\/p>\n<p>\n    A small passive may heat rapidly. A shield can, large connector or heavy copper<br \/>\n    region may lag behind. If heating is too aggressive, the board and its components<br \/>\n    experience different expansion rates. Paste can also react poorly to rapid solvent<br \/>\n    evaporation.\n  <\/p>\n<p>\n    A more gradual rise gives the assembly time to absorb heat while limiting the<br \/>\n    temperature difference between fast-heating and slow-heating locations.\n  <\/p>\n<h3>Soak is about thermal equalization and flux behaviour<\/h3>\n<p>\n    The soak region gives colder areas of the circuit board time to catch up before<br \/>\n    solder reaches its liquid state. It also affects flux activation.\n  <\/p>\n<p>\n    Longer is not automatically safer. Holding the assembly hot for too long may<br \/>\n    consume flux activity before the solder reaches the point where strong wetting is<br \/>\n    required.\n  <\/p>\n<p>\n    The profile therefore has to balance two competing needs: reducing board-wide<br \/>\n    temperature variation without exhausting the chemistry that supports soldering.\n  <\/p>\n<h3>Reflow has both a temperature requirement and a time requirement<\/h3>\n<p>\n    Once the solder alloy passes its liquidus temperature, the paste becomes molten<br \/>\n    and the joint forms.\n  <\/p>\n<p>\n    A profile that barely crosses liquidus may fail to provide enough thermal exposure<br \/>\n    for reliable wetting at the coldest location. A profile with excessive peak<br \/>\n    temperature or excessive time above liquidus can stress components, accelerate<br \/>\n    intermetallic growth and increase oxidation.\n  <\/p>\n<p>\n    A useful profile therefore has a process window rather than a single magic<br \/>\n    temperature.\n  <\/p>\n<p>\n    For example, imagine a hypothetical lead-free assembly profiled with two<br \/>\n    thermocouples. TC1 on a small passive reaches a 241&deg;C peak, while TC2 beside a<br \/>\n    thermally heavy connector reaches only 222&deg;C. Looking only at TC1 would make<br \/>\n    the run appear comfortably hot. The 19&deg;C difference shows why the cold<br \/>\n    location must also be characterized.\n  <\/p>\n<h3>Cooling remains part of the soldering process<\/h3>\n<p>\n    Once the assembly leaves the hottest oven zones, molten solder has to solidify.\n  <\/p>\n<p>\n    Cooling affects solder joint microstructure and thermal stress. It also determines<br \/>\n    how quickly downstream handling can begin safely. The process is therefore not<br \/>\n    finished at peak temperature.\n  <\/p>\n<h2>Profile the assembly, not the oven recipe<\/h2>\n<p>\n    Oven zone settings are machine inputs. A thermal profile is evidence of what the<br \/>\n    assembly actually experienced.\n  <\/p>\n<p>\n    That difference becomes significant on boards with uneven thermal mass. Two<br \/>\n    printed circuit boards can pass through the same oven recipe and produce<br \/>\n    materially different component temperatures.\n  <\/p>\n<p>\n    Profile development normally places thermocouples at locations selected to<br \/>\n    represent meaningful thermal extremes. Useful positions include:\n  <\/p>\n<ul>\n<li>A small, fast-heating component.<\/li>\n<li>A large or thermally massive package.<\/li>\n<li>A location connected to a broad copper area.<\/li>\n<li>A region close to a board edge.<\/li>\n<li>A location expected to be thermally shielded.<\/li>\n<li>A package whose temperature limit needs close observation.<\/li>\n<\/ul>\n<p>\n    Attaching every thermocouple to an easy pad near the centre of the PCB defeats<br \/>\n    much of the purpose. The measurement plan should challenge the recipe.\n  <\/p>\n<p>\n    The engineering question is not, &#8220;What are the oven zones set to?&#8221; It is,<br \/>\n    &#8220;What did the hottest and coldest relevant parts of this assembly experience?&#8221;\n  <\/p>\n<h2>Board A and Board B show why the same layer count can require different reflow control<\/h2>\n<p>\n    Layer count alone does not describe the thermal load presented to a reflow oven.\n  <\/p>\n<p>Consider two hypothetical eight-layer assemblies.