Reflow soldering

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Example Ramp to Spike thermal profile. Example Ramp to Spike thermal profile.png
Example Ramp to Spike thermal profile.
Example of reflow soldering thermal profile. RSS Components of a Profile1.svg
Example of reflow soldering thermal profile.

Reflow soldering is a process in which a solder paste (a sticky mixture of powdered solder and flux) is used to temporarily attach anywhere from one to thousands of tiny electrical components to their contact pads, after which the entire assembly is subjected to controlled heat. The solder paste reflows in a molten state, creating permanent solder joints. Heating may be accomplished by passing the assembly through a reflow oven, under an infrared lamp, or (mainly for prototyping) by soldering individual joints with a hot air pencil.

Contents

Reflow soldering with long industrial convection ovens is the preferred method of soldering surface mount technology (SMT) components to a printed circuit board (PCB). Each segment of the oven has a regulated temperature, according to the specific thermal requirements of each assembly. Reflow ovens meant specifically for the soldering of surface mount components may also be used for through-hole components by filling the holes with solder paste and inserting the component leads through the paste. Wave soldering however, has been the common method of soldering multi-leaded through-hole components onto a circuit board designed for surface-mount components.

When used on boards containing a mix of SMT and plated through-hole (PTH) components, through-hole reflow, when achievable by specifically modified paste stencils, may allow for the wave soldering step to be eliminated from the assembly process, potentially reducing assembly costs.[ clarification needed ] While this may be said of lead-tin solder pastes used previously, lead-free solder alloys such as SAC present a challenge in terms of the limits of oven temperature profile adjustment and requirements of specialized through-hole components that must be hand soldered with solder wire or cannot reasonably withstand the high temperatures directed at circuit boards as they travel on the conveyor of the reflow oven. The reflow soldering of through-hole components using solder paste in a convection oven process is called intrusive soldering.

The goal of the reflow process is for the solder paste to reach the eutectic temperature at which the particular solder alloy undergoes a phase change to a liquid or molten state. At this specific temperature range, the molten alloy demonstrates properties of adhesion. Molten solder alloy behaves much as water, with properties of cohesion and adhesion. With sufficient flux, in the state of liquidus, molten solder alloys will exhibit a characteristic called "wetting."

Wetting is a property of the alloy when within its specific eutectic temperature range. Wetting is a necessary condition for the formation of solder joints that meet the criteria as "acceptable" or "target" conditions, while "non-conforming" is considered defective according to IPC.

The reflow oven temperature profile is suited for characteristics of a particular circuit board assembly, the size and depth of the ground plane layer within the board, the number of layers within the board, the number and size of the components, for example. The temperature profile for a particular circuit board will allow for reflow of solder onto the adjoining surfaces, without overheating and damaging the electrical components beyond their temperature tolerance. In the conventional reflow soldering process, there are usually four stages, called "zones", each having a distinct thermal profile: preheat, thermal soak (often shortened to just soak), reflow, and cooling.

Preheat zone

Preheat is the first stage of the reflow process. During this reflow phase, the entire board assembly climbs towards a target soak or dwell temperature. The main goal of the preheat phase is to get the entire assembly safely and consistently to a soak or pre-reflow temperature. Preheat is also an opportunity for volatile solvents in the solder paste to outgas. For paste solvents to be properly expelled and the assembly to safely reach pre-reflow temperatures the PCB must be heated in a consistent, linear manner. An important metric for the first phase of the reflow process is the temperature slope rate or rise vs time. This is often measured in degrees Celsius per second, °C/s. Many variables factor into a manufacturer's target slope rate. These include: target processing time, solder paste volatility, and component considerations. It is important to account for all these process variables, but in most cases sensitive component considerations are paramount. "Many components will crack if their temperature is changed too quickly. The maximum rate of thermal change that the most sensitive components can withstand becomes the maximum allowable slope"[ clarification needed ]. However, if thermally sensitive components are not in use and maximizing throughput is of great concern, aggressive slope rates may be tailored to improve processing time. For this reason, many manufacturers push these slope rates up to the maximum common allowable rate of 3.0 °C/s. Conversely, if a solder paste containing particularly strong solvents is being used, heating the assembly too fast can easily create an out of control process. As the volatile solvents outgas they may splatter solder off the pads and onto the board. Solder-balling is the main concern of violent outgassing during the preheat phase. Once a board has been ramped up to temperature in the preheat phase it is time to enter the soak or pre-reflow phase.

