A reflow oven is a machine used primarily for reflow soldering of surface mount electronic components to printed circuit boards (PCB).
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- 1Types of reflow ovens
Types of reflow ovens[edit]
Infrared and convection ovens[edit]
A convection industrial reflow oven.
Reflow Solder Process
[1] The oven contains multiple zones, which can be individually controlled for temperature. Generally there are several heating zones followed by one or more cooling zones. The PCB moves through the oven on a conveyor belt, and is therefore subjected to a controlled time-temperature profile.
The heat source is normally from ceramic infrared heaters, which transfers the heat to the assemblies by means of radiation. Ovens which also use fans to force heated air towards the assemblies (which are usually used in combination with ceramic infrared heaters) are called infrared convection ovens.
Some ovens are designed to reflow PCBs in an oxygen-free atmosphere. Nitrogen (N2) is a common gas used for this purpose. This minimizes oxidation of the surfaces to be soldered. The nitrogen reflow oven takes a few minutes to reduce Oxygen concentration to acceptable levels within the chamber. Thus nitrogen ovens typically have nitrogen injection in at all times which decreases defect rates.
Vapour phase oven[edit]
The heating of the PCBs is sourced by thermal energy emitted by the phase transition of a heat transfer liquid (e. g. PFPE) condensing on the PCBs. The liquid used is chosen with a desired boiling point in mind to suit the solder alloy to be reflowed.
Some advantages of vapour phase soldering are:
- High energy efficiency due to the high heat transfer coefficient of vapour phase media
- Soldering is oxygen-free. There is no need for any protective gas (e.g. nitrogen)
- No overheating of assemblies. The maximum temperature assemblies can reach is limited by the boiling point of the medium.
This is also known as condensation soldering.
Thermal profiling[edit]
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. [2][3]
Example of a modern thermal profiler
See also[edit]
References and further reading[edit]
- ^Girouard, Roland. 'Mark5 Reflow Oven'. Heller Industries Website. Heller Industries Inc. Retrieved 28 September 2012.
- ^http://www.ipc.org/TOC/IPC-7530.pdf
- ^'Modern thermal profiling device'. Solderstar Website. Solderstar. Retrieved 28 September 2018.
- General references
- 'T.Bazouni: Reflow Soldering'. Archived from the original on 2008-06-18. Retrieved 2008-04-11.
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Reflow_oven&oldid=886279247'
Compact datalogger used for the capture of thermal profiles from a reflow oven
A graphical representation of the Process Window Index for a thermal profile
A thermal profile is a complex set of time-temperature data typically associated with the measurement of thermal temperatures in an oven (ex: reflow oven). The thermal profile is often measured along a variety of dimensions such as slope, soak, time above liquidus (TAL), and peak.
A thermal profile can be ranked on how it fits in a process window (the specification or tolerance limit).[1] Raw temperature values are normalized in terms of a percentage relative to both the process mean and the window limits. The center of the process window is defined as zero, and the extreme edges of the process window are ±99%.[1] A Process Window Index (PWI) greater than or equal to 100% indicates the profile is outside of the process limitations. A PWI of 99% indicates that the profile is within process limitations, but runs at the edge of the process window.[1] For example, if the process mean is set at 200 °C with the process window calibrated at 180 °C and 220 °C respectively, then a measured value of 188 °C translates to a process window index of −60%.
The method is used in a variety of industrial and laboratory processes,[2] including electronic component assembly, optoelectronics,[3] optics,[4] biochemical engineering,[5] food science,[6] decontamination of hazardous wastes,[citation needed] and geochemical analysis.[7]
- 1Soldering of electronic products
- 1.1Reflow process
Soldering of electronic products[edit]
One of the major uses of this method is soldering of electronic assemblies. There are two main types of profiles used today: The Ramp-Soak-Spike (RSS) and the Ramp to Spike (RTS). In modern systems, quality management practices in manufacturing industries have produced automatic process algorithms such as PWI, where soldering ovens come preloaded with extensive electronics and programmable inputs to define and refine process specifications. By using algorithms such as PWI, engineers can calibrate and customize parameters to achieve minimum process variance and a near zero defect rate.