<\/p>\n<table>\n<thead>\n<tr>\n<th>Decision factor<\/th>\n<th>Board A<\/th>\n<th>Board B<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Layer count<\/td>\n<td>8<\/td>\n<td>8<\/td>\n<\/tr>\n<tr>\n<td>Board thickness<\/td>\n<td>1.6 mm<\/td>\n<td>1.6 mm<\/td>\n<\/tr>\n<tr>\n<td>Smallest featured package<\/td>\n<td>0.5 mm pitch QFN<\/td>\n<td>0.5 mm pitch QFN<\/td>\n<\/tr>\n<tr>\n<td>Copper distribution<\/td>\n<td>Moderate, relatively even<\/td>\n<td>Large internal and external copper areas<\/td>\n<\/tr>\n<tr>\n<td>Large components<\/td>\n<td>Few<\/td>\n<td>Large connector and shielded power section<\/td>\n<\/tr>\n<tr>\n<td>Hidden solder joints<\/td>\n<td>QFN thermal pad<\/td>\n<td>QFN thermal pad plus BGA<\/td>\n<\/tr>\n<tr>\n<td>Thermal behaviour<\/td>\n<td>More uniform<\/td>\n<td>Larger hot-to-cold temperature spread expected<\/td>\n<\/tr>\n<tr>\n<td>Inspection burden<\/td>\n<td>SPI, AOI, targeted X-ray<\/td>\n<td>SPI, AOI, broader X-ray attention<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>\n    Both boards have eight layers and the same nominal thickness, but Board B can take<br \/>\n    more engineering effort to profile.\n  <\/p>\n<p>\n    Its large copper regions conduct heat away from local solder joints. The connector<br \/>\n    body and shielded section add thermal mass. The BGA adds solder joints that<br \/>\n    automated optical inspection (AOI) cannot directly see.\n  <\/p>\n<p>\n    That changes cost without changing the layer count. Board B can require more<br \/>\n    profiling work, more thermocouple placement thought and more X-ray inspection<br \/>\n    coverage before the process window is demonstrated.\n  <\/p>\n<p>\n    This is why assembly pricing cannot be derived reliably from layer count or<br \/>\n    component count alone.\n  <\/p>\n<h2>Thermal mass creates the profile problem engineers need to anticipate<\/h2>\n<p>\n    Copper is an efficient heat conductor. That is valuable electrically and thermally<br \/>\n    in operation, but it changes how a circuit board responds inside the oven.\n  <\/p>\n<p>\n    A pad connected to a substantial ground or power structure can transfer heat into<br \/>\n    the surrounding copper faster than an isolated signal pad. Large components add<br \/>\n    another heat sink. Small packages, meanwhile, can heat much faster.\n  <\/p>\n<p>\n    The resulting temperature difference across the PCB is often more important than<br \/>\n    the temperature at a single measurement point.\n  <\/p>\n<p>\n    Take a hypothetical 2.0 mm thick, six-layer board with a QFN near the centre and<br \/>\n    a large power connector beside a wide copper region. Assume the QFN location peaks<br \/>\n    at 239&deg;C while the connector-side joint reaches 226&deg;C. Increasing oven heat<br \/>\n    may improve the cold location, but it also drives the QFN hotter.\n  <\/p>\n<p>\n    The better response may involve changing zone balance, conveyor speed, soak<br \/>\n    behaviour or a combination of those variables. The objective is to bring both<br \/>\n    locations inside a usable process window rather than maximizing peak temperature<br \/>\n    everywhere.\n  <\/p>\n<p>\n    This same mechanism explains why copying an established profile from a visually<br \/>\n    similar board can fail. Copper architecture that is invisible from the assembled<br \/>\n    top view can materially change the thermal response.\n  <\/p>\n<h2>Fine-pitch packages make paste control inseparable from reflow control<\/h2>\n<p>\n    As lead pitch and pad geometry shrink, the acceptable margin around solder paste<br \/>\n    deposition becomes smaller.\n  <\/p>\n<p>\n    A 0.3 mm pitch ball grid array (BGA), an 01005 passive and a large <a href=\"https:\/\/rlxsolutions.com\/blog\/through-hole-pcb-assembly-soldering\/\">through-hole<\/a><br \/>\n    connector may all appear on the same mixed-technology assembly, but their process<br \/>\n    sensitivities are different. Reflow settings have to respect the thermal<br \/>\n    requirements of the whole assembly while stencil design and printing control the<br \/>\n    amount of solder presented to each surface-mount joint.