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[9]:14 The most commonly used process limit is 4C/s, though many component and solder paste manufacturers specify the value as 2C/s. Thus, we can calculate the volume of flux left after reflow: (92.6-88.5)/ (100-88.5) = 35.4% of total flux. 4:15 from the start). Even when focusing on the thermal profile there are yet more challenges. Using a carefully designed profile will result in a repeatable process which will consistently deliver the required results - It is worth the extra time and effort. using the spacers, the eC-reflow-mate has the same characteristics of an oven The two most common settings for establishing a recipe for a good reflow profile are the zone set-points and conveyor speed. a large component, or clamped inside a via near this ground pin. bottom temperature can be made equal to the top temperature (Follow Your profile will depend upon many factors including paste chemistry, customer requirements, component limitations, oven characteristics, board layout, etc. The last zone is a cooling zone to gradually cool the processed board and solidify the solder joints. the right). The results can be very useful to researchers and scientists in the surface mount technology (SMT). the total reflow period. PCBs with a HAL finish (hot air leveled) require less solder paste Molten solder alloy behaves much as water, with properties of cohesion and adhesion. The next challenge is the reflow process window. practice to set the cool-down period up to opening the door to be just 1 second. Know where the ground planes are, where there are top and bottom components with different heat tolerances and so on. The combination of these factors puts a lot of pressure on the solder paste flux to remove surface oxides. expansion and contraction of components that warm up and cool down during their A PCB frame is available as It is important to monitor the process temperature to keep it from exceeding this limit. The flux reduces surface tension at the juncture of the metals to accomplish metallurgical bonding, allowing the individual solder powder spheres to combine. This is an expensive solution and may be hard to justify. To give an indication of a temperature profile in practice, here is a screen shot This is a significant part of efficient design services that require advanced board manufacturing and assembly. it is activated, the TAL should be reached within roughly 3 minutes or monitor the temperature curves and to check these against the profile We need to share this knowledge. Consenting to these technologies will allow us to process data such as browsing behavior or unique IDs on this site. In software you can easily control what you may or may not want to change, including optimization with the fastest belt speed for higher throughput or least amount of zone set-point changes for faster changeover. With offices across the globe, KIC is ready to become your partner in oven based manufacturing. should be compensated in the profile. Voiding Note that this differs from the default value of 24 hours given in J-STD-020A. The relationship between reflow profile parameters (soak temperature and soak time, reflow temperature and reflow time, cooling rate) and shear performance of Sn3.0Ag0.5Cu/Cu solder joint was . 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). An alloy may have This is directly proportional to the input temperatures set on the profile. Linear ramp to spike profiles are also commonly used and are typically a good starting place for most SMT processes. The Ramp to Peak, also known as Tamp to Spike Tent and/or Straight Ramp (depending on the solder paste company) is that which is currently being used to reduce voiding. A slow ramp rate also increases the oxidation of the solder powder particles inhibiting the coalescence of these oxidized particles. This situation can lead to graping. Even if youre going to experiment, you can see the results and make decisions quickly, so more uptime, and greater cost savings. when using no-clean solder paste (no-clean paste has less active flux). for a conveyor belt oven is that a change in the parameter for one zone typically Also, resistors are more prone than capacitors to graping. Often a contract manufacturer is given specific materials to use when assembling a board, so they cant just change the paste without a new qualification process, which lets face it, is costly and time consuming. A particular difference is the influence of thermal inertia. Fine-tuning a profile for specific components limitations should be considered, Include TCs on void-susceptible components when profiling, Balance TCs across components and PCB challenges, Use available tools to improve the time and accuracy of finding an optimum profile. KIC is a technology company working to make reflow ovens smarter through thermal profiling, automation and optimization solutions. The benefits of not having attached TCs surpass the convenience of not having to instrument a PCB every time a new profile is needed. The design of the PCB assembly will influence the machine selection and what options are added to the specification. Modern circuit assembly techniques that use reflow soldering do not necessarily allow the solder to flow more than once. Preheat zone [ edit] Preheat is the first stage of the reflow process. Troubleshooting The possible resulting surface oxidation can lead to voiding, graping, head-in-pillow, and other defects. temperature at 245C). The lower PCBs based on Insulated Metal Substrate (IMS) are often used. Therefore, a ramp-to-peak (RTP) profile with little or no soak, a slow ramp rate of 1-1.5C/second, and low peak temperature (this may mean extending the TAL slightly) will provide best results. Placement is also a key element in the voiding problem and an important place to look for problems. For PCBs with components on both sides, the bottom heater The temperature profile is in part mandated by the proper reflow, the advise is to lengthen the reflow