Reflow process[edit]
In soldering, a thermal profile is a complex set of time-temperature values for a variety of process dimensions such as slope, soak, TAL, and peak.[8]Solder paste contains a mix of metal, flux, and solvents that aid in the phase change of the paste from semi-solid, to liquid to vapor; and the metal from solid to liquid. For an effective soldering process, soldering must be carried out under carefully calibrated conditions in a reflow oven. Convection Reflow Oven Detailed Description
There are two main profile types used today in soldering:
- The Ramp-Soak-Spike (RSS)
- Ramp to Spike (RTS)
Profile temperature data plotted against reflow oven settings
Ramp-Soak-Spike[edit]
Ramp-Soak-Spike characteristics
Ramp is defined as the rate of change in temperature over time, expressed in degrees per second.[9]:14 The most commonly used process limit is 4 °C/s, though many component and solder paste manufacturers specify the value as 2 °C/s. Many components have a specification where the rise in temperature should not exceed a specified temperature per second, such as 2 °C/s. Rapid evaporation of the flux contained in the solder paste can lead to defects, such as lead lift, tombstoning, and solder balls. Additionally, rapid heat can lead to steam generation within the component if the moisture content is high, resulting in the formation of microcracks.[9]:16
In the soak segment of the profile, the solder paste approaches a phase change. The amount of energy introduced to both the component and the PCB approaches equilibrium. In this stage, most of the flux evaporates out of the solder paste. The duration of the soak varies for different pastes. The mass of the PCB is another factor that must be considered for the soak duration. An over-rapid heat transfer can cause solder splattering and the production of solder balls, bridging and other defects. If the heat transfer is too slow, the flux concentration may remain high and result in cold solder joints, voids and incomplete reflow.[9]:16
After the soak segment, the profile enters the ramp-to-peak segment of the profile, which is a given temperature range and time exceeding the melting temperature of the alloy. Successful profiles range in temperature up to 30 °C higher than liquidus, which is approximately 183 °C for eutectic and approximately 217 °C for lead-free.[9]:16–17
The final area of this profile is the cooling section. A typical specification for the cool down is usually less than −6 °C/s (falling slope).[9]:17
Ramp-to-Spike[edit]
Ramp-To-Spike characteristics
The Ramp to Spike (RTS) profile is almost a linear graph, starting at the entrance of the process and ending at the peak segment, with a greater Δt (change in temperature) in the cooling segment. While the Ramp-Soak-Spike (RSS) allows for about 4 °C/s, the requirements of the RTS is about 1–2 °C/s. These values depend on the solder paste specifications. The RTS soak period is part of the ramp and is not as easily distinguishable as in RSS. The soak is controlled primarily by the conveyor speed. The peak of the RTS profile is the endpoint of the linear ramp to the peak segment of the profile. The same considerations about defects in an RSS profile also apply to an RTS profile.[9]:18
When the PCB enters the cooling segment, the negative slope generally is steeper than the rising slope.[9]:18
Thermocouple attachments[edit]
Thermocouples (or TCs) are two dissimilar metals joined by a welded bead. For a thermocouple to read the temperature at any given point, the welded bead must come in direct contact with the object whose temperatures need to be measured. The two dissimilar wires must remain separate, joined only at the bead; otherwise, the reading is no longer at the welded bead but at the position where the metals first make contact, rendering the reading invalid.[9]:20
A zigzagging thermocouple reading on a profile graph indicates loosely attached thermocouples. For accurate readings, thermocouples are attached to areas that are dissimilar in terms of mass, location and known trouble spots. Additionally, they should be isolated from air currents. Finally, the placement of several thermocouples should range from populated to less populated areas of the PCB for the best sampling conditions.[9]:20
Several methods of attachment are used, including epoxy, high-temperature solder, Kapton and aluminum tape, each with various levels of success for each method.[10]
Epoxies are good at securing TC conductors to the profile board to keep them from becoming entangled in the oven during profiling. Epoxies come in both insulator and conductor formulations The specs need to be checked otherwise an insulator can play a negative role in the collection of profile data. The ability to apply this adhesive in similar quantities and thicknesses is difficult to measure in quantitative terms. This decreases reproducibility. If epoxy is used, properties and specifications of that epoxy must be checked. Epoxy functions within a wide range of temperature tolerances.