Thermal soak zone

The second section, thermal soak, is typically a 60 to 120 second exposure for removal of solder paste volatiles and activation of the fluxes, where the flux components begin oxide reduction on component leads and pads. Too high a temperature can lead to solder spattering or balling as well as oxidation of the paste, the attachment pads and the component terminations. Similarly, fluxes may not fully activate if the temperature is too low. At the end of the soak zone a thermal equilibrium of the entire assembly is desired just before the reflow zone. A soak profile is suggested to decrease any delta T between components of varying sizes or if the PCB assembly is very large. A soak profile is also recommended to diminish voiding in area array type packages. [1]

Reflow zone

The third section, the reflow zone, is also referred to as the "time above reflow" or "temperature above liquidus" (TAL), and is the part of the process where the maximum temperature is reached. An important consideration is peak temperature, which is the maximum allowable temperature of the entire process. A common peak temperature is 20–40 °C above liquidus. [1] This limit is determined by the component on the assembly with the lowest tolerance for high temperatures (the component most susceptible to thermal damage). A standard guideline is to subtract 5 °C from the maximum temperature that the most vulnerable component can sustain to arrive at the maximum temperature for process. It is important to monitor the process temperature to keep it from exceeding this limit. Additionally, high temperatures (beyond 260 °C) may cause damage to the internal dies of SMT components as well as foster intermetallic growth. Conversely, a temperature that is not hot enough may prevent the paste from reflowing adequately.

An example of a commercial reflow oven. Mark5 1826 Reflow Oven.jpg
An example of a commercial reflow oven.

[2]

Example of a modern thermal profiler Example thermal profiler.png
Example of a modern thermal profiler

Time above liquidus (TAL), or time above reflow, measures how long the solder is a liquid. The flux reduces surface tension at the juncture of the metals to accomplish metallurgical bonding, allowing the individual solder powder spheres to combine. If the profile time exceeds the manufacturer's specification, the result may be premature flux activation or consumption, effectively "drying" the paste before formation of the solder joint. An insufficient time/temperature relationship causes a decrease in the flux's cleaning action, resulting in poor wetting, inadequate removal of the solvent and flux, and possibly defective solder joints. Experts usually recommend the shortest TAL possible, however, most pastes specify a minimum TAL of 30 seconds, although there appears to be no clear reason for that specific time. One possibility is that there are places on the PCB that are not measured during profiling, and therefore, setting the minimum allowable time to 30 seconds reduces the chances of an unmeasured area not reflowing. A high minimum reflow time also provides a margin of safety against oven temperature changes. The wetting time ideally stays below 60 seconds above liquidus. Additional time above liquidus may cause excessive intermetallic growth, which can lead to joint brittleness. The board and components may also be damaged at extended temperature over liquidus, and most components have a well-defined time limit for how long they may be exposed to temperatures over a given maximum. Too little time above liquidus may trap solvents and flux and create the potential for cold or dull joints as well as solder voids.

Cooling zone

The last zone is a cooling zone to gradually cool the processed board and solidify the solder joints. Proper cooling inhibits excess intermetallic formation or thermal shock to the components. Typical temperatures in the cooling zone range from 30–110 °C (86–230 °F). A fast cooling rate is chosen to create a fine grain structure that is most mechanically sound. [1] Unlike the maximum ramp-up rate, the ramp–down rate is often ignored. The ramp rate is less critical above certain temperatures, however, the maximum allowable slope for any component should apply whether the component is heating up or cooling down. A cooling rate of 4 °C/s is commonly suggested. It is a parameter to consider when analyzing process results.

Etymology

The term "reflow" is used to refer to the temperature above which a solid mass of solder alloy is certain to melt (as opposed to merely soften). If cooled below this temperature, the solder will not flow. Warmed above it once more, the solder will flow again—hence "re-flow".

Modern circuit assembly techniques that use reflow soldering do not necessarily allow the solder to flow more than once. They guarantee that the granulated solder contained in the solder paste surpasses the reflow temperature of the solder involved.

Thermal profiling

Thermal profiling is the act of measuring several points on a circuit board to determine the thermal excursion it takes through the soldering process. In the electronics manufacturing industry, SPC (Statistical Process Control) helps determine if the process is in control, measured against the reflow parameters defined by the soldering technologies and component requirements. [3] [4] Modern software tools allow a profile to be captured, then automatically optimized using a mathematical simulation, which greatly reduces the time needed to establish optimal settings for the process. [5]

See also

Related Research Articles

<span class="mw-page-title-main">Solder</span> Alloy used to join metal pieces

Solder is a fusible metal alloy used to create a permanent bond between metal workpieces. Solder is melted in order to wet the parts of the joint, where it adheres to and connects the pieces after cooling. Metals or alloys suitable for use as solder should have a lower melting point than the pieces to be joined. The solder should also be resistant to oxidative and corrosive effects that would degrade the joint over time. Solder used in making electrical connections also needs to have favorable electrical characteristics.