zone (but keep the peak operational life). top layer (the component layer), and they are only suitable for Reflowing Solder Paste - V-One - Voltera To cool down more quickly, the oven opens the door at some pre-set temperature; Ultimately, quality requirements should drive the process, not adherence to these guidelines., From the July / August 2019 issue of SMT Today magazine, https://kicthermal.com/wp-content/uploads/2019/03/Optimized-Reflow-Profiling-to-Minimize-Voiding.pdf, https://kicthermal.com/wp-content/uploads/2019/03/Optimized-Reflow-Profiling-to-Minimize-Voiding-v3-Final.pdf. Mixed assemblies using a combination of tin-lead and lead-free alloys typically generate more voiding than a totally lead-free assembly, and certainly more so than a totally SnPb assembly. Obviously, when a PCB has components on both sides, you have no other option than Reflow a 40s to 80s window. In addition, a reflow profile that doesnt exhaust the flux chemistry and decreases solder paste slump will also help to minimize HiP. Common solder is an alloy from metals tin, silver, lead and copper. 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 ). This decreases reproducibility. This field is for validation purposes and should be left unchanged. The shelf life of solder paste is generally 6 to 9 months. Many components have a specification where the rise in temperature should not exceed a specified temperature per second, such as 2C/s. Best Practices Reflow Profiling for Lead-Free SMT Assembly, Authored by: Ed Briggs and Ronald C. Lasky, Ph.D.,PE. so that an intermetallic compound (IMC) is able to form. The purpose of the frame is to minimize the contact area between In a conveyor belt oven, the distance between the heating zones is fixed and measurements. A ramp-to-peak profile is a linear ramp to the peak (max) temperature. The PCB temperature sensor (the external sensor) is a K-type The occurrence of graping has increased due to higher lead-free reflow temperatures, decreased volume of the printed paste deposit, and finer particle size of the powder for the solder pastes required. This thermal stress, depending on the differences in CTE (coefficient of thermal expansion) of the joining surfaces, can fracture or tear the solder joint. and that is not covered with solder-resist. This decreasing ratio of flux to powder means there is less flux available to remove oxides from the joining surfaces and the solder powder particles within the solder paste itself. With offices across the globe, KIC is ready to become your partner in oven based manufacturing. Tim ONeil, AIM. This paper will discuss techniques to optimize the reflow profile to minimize such defects. These are the key areas of the thermal profile that can be changed to reduce voiding. 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. Conversely, a shorter profile, a shorter TAL, and a lower peak temperature results in a more mobile flux remnant. Another disadvantage to Kapton tape is that at temperatures above 200C the tape becomes elastic and, hence, the TCs have a tendency to lift off the substrate surface. compound, to further improve the thermal insulation. of solder paste should be kept in a cool storage (1C to 6C), but 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. Different kinds of PCBs need different profiles. solder-resisting thermocouple, it is typically impossible or at least Most people already have thermal profilers in their factory, so use them. observation of the bulk microstructures of the bumps shows that reflow profile 4, which has low peak temperature, short time above liquidus and fast cooling rate, forms the finest. In short, the thermal profile! Residue Discoloration The eC-reflow-mate has a grid plate and comes with a set of spacers. above TAL, because solder both coalesces better with the copper and wets PDF Preconditioning of SMT Packages - Texas Instruments India A Model Study of Profiling for Voiding Control at lead-free Reflow Soldering, by Dr. Yan Liu, William Manning, Dr. Benlih Huang, Dr. Ning-Cheng Lee, Indium Corp. the flux in the solder paste sets an upper bound on the duration of the soak zone. A stencil with . The SMT reflow profile can be broken down into 4 phases or regions: preheat, pre-reflow, reflow, and cooling. the decision to open the door is determined by the PCB temperature (the highest They vary from small PCB assemblies with miniaturization of parts and often a BGA to very large, heavy double-sided PCB assemblies with ground planes, heat sinks, and densely populated. With the smaller print deposit, the surface area exposed to the reflow oven environment increases in relation to the total amount of solder paste deposited. PDF Section 3: Reflow Profiling - Purdue University One in favor of a lower peak and one in favor of a higher peak, the greatest consensus being the higher peak. that it lets heat flow away from the PCB, an advantage is that heat is spread more Pad designs, which include via/micro-via or pads designed for smaller components, such as 01005 and 0.4mm pitch BGAs, increase voiding even further. profiles, all solder pastes left between 35 to 60% of residues on the assembly after reflow soldering, whereas between 30 to 55% remained for the 5-min. The eight (8) reflow profiles studied were derived from the Taguchi DOE which also included three factorial interactions. Lead-free Reflow Profile: Soaking type vs. Slumping type The maximum temperature ramp (for heating up) is roughly 1.5C/s. Soak Time. For both types of solder paste, the activator portion of the flux chemistry removes existing surface oxides on the mating surfaces, as well as the powder particles within the solder paste itself. Key focus to reduce/eliminate voiding is the reflow profile. Because the flux residue is affected by temperature, a reflow profile with a shorter TAL, lower peak temperature, faster ramp rate, and no soak provides the best results. You may have attached TCs to several parts that use the solder paste spec but for your QFN, plastic connector, LEDs, you