The properties of solder used for TC attachment differ from that of electrically connective solder. High temperature solder is not the best choice to use for TC attachment for several reasons. First, it has the same drawbacks as epoxy – the quantity of solder needed to adhere the TC to a substrate varies from location to location. Second, solder is conductive and may short-circuit TCs. Generally, there is a short length of conductor that is exposed to the temperature gradient. Together, this exposed area, along with the physical weld produce an Electromotive Force (EMF). Conductors and the weld are placed in a homogeneous environment within the temperature gradient to minimize the effects of EMF.
Kapton tape is one of the most widely used tapes and methods for TC and TC conductor attachment. When several layers are applied, each layer has an additive effect on the insulation and may negatively impact a profile. A disadvantage of this tape is that the PCB has to be very clean and smooth to achieve an airtight cover over the thermocouple weld and conductors. Another disadvantage to Kapton tape is that at temperatures above 200 °C the tape becomes elastic and, hence, the TCs have a tendency to lift off the substrate surface. The result is erroneous readings characterized by jagged lines in the profile.
Aluminum tape comes in various thicknesses and density. Heavier aluminum tape can defuse the heat transfer through the tape and act as an insulator. Low density aluminum tape allows for heat transfer to the EMF-producing area of the TC. The thermal conductivity of the aluminum tape allows for even conduction when the thickness of the tape is fairly consistent in the EMF-producing area of the thermocouple.
Virtual profiling[edit]
Virtual profiling is a method of creating profiles without attaching the thermocouples (TCs) or having to physically instrument a PCB each and every time a profile is run for the same production board. All the typical profile data such as slope, soak, TAL, etc., that are measured by instrumented profiles are gathered by using virtual profiles. The benefits of not having attached TCs surpass the convenience of not having to instrument a PCB every time a new profile is needed.
Virtual profiles are created automatically, for both reflow or wave solder machines. An initial recipe setup is required for modeling purposes, but once completed, profiling can be made virtual. As the system is automatic, profiles can be generated periodically or continuously for each and every assembly. SPC charts along with CpK can be used as an aid when collecting a mountain of process-related data. Automated profiling systems continuously monitor the process and create profiles for each assembly. As barcoding becomes more common with both reflow and wave processes, the two technologies can be combined for profiling traceability, allowing each generated profile to be searchable by barcode. This is useful when an assembly is questioned at some time in the future. As a profile is created for each assembly, a quick search using the PCB’s barcode can pull up the profile in question and provide evidence that the component was processed in spec. Additionally, tighter process control can be achieved when combining automated profiling with barcoding, such as confirming that the correct process has been input by the operator before launching a production run.[11][12]
External links[edit]
References[edit]
- ^ abc'A Method for Quantifying Thermal Profile Performance'. KIC Thermal. Archived from the original on 2010-09-30. Retrieved 2010-09-30.
- ^Pearce, Ray 'Process improvement through thermal profiling: the goal of thermal profiling is to always increase quality and reduce waste. Three case histories--covering powder coating, baking and solder reflow applications ' Process Heating, 01-JAN-05 [1]
- ^'High performance thermal profiling of photonic integrated circuits'
- ^Kapusta, Evelyn (2005), Using Thermal Profiling to Monitor Optical Feedback in Semiconductor Lasers (Thesis)
- ^K. Gill, M. Appleton and G. J. Lye 'Thermal profiling for parallel on-line monitoring of biomass growth in miniature stirred bioreactors' Biotechnology Letters Volume 30, Number 9 / September, 2008 [2]
- ^B. Strahm & B, Plattner, 'Thermal profiling: Predicting processing characteristics of feed materials:' [3]Archived November 17, 2006, at the Wayback Machine
- ^Arehart, Greg B.; Donelick, Raymond A. (2006). 'Thermal and isotopic profiling of the Pipeline hydrothermal system: Application to exploration for Carlin-type gold deposits'. Journal of Geochemical Exploration. 91 (1–3): 27–40. doi:10.1016/j.gexplo.2005.12.005. ISSN0375-6742.
- ^Houston, Paul N; Brian J. Louis; Daniel F. Baldwin; Philip Kazmierowicz. 'Taking the Pain Out of Pb-free Reflow'(PDF). Lead-Free Magazine. p. 3. Retrieved 2008-12-10.
- ^ abcdefghiO'Leary, Brian; Michael Limberg (2009). Profiling Guide. DiggyPod. ISBN978-0-9840903-0-3.
- ^TC Attachment Methods '[4]'
- ^Automatic Profiling video (Video). KIC Thermal.