\n  <\/p>\n<p>\n    This is where SPI becomes especially valuable. Catching insufficient, excessive<br \/>\n    or displaced paste before reflow prevents the oven from converting a printing<br \/>\n    problem into a finished solder defect.\n  <\/p>\n<p>\n    For packages with bottom-side terminations, the control chain continues after<br \/>\n    reflow. A BGA joint is hidden beneath the package. A QFN centre thermal pad is also<br \/>\n    difficult to assess by surface appearance alone.\n  <\/p>\n<p>\n    Automated X-ray inspection (AXI) allows those joints and thermal pads to be<br \/>\n    evaluated for conditions that AOI cannot see directly, including void distribution<br \/>\n    and hidden connection quality.\n  <\/p>\n<p>\n    The practical sequence is therefore connected: stencil design influences deposit<br \/>\n    geometry, SPI verifies the print, reflow forms the joint, and AXI checks hidden<br \/>\n    outcomes.\n  <\/p>\n<h2>Surface finish changes the soldering interface<\/h2>\n<p>\n    Reflow does not join solder directly to bare PCB laminate. It joins the component<br \/>\n    termination to a prepared pad surface, and that surface finish affects<br \/>\n    solderability, planarity and process behaviour.\n  <\/p>\n<p>\n    Common printed circuit board finishes include hot air solder levelling (HASL),<br \/>\n    electroless nickel immersion gold (ENIG), electroless nickel electroless palladium<br \/>\n    immersion gold (ENEPIG), immersion silver, immersion tin and organic solderability<br \/>\n    preservative (OSP).\n  <\/p>\n<p>\n    Finish selection should therefore happen with assembly in mind.\n  <\/p>\n<p>\n    Planarity becomes more significant as component pitch decreases. Storage history<br \/>\n    and surface condition also matter because the soldering process depends on<br \/>\n    wettable surfaces. A thermally correct profile cannot fully compensate for a<br \/>\n    compromised soldering interface.\n  <\/p>\n<p>\n    The engineer specifying the bare board and the engineer responsible for assembly<br \/>\n    should not treat those choices as separate systems. Reflow is where decisions from<br \/>\n    both stages meet.\n  <\/p>\n<h2>Moisture-sensitive components need control before heating<\/h2>\n<p>\n    Some electronic packages absorb moisture during exposure to ambient conditions.<br \/>\n    Reflow raises the package temperature quickly, which can turn trapped moisture<br \/>\n    into internal vapour pressure.\n  <\/p>\n<p>\n    The resulting damage mechanism is different from a poor solder joint, but it<br \/>\n    occurs during the same thermal process.\n  <\/p>\n<p>\n    Moisture-sensitive device handling therefore belongs beside profile control.<br \/>\n    Storage, exposure history and dry handling need to be managed before the<br \/>\n    components enter the oven.\n  <\/p>\n<p>\n    Electrostatic discharge (ESD) control is similarly upstream. An assembly can<br \/>\n    emerge from reflow with visually acceptable solder joints while containing a<br \/>\n    component damaged by improper handling before placement.\n  <\/p>\n<p>\n    Good reflow control does not replace ESD and moisture-sensitive device controls.<br \/>\n    It assumes those controls are already working.\n  <\/p>\n<h2>Inspection separates solder appearance from assembly evidence<\/h2>\n<p>\n    A solder joint that looks acceptable from one viewing angle does not prove the<br \/>\n    assembly process is under control.\n  <\/p>\n<p>Different inspection methods answer different questions:<\/p>\n<ul>\n<li>\n      <strong>SPI before reflow:<\/strong><br \/>\n      Was the intended amount of solder paste deposited in the intended position?\n    <\/li>\n<li>\n      <strong>AOI after reflow:<\/strong><br \/>\n      Are visible components and solder joints present, aligned and visually acceptable?\n    <\/li>\n<li>\n      <strong>AXI:<\/strong><br \/>\n      What happened underneath packages such as BGA, land grid array (LGA) and QFN devices?\n    <\/li>\n<li>\n      <strong>Manual visual inspection:<\/strong><br \/>\n      Does workmanship meet the specified IPC-A-610 Class 2 or Class 3 criteria?