may set up a separate process windows based on the component spec. Epoxies are good at securing TC conductors to the profile board to keep them from becoming entangled in the oven during profiling. the temperature rise from soak zone to reflow zone is 95C. Reflow Profile: Two unique profile families are depicted below; both can be used in rampspike or ramp-soak- -spike applications, and they each have similar reflow temperatures. Trial and error seldom feel like a good solution. The quick tool of establishing a process window for each TC/component will force the automated prediction software to look for an oven recipe that results in an in-spec profile for all the TCs are within their respective individual process windows, saving time and money. This is especially important for lead-free solders (SAC305), Low density aluminum tape allows for heat transfer to the EMF-producing area of the TC. Many people will drill below a Ball Grid Array, for example, and attach a TC at the ball. In comparison, a batch oven There are various types of reflow soldering machine available depending on the required line speed and design/material of the PCB assemblies to be processed. oven door opens for accelerated cooling (free air cooling). For impractical, to solder the temperature sensor on the PCB after the pick & place The duration of the soak varies for different pastes. adjusting the points in the profile curve, or by enabling and setting the A virtual reflow profile is based upon soldering theory, the recommended solder profile from the solder paste manufacturer, size, thickness, cooper weight, layers of the board and size, and the density of the components. temperature differences across the PCB. The solder paste reflows in a molten state, creating permanent solder joints. [9]:16, In the soak segment of the profile, the solder paste approaches a phase change. Therefore, some understanding of the reflow profile is useful in determining the best method(s) for optimizing the reflow process in these challenging conditions. However, making a change to a new solder paste is not a drop in solution nor is it always the correct solution. aluminium; more recently, Eurocircuits provides spacers from a (white) synthetic profile also depends on the PCB, and therefore the suitability of a profile must 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. Thermal profiling is the act of measuring several points on a circuit board to determine the thermal excursion it takes through the soldering process. Fill the Void by Tony Lentz & Greg Smith, FCT Assembly. Head-in-Pillow Without a subpoena, voluntary compliance on the part of your Internet Service Provider, or additional records from a third party, information stored or retrieved for this purpose alone cannot usually be used to identify you. Too often people have a voiding issue, call their solder paste company and when asked what the thermal profile is, the response is I didnt profile. Pots [1] Raw temperature values are normalized in terms of a percentage relative to both the process mean and the window limits. How to optimize the reflow profile? - Bittele - 7pcb.com Basically, there are five stages in the profile. Machine options that are usually available are as follows:-, In order to create an acceptable reflow profile each assembly needs to be considered separately as there are many different aspects that can affect how the reflow oven is programmed. PDF PROFILE SUPPLEMENT FOR LEAD-free ALLOYS - AIM Solder ovens that we are aware of, can be monitored with a computer. A thermal profile can be ranked on how it fits in a process window (the specification or tolerance limit). ABSTRACT This situation makes reducing voids more difficult. 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. Free air cooling is sufficient. A soak profile displays some "plateau" within a limited temperature range, before the alloy reflows. The Art of Thermal Profiling If epoxy is used, properties and specifications of that epoxy must be checked. Graping is much less prevalent in solder mask defined pads, which confines the amount of flux spreading that can occur. the pads and component pins better when it is hotter thereby creating This sensor should measure the temperature of the relevant cold Since cooling is typically already slow in a batch oven, this to a temperature above the melting point (the temperature above liquidus, This application note assumes an option for the eC-reflow-mate. SMT Today, November 2015 issue. A fine grain structure provides the most reliable mechanical bond. reflow oven from Eurocircuits. In the reflow zone, the temperature rises at a rate of approximately 2C FEATURES & BENEFITS Maximizes reflow yield for lead-free processing, allowing full alloy coalescence at circular dimensions as small as 0.25mm (0.010") with 0.100mm (4mil) stencil thickness. on a ground trace/zone near a large component. For an effective soldering process, soldering must be carried out under carefully calibrated conditions in a reflow oven. Factors such as:-, If the reflow profile has insufficient heat applied during the reflow stage there will be solder joints seen similar to the images below:-, There are a number of aspects of PCB design that have an influence on how well an assembly will reflow. Reflow soldering is the most widely used method of attaching surface mount components to printed circuit boards (PCBs). Having the ability to build a reflow profile around the entire populated PCB is critical to achieving the solder paste recommendations, maintaining the component specifications and reducing voiding. 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. The exposed powder particles then become oxidized and with no flux to protect or remove the oxides, these particles do not coalesce into the solder joint. Graph of ramp rate at 0.5 C/Sec Total time to achieve reflow 20 seconds Solder temperature Liquid 217 C T 10 C Solid The difficulty of calibrating a profile

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