<span class="mw-page-title-main">Ball grid array</span> Surface-mount packaging that uses an array of solder balls

A ball grid array (BGA) is a type of surface-mount packaging used for integrated circuits. BGA packages are used to permanently mount devices such as microprocessors. A BGA can provide more interconnection pins than can be put on a dual in-line or flat package. The whole bottom surface of the device can be used, instead of just the perimeter. The traces connecting the package's leads to the wires or balls which connect the die to package are also on average shorter than with a perimeter-only type, leading to better performance at high speeds.

<span class="mw-page-title-main">Surface-mount technology</span> Method for producing electronic circuits

Surface-mount technology (SMT), originally called planar mounting, is a method in which the electrical components are mounted directly onto the surface of a printed circuit board (PCB). An electrical component mounted in this manner is referred to as a surface-mount device (SMD). In industry, this approach has largely replaced the through-hole technology construction method of fitting components, in large part because SMT allows for increased manufacturing automation which reduces cost and improves quality. It also allows for more components to fit on a given area of substrate. Both technologies can be used on the same board, with the through-hole technology often used for components not suitable for surface mounting such as large transformers and heat-sinked power semiconductors.

<span class="mw-page-title-main">Brazing</span> Metal-joining technique

Brazing is a metal-joining process in which two or more metal items are joined by melting and flowing a filler metal into the joint, with the filler metal having a lower melting point than the adjoining metal.

<span class="mw-page-title-main">Flux (metallurgy)</span> Chemical used in metallurgy for cleaning or purifying molten metal

In metallurgy, a flux is a chemical cleaning agent, flowing agent, or purifying agent. Fluxes may have more than one function at a time. They are used in both extractive metallurgy and metal joining.

<span class="mw-page-title-main">Reflow oven</span>

A reflow oven is a machine used primarily for reflow soldering of surface mount electronic components to printed circuit boards (PCBs).

<span class="mw-page-title-main">Wave soldering</span> Electronics soldering process

Wave soldering is a bulk soldering process used for the manufacturing of printed circuit boards. The circuit board is passed over a pan of molten solder in which a pump produces an upwelling of solder that looks like a standing wave. As the circuit board makes contact with this wave, the components become soldered to the board. Wave soldering is used for both through-hole printed circuit assemblies, and surface mount. In the latter case, the components are glued onto the surface of a printed circuit board (PCB) by placement equipment, before being run through the molten solder wave. Wave soldering is mainly used in soldering of through hole components.

<span class="mw-page-title-main">Rework (electronics)</span> Refinishing operation of an electronic printed circuit board assembly

In electronics, rework is the repair or refinish of a printed circuit board (PCB) assembly, usually involving desoldering and re-soldering of surface-mounted electronic components (SMD). Mass processing techniques are not applicable to single device repair or replacement, and specialized manual techniques by expert personnel using appropriate equipment are required to replace defective components; area array packages such as ball grid array (BGA) devices particularly require expertise and appropriate tools. A hot air gun or hot air station is used to heat devices and melt solder, and specialised tools are used to pick up and position often tiny components. A rework station is a place to do this work—the tools and supplies for this work, typically on a workbench. Other kinds of rework require other tools.

The solderability of a substrate is a measure of the ease with which a soldered joint can be made to that material. Good solderability requires wetting of the substrate by the solder.

<span class="mw-page-title-main">Solder paste</span> Material used in the manufacture of printed circuit boards

Solder paste is used in the manufacture of printed circuit boards to connect surface mount components to pads on the board. It is also possible to solder through-hole pin in paste components by printing solder paste in and over the holes. The sticky paste temporarily holds components in place; the board is then heated, melting the paste and forming a mechanical bond as well as an electrical connection. The paste is applied to the board by jet printing, stencil printing or syringe; then the components are put in place by a pick-and-place machine or by hand.