\n    <\/li>\n<li>\n      <strong>Electrical test:<\/strong><br \/>\n      Does the assembled circuit connect and operate as intended?\n    <\/li>\n<\/ul>\n<p>\n    Inspection depth also changes with the build requirement. IPC-A-610 Class 3 calls<br \/>\n    for tighter workmanship expectations than Class 2, so the required class needs to<br \/>\n    be identified before the manufacturing plan is finalized.\n  <\/p>\n<p>\n    The failure mode here is contractual as much as technical. If the build class,<br \/>\n    inspection requirements or test expectations are left ambiguous until finished<br \/>\n    assemblies exist, there may be no agreed basis for release.\n  <\/p>\n<h2>First article approval should freeze a demonstrated process, not just approve one board<\/h2>\n<p>\n    A first article inspection (FAI) is most useful when it connects the physical<br \/>\n    assembly to the process used to manufacture it.\n  <\/p>\n<p>\n    That means the accepted first article should correspond to controlled production<br \/>\n    information such as the bill of materials, pick-and-place data, assembly drawing,<br \/>\n    stencil configuration and approved process settings.\n  <\/p>\n<p>\n    Suppose a hypothetical 12-layer board has a 0.4 mm pitch BGA, two QFNs and a dense<br \/>\n    passive field. The first article passes SPI, post-reflow AOI and X-ray inspection,<br \/>\n    and the functional test passes. If a later lot uses a materially different stencil<br \/>\n    or reflow recipe without controlled review, the first article no longer represents<br \/>\n    the same manufacturing condition.\n  <\/p>\n<p>\n    The purpose of FAI is therefore broader than finding a cosmetically good board. It<br \/>\n    establishes a reference point before the production lot is released.\n  <\/p>\n<h2>Control changes deliberately when defects appear<\/h2>\n<p>\n    Reflow troubleshooting works best when the defect is traced back to the stage<br \/>\n    capable of creating it.\n  <\/p>\n<table>\n<thead>\n<tr>\n<th>Observed problem<\/th>\n<th>First process areas to investigate<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Bridging<\/td>\n<td>Paste volume, stencil aperture, print alignment, placement, reflow behaviour<\/td>\n<\/tr>\n<tr>\n<td>Opens<\/td>\n<td>Paste deposition, component coplanarity, placement, wetting, thermal exposure<\/td>\n<\/tr>\n<tr>\n<td>Tombstoning<\/td>\n<td>Pad balance, paste symmetry, placement and local heating difference<\/td>\n<\/tr>\n<tr>\n<td>BGA concern<\/td>\n<td>Print, placement, thermal profile and X-ray evidence<\/td>\n<\/tr>\n<tr>\n<td>QFN thermal-pad voiding<\/td>\n<td>Paste pattern, deposited volume, package geometry and reflow behaviour<\/td>\n<\/tr>\n<tr>\n<td>Repeated cold joints in one region<\/td>\n<td>Local thermal mass and actual measured profile<\/td>\n<\/tr>\n<tr>\n<td>Random visible defects<\/td>\n<td>\n          Print repeatability, placement stability and process control before changing<br \/>\n          the recipe\n        <\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>\n    The table identifies starting points, not automatic diagnoses. Several mechanisms<br \/>\n    can create a similar visual result.\n  <\/p>\n<p>\n    The useful discipline is to change one meaningful variable with evidence behind<br \/>\n    it. Repeatedly raising peak temperature without checking paste deposition or local<br \/>\n    board temperature can hide the cause while shrinking the thermal margin available<br \/>\n    to other components.\n  <\/p>\n<h2>Give the assembler enough data to control reflow before production<\/h2>\n<p>\n    Manufacturing control improves when the assembly package makes intent explicit.\n  <\/p>\n<p>\n    For a <a href=\"https:\/\/rlxsolutions.com\/blog\/pcb-assembly-guide\/\">PCB assembly<\/a> build, the manufacturing package should include the bill of<br \/>\n    materials, pick-and-place or centroid data, assembly drawing and the fabrication<br \/>\n    information needed for the bare PCB. Stackup and impedance information also belong<br \/>\n    with the fabrication package when applicable.