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SMT Reflow soldering process is the most widely used method of attaching surface mount components(SMC) to printed circuit boards (PCBs). The aim of the process is to form acceptable solder joints between SMC and PCB. The reflow soldering process normally adopt following steps:
1.Print Solder paste on the PCB.
Solder paste is a kind mixture of solder alloy and flux. For reflow soldering normally adopt SnAgCu(Sn 99%, Ag 0.3%, Cu 0.7%) alloy,Make the SnAgCu to the small balls(1#?2#:75-150um?3#:25-45um,4#:20-38um?5#:10-20um) and mixed it with the flux?surface active agent and other things that can be helpful for soldering.
There are three method to print the solder paste on PCB, Manually, use semi auto solder printer or use the full automatic online solder paste printer. If use the full automatic SMT stencil printer, then it need a PCB loader to send the PCB to it automaticly.
After print the solder paste to the PCB solder pat by stencil printer(manual,semi auto or automatic), then send the PCB to the pick and place machine(chip mounter, surface mounter) to pick and place the surface mount component(SMC/SMD) to the PCB, the solder paste will stick the SMC/SMD to the PCB. At this status, the solder paste still a mixture with the solder balls and flux. not a solder joint between PCB and SMC/SMD.
A reflow oven is the machine used to change the solder paste status from mixture to solder joint. by first pre-heating the components/PCB/solder paste to the soldering temperature, the flux will be activiate and move the oxidated surface of the PCB and SMC/SMD to let the solder easy joint the PCB and joint the SMC/SMD. and then melting the solder(solder melting temperature 230? to 250?,there are also low temperature solder 140? and high temperature solder above 350?), after the solder cooling down, it will form the stable solder joints between SMC/SMD and PCB.
The key aspects that lead to an effective SMT reflow soldering process are as follows:
There are various types of reflow soldering machine available depending on the required line speed(output capacity),component size and components polulation of the PCB assemblies to be processed. The selected oven needs to be of a suitable size to handle the production rate of the pick and place equipment.
Nowadays the technology development required put more and more components on a common size PCB, the economical way is to develop the components and make it smaller, So multi functions, system in one chip, 144pins QFP, BGA 0201 01005 components had been invented and used widely in the SMT soldering process. this leaved more few space tween the chips and the pins. This kind components and chips need high accuracy solder paste printer,high speed pick and place machine, long heating zones reflow oven.
High accuracy solder paste printer means automatci solder paste printer, As manual and semi auto solder paste printer is adjusted by manual, accuracy is low, and will lost accuracy when doing printing, Automatic online solder paste printer(smt stencil printer) is adjusted by computer, automaticly, it use high accuracy camera to read the mark point on the PCB ,and can adjusting printing position everytime. it can reach 0.01mm printing accuracy for each printing.
High speed pick and place machine siemens X4I can reach a speed of 200000CPH(chip per hour) and can pick and place 01005 components. The price is also very high, 400,000USD for each machine, add the feeders around 700,000USD.
High speed pick and place machine and small components, high components polulation asked a high standard for reflow ovens, it need the reflow oven temperature ramup accuracy, slowly and temperature diviation equally on PCB to avoid any thermal shock to the components(fast temperature ramup will damage the comonents), when doing this, Pick and place machine also need the output capacity of the reflow oven is big, so nowadays the 10 heating zones reflow oven an 12 heating zones reflow oven become the widely used reflow oven in SMT assembly line.
In order to create a reflow profile thermocouples are connected to a sample assembly (usually with high temperature solder) in a number of locations to measure the range of temperatures across the PCB. It is recommended to have at least one thermocouple located on a pad towards the edge of the PCB and one thermocouple located on a pad towards the middle of the PCB. Ideally more thermocouples should be used to measure the full range of temperatures across the PCB – known as ‘Delta T’.
Within a typical reflow soldering profile there are usually four stages – Preheat, soak, reflow and cooling. The main aim being to transfer enough heat into the assembly to melt the solder and form the solder joints without causing any damage to components or PCB.
Preheat – During this phase the components, PCB and solder are all heated to a specified soak or dwell temperature being careful not to heat too quickly (usually no more than 2ºC/second - check solder paste datasheet). Heating too quickly can cause defects such as components to crack and the solder paste to splatter causing solder balls during reflow.
The earlier process steps within surface mount assembly are critical to an effective reflow soldering process. Thesolder paste printing process is key to ensure a consistent deposit of solder paste onto the PCB. Any fault at this stage will lead to undesired results and so complete control of this process along with effective stencil design is needed.
QISMT Electronic Co.,ltd provide a full SMT assembly line solutions, including SMT Reflow Oven, Wave Soldering Machine, Pick and Place Machine, Insert Plug-in machine, SMT Stencil Printer, SMT AOI SPI Machine, SMT Peripheral Equipment, SMT Production line, SMT Spare Parts etc any kind SMT machines you may need, please contact us for more information: wechat whatsapp:+8617748683433, Skype: qismteric, Email:firstname.lastname@example.org
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