- ^https://www.youtube.com/watch?v=5zmx9T54XHA
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Thermal_profiling&oldid=886226886'
Reflow ovens are used during Surface Mount Technology (SMT) manufacturing or in Semiconductor Packaging processes. Typically, the reflow oven is part of an electronics assembly manufacturing line and is preceded by printing and placement machines. The printing machine prints solder paste on the board and the placement machine places components onto the printed solder paste.
Setting Up a Reflow Oven
Setting up a reflow oven requires knowledge of the solder paste being used in the assembly. Does the paste require a Nitrogen (low Oxygen) atmosphere during heating? What are the reflow specifications including peak temperature, Time Above Liquidus (TAL), etc? Once these process characteristics are known the Process Engineer can endeavor to set up the reflow oven recipe with the goal of achieving a certain reflow profile. The reflow oven recipe refers to the oven settings, including the zone temperatures, convection rates and gas flow rates. The reflow profile is the temperature that the board “sees” during the reflow process. A number of factors need to be considered when developing a reflow process. How big/massive is the board? Are there very small components on the board that could be dislodged by high convection rates? What is the maximum component temperature limit? Will fast ramp rates be problematic? What is the desired profile shape (traditional cash register or a straight ramp)?
Reflow Oven Recipe Set Up and Oven Profiling
Many reflow ovens feature automatic recipe set up software allowing the oven to create a starting point recipe based on the board characteristics and solder paste specification. This starting point recipe can be further refined to center the profile in the process window by profiling the oven using a thermal recorder or trailing thermocouple wires. Oven set points can be adjusted up/down based on the actual thermal profile versus the solder paste specification and the board/component temperature limits. If automatic recipe set up is not available, the process engineer can use a default reflow profile and adjust the recipe to center the process through profiling. Automatic set-up usually provides a better starting point and reduces the iterations/adjustments required.
Once a centered profile has been achieved the process can be further qualified by running multiple profiles and calculating a process Cpk. This Cpk value will allow the reflow oven process engineer to determine whether the process is centered and repeatable and thus ready for production.
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Pyramax™ reflow ovens provide optimized lead-free processing for the ultimate in productivity and efficiency. BTU’s exclusive closed loop convection control provides precise heating and cooling, programmable heat transfer, and reduced nitrogen consumption, adding up to the lowest Cost of Ownership in the industry. With 6, 8, 10 and 12-zone air or nitrogen models, 350oC maximum temperature and a comprehensive menu of options, Pyramax™ reflow ovens are the industry’s most versatile performers and best value. For high-volume, high-mix manufacturers the Pyramax is now available as a dual chamber reflow oven – the ZeroTurn. The ZeroTurn eliminates recipe changeover time by allowing two different thermal profiles to be ready simultaneously.
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Temperature To Reflow Solder Oven Replacement
Example Ramp to Spike thermal profile thermal profile.
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 one or 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 or under an infrared lamp or by soldering individual joints [unconventionally] with a desoldering hot air pencil.
Reflow soldering with long industrial convection ovens is the preferred method of soldering surface mount components to a printed circuit board or 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 such as through-hole connectors or highly application specific through-hole components, onto a circuit board designed for surface-mount type components.
When used on boards containing a mix of SMT and 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. 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[edit]
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”. 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/Second. 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[edit]
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[edit]
The third section, the reflow zone, is also referred to as the “time above reflow” or “time 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 isn’t hot enough may prevent the paste from reflowing adequately.
An example of a commercial reflow oven.
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 times 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[edit]
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 30–100 °C (86–212 °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. It may be that 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[edit]
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[edit]
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[edit]
- Restriction of Hazardous Substances Directive (RoHS)
References[edit]
- ^ abcProfiling Basics – Reflow Phases
- ^Girouard, Roland. 'Mark5 Reflow Oven'. Heller Industries Website. Heller Industries Inc. Retrieved 28 September 2012.
- ^http://www.ipc.org/TOC/IPC-7530.pdf
- ^http://www.solderstar.com/files/5214/3567/7718/SolderStar_Reflow_Solutions.pdf
- ^https://www.youtube.com/watch?v=403RFnmL2hk
External links[edit]
- 'T.Bazouni: Reflow Soldering (Reflow Ovens)'. Archived from the original on 2008-06-18. Retrieved 2008-04-11.
* Guide to Pin In Hole Intrusive Reflow, Design and Assembly Ebook - Bob Willis
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