\n  <\/p>\n<p>\n    Beyond the basic files, flag requirements that affect the assembly process. These<br \/>\n    can include the required workmanship class, critical component handling needs,<br \/>\n    test requirements, programming requirements and any areas where thermal or<br \/>\n    inspection behaviour deserves special attention.\n  <\/p>\n<p>\n    A dense BGA field next to a large power section should not be discovered only<br \/>\n    after the first profile run. Neither should a customer requirement for functional<br \/>\n    test or serial traceability.\n  <\/p>\n<p>\n    An <a href=\"https:\/\/rlxsolutions.com\/engineering-design.html\">engineer-reviewed design for manufacturability<\/a> (DFM) and design for assembly<br \/>\n    (DFA) review is the point to identify those process interactions before production<br \/>\n    material is committed.\n  <\/p>\n<h2>Reflow control is a system, not a temperature setting<\/h2>\n<p>\n    The most transferable rule is to resist treating the oven recipe as the whole<br \/>\n    soldering process.\n  <\/p>\n<p>\n    Paste deposition determines how much solder enters the joint. Placement determines<br \/>\n    where the terminations meet the paste. Board construction and component mass<br \/>\n    determine how heat moves through the assembly. The thermal profile determines how<br \/>\n    that solder and flux experience heat. Inspection then shows whether the process<br \/>\n    produced the intended result.\n  <\/p>\n<p>\n    A process can therefore fail even while the oven itself is operating exactly as<br \/>\n    programmed.\n  <\/p>\n<p>\n    For engineers, the useful questions are concrete: Where are the hot and cold<br \/>\n    locations? Which joints are hidden? What does SPI show before reflow? Which<br \/>\n    component or copper region drives the thermal load? What evidence releases the<br \/>\n    first article?\n  <\/p>\n<p>Those questions produce a controllable <a href=\"https:\/\/rlxsolutions.com\/pcb-assembly.html\">assembly process<\/a>.<\/p>\n<h2>Put reflow requirements into the build package before quoting<\/h2>\n<p>\n    If your board is moving from prototype into controlled production, RLX reviews<br \/>\n    assembly requirements as part of every quote. Send the Gerbers or other accepted<br \/>\n    manufacturing files, BOM, pick-and-place data and assembly drawing so profiling,<br \/>\n    inspection, testing and first-article requirements can be considered together<br \/>\n    rather than after the build starts.\n  <\/p>\n<section class=\"faq\">\n<h2>Reflow Soldering FAQ<\/h2>\n<h3>What information does an assembler need to develop a reflow process?<\/h3>\n<p>\n      The assembler needs the PCB manufacturing data, bill of materials, pick-and-place<br \/>\n      data and assembly drawing, along with any requirements that affect workmanship,<br \/>\n      testing or component handling. Thermal behaviour also depends on the physical<br \/>\n      board and component population, so the actual assembly configuration matters<br \/>\n      more than an isolated oven temperature target.\n    <\/p>\n<h3>Why is SPI performed before reflow?<\/h3>\n<p>\n      Solder paste inspection checks paste volume, area and offset before parts are<br \/>\n      soldered. That matters because many downstream defects begin during stencil<br \/>\n      printing. If a pad receives too much, too little or misplaced paste, changing<br \/>\n      the reflow temperature does not necessarily correct the underlying problem.<br \/>\n      Detecting the condition before placement and reflow prevents unnecessary defect<br \/>\n      creation.\n    <\/p>\n<h3>Why do BGA and QFN packages need X-ray inspection?<\/h3>\n<p>\n      Their important solder connections are partly or completely hidden beneath the<br \/>\n      package. Automated optical inspection can assess visible placement and solder<br \/>\n      features, but it cannot directly view every concealed joint. X-ray inspection<br \/>\n      provides additional evidence for BGA, LGA and QFN assemblies, including<br \/>\n      evaluation of hidden connections and voiding beneath thermal pads.\n    <\/p>\n<h3>Does every PCB use the same reflow profile?