<span class="mw-page-title-main">Selective soldering</span>

Selective soldering is the process of selectively soldering components to printed circuit boards and molded modules that could be damaged by the heat of a reflow oven or wave soldering in a traditional surface-mount technology (SMT) or through-hole technology assembly processes. This usually follows an SMT oven reflow process; parts to be selectively soldered are usually surrounded by parts that have been previously soldered in a surface-mount reflow process, and the selective-solder process must be sufficiently precise to avoid damaging them.

<span class="mw-page-title-main">Dip soldering</span> Solder by immersion in a bath of molten solder

Dip soldering is a small-scale soldering process by which electronic components are soldered to a printed circuit board (PCB) to form an electronic assembly. The solder wets to the exposed metallic areas of the board, creating a reliable mechanical and electrical connection.

Process Window Index (PWI) is a statistical measure that quantifies the robustness of a manufacturing process, e.g. one which involves heating and cooling, known as a thermal process. In manufacturing industry, PWI values are used to calibrate the heating and cooling of soldering jobs while baked in a reflow oven.

<span class="mw-page-title-main">Thermal profiling</span>

A thermal profile is a complex set of time-temperature data typically associated with the measurement of thermal temperatures in an oven. The thermal profile is often measured along a variety of dimensions such as slope, soak, time above liquidus (TAL), and peak.

<span class="mw-page-title-main">Soldering</span> Process of joining metal pieces with heated filler metal

Soldering is a process of joining two metal surfaces together using a filler metal called solder. The soldering process involves heating the surfaces to be joined and melting the solder, which is then allowed to cool and solidify, creating a strong and durable joint.

Tin-silver-copper, is a lead-free (Pb-free) alloy commonly used for electronic solder. It is the main choice for lead-free surface-mount technology (SMT) assembly in the industry, as it is near eutectic, with adequate thermal fatigue properties, strength, and wettability. Lead-free solder is gaining much attention as the environmental effects of lead in industrial products is recognized, and as a result of Europe's RoHS legislation to remove lead and other hazardous materials from electronics. Japanese electronics companies have also looked at Pb-free solder for its industrial advantages.

<span class="mw-page-title-main">Graping</span> Phenomenon occurring to unreflowed [[solder]].

Graping is a phenomenon marked by the appearance of unreflowed solder particles on top of the solder mass. The solder that is partially coalesced resembles a cluster of grapes, hence the derivation of the phenomenon’s name.

Glass frit bonding, also referred to as glass soldering or seal glass bonding, describes a wafer bonding technique with an intermediate glass layer. It is a widely used encapsulation technology for surface micro-machined structures, e.g., accelerometers or gyroscopes. This technique utilizes low melting-point glass and therefore provides various advantages including that viscosity of glass decreases with an increase of temperature. The viscous flow of glass has effects to compensate and planarize surface irregularities, convenient for bonding wafers with a high roughness due to plasma etching or deposition. A low viscosity promotes hermetically sealed encapsulation of structures based on a better adaption of the structured shapes. Further, the coefficient of thermal expansion (CTE) of the glass material is adapted to silicon. This results in low stress in the bonded wafer pair. The glass has to flow and wet the soldered surfaces well below the temperature where deformation or degradation of either of the joined materials or nearby structures occurs. The usual temperature of achieving flowing and wetting is between 450 and 550 °C.

Stencil printing is the process of depositing solder paste on the printed wiring boards (PWBs) to establish electrical connections. It is immediately followed by the component placement stage. The equipment and materials used in this stage are a stencil, solder paste, and a printer.

In the assembly of integrated circuit packages to printed circuit boards, a head-in-pillow defect, also called ball-and-socket, is a failure of the soldering process. For example, in the case of a ball grid array (BGA) package, the pre-deposited solder ball on the package and the solder paste applied to the circuit board may both melt, but the melted solder does not join. A cross-section through the failed joint shows a distinct boundary between the solder ball on the part and the solder paste on the circuit board, rather like a section through a head resting on a pillow.

References

  1. 1 2 3 Profiling Basics – Reflow Phases
  2. ↑ Girouard, Roland. "Mark5 Reflow Oven". Heller Industries Website. Heller Industries Inc. Retrieved 11 March 2024.
  3. ↑ http://www.ipc.org/TOC/IPC-7530.pdf [ bare URL PDF ]
  4. ↑ http://www.solderstar.com/files/5214/3567/7718/SolderStar_Reflow_Solutions.pdf [ bare URL PDF ]
  5. ↑ "AutoSeeker - Reflow Thermal Profile Optimisation Software". YouTube .

http://www.idc-online.com/technical_references/pdfs/electronic_engineering/Electronics_Manufacture_Intrusive_Reflow.pdf

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