<\/h3>\n<p>\n      No. The measured profile depends on the complete assembly, including board<br \/>\n      thickness, copper distribution, component mass, package construction, solder<br \/>\n      paste system and surface finish. Two circuit boards with similar dimensions can<br \/>\n      respond differently if one contains heavy copper areas or thermally massive<br \/>\n      components. Profile development therefore needs measurements from the assembly<br \/>\n      being manufactured.\n    <\/p>\n<h3>What is the difference between IPC-A-610 Class 2 and Class 3 in assembly?<\/h3>\n<p>\n      Both classes define workmanship expectations for electronic assemblies, but<br \/>\n      Class 3 applies tighter acceptance requirements than Class 2. The required class<br \/>\n      affects how solder joints, placement and workmanship are assessed. It should be<br \/>\n      identified before manufacturing so inspection and documentation can be planned<br \/>\n      around the intended acceptance level rather than decided after boards are<br \/>\n      assembled.\n    <\/p>\n<h3>How is a new assembly approved before the production lot runs?<\/h3>\n<p>\n      A first article provides an early manufactured unit that can be inspected and<br \/>\n      tested before production release. Relevant checks can include solder paste<br \/>\n      inspection, post-reflow AOI, X-ray for hidden joints, manual workmanship<br \/>\n      inspection and electrical or functional testing where specified. Approval ties<br \/>\n      the physical result to the manufacturing data and process used to build it.\n    <\/p>\n<\/section>\n","protected":false},"excerpt":{"rendered":"<p>Reflow soldering joins surface-mount components to a printed circuit board by printing solder paste, placing parts, and heating the assembly [&hellip;]<\/p>\n","protected":false},"author":2,"featured_media":207,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"nf_dc_page":"","site-sidebar-layout":"default","site-content-layout":"","ast-site-content-layout":"default","site-content-style":"default","site-sidebar-style":"default","ast-global-header-display":"","ast-banner-title-visibility":"","ast-main-header-display":"","ast-hfb-above-header-display":"","ast-hfb-below-header-display":"","ast-hfb-mobile-header-display":"","site-post-title":"","ast-breadcrumbs-content":"","ast-featured-img":"","footer-sml-layout":"","ast-disable-related-posts":"","theme-transparent-header-meta":"","adv-header-id-meta":"","stick-header-meta":"","header-above-stick-meta":"","header-main-stick-meta":"","header-below-stick-meta":"","astra-migrate-meta-layouts":"default","ast-page-background-enabled":"default","ast-page-background-meta":{"desktop":{"background-color":"var(--ast-global-color-5)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"ast-content-background-meta":{"desktop":{"background-color":"var(--ast-global-color-4)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"tablet":{"background-color":"var(--ast-global-color-4)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""},"mobile":{"background-color":"var(--ast-global-color-4)","background-image":"","background-repeat":"repeat","background-position":"center center","background-size":"auto","background-attachment":"scroll","background-type":"","background-media":"","overlay-type":"","overlay-color":"","overlay-opacity":"","overlay-gradient":""}},"footnotes":""},"categories":[17],"tags":[],"class_list":["post-208","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-pcb-assembly"],"_links":{"self":[{"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/posts\/208","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/comments?post=208"}],"version-history":[{"count":0,"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/posts\/208\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/media\/207"}],"wp:attachment":[{"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/media?parent=208"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/categories?post=208"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rlxsolutions.com\/blog\/wp-json\/wp\/v2\/tags?post=208"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}