Reflow Soldering Profile

Reflow Soldering Profile. Reflow Soldering Profile above 20* seconds 30* seconds Rampdown rate (Tp to TL) 6°C/second max 6°C/second max Time 25°C to peak temperature 6 minutes max 8 minutes max * Tolerance for peak profile temperature (TP) is defined as a supplier minimum and a user maximum IPC020d51 Time e Tc5°C t Max Ramp Up Rate = 3°C/s Max Ramp Down Rate = 6°C/s File Size 331KBPage Count 2.

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.

Reflow Soldering Process SURFACE MOUNT PROCESS

Characteristics of The “EcReflowMate”PCB Frame Versus Grid Plate Versus PCB SpacersPositioning The PCB Temperature SensorSolder Paste PropertiesProfile CharacteristicsProfile RecommendationsIdeal Profile Versus Measured ProfileClosing RemarksFurther ReadingThe Eurocircuits eCreflowmateis a batch oven with quartz infrared heaters on the top and bottom plus convection(controlled air flow) The oven has two floating temperature sensors (top and bottom)and a provision for one sensor to clamp/attach onto the PCB (the “external”sensor) Eurocircuits documents that the infrared lamps take 20 to 30 seconds to heat upto the desired temperature The oven firmware switches the infrared lamps off8 °C before reaching the desired temperature to compensate for thethermal inertia of the lamps The maximum temperature ramp (for heating up) is roughly 15 °C/s Thecool down temperature ramp for the oven is roughly 08 °C/s but thePCB cools down slower (due to thermal inertia) At a preset temperature theoven door opens for accelerated cooling (free air cooling) The PCB temperature sensor (the “external” sensor) is a Ktypethermocouple with semirigid wires The sensor can be taped onto a PCB butshould not be soldered In some reflow ovens the PCBs are snapped in a frame that touches the PCB onlyon the edges The purpose of the frame is to minimize the contact area betweenthe PCB and the oven to avoid heat from the PCB to flow away Other reflow ovenshave a grid plate onto which you place the PCBs The disadvantage of a grid isthat it lets heat flow away from the PCB an advantage is that heat is spread moreevenly across the bottom of the PCB The eCreflowmate has a grid plate and comes with a set of spacers Whenusing the spacers the eCreflowmate has the same characteristics of an oventhat clamps the PCB in a frame Originally the spacers were of (blue coated)aluminium more recently Eurocircuits provides spacers from a (white) syntheticcompound to further improve the thermal insulation A PCB frame is available asan option for the eCreflowmate A side effect of using spacers on a grid (or using a frame) is that the PCBcools down slower Since cooling is typically already slow in a ba Many batch reflow ovens use a thermocouple sensor to measure the temperature ofthe PCB This sensor should measure the temperature of the relevantcoldspot on the PCB Ideally therefore the sensor is soldered on the ground pin ofa large component or clamped inside a via near this ground pin Not all thermocouples withstand soldering and even when using an appropriatesolderresisting thermo­couple it is typically impossible —or at leastimpractical to solder the temperature sensor on the PCB after the pick & placeprocess or to find a via that is nearby with a hole that has the right sizeand that is not covered with solderresist In practice the sensor is taped (or clamped) on a vacant spot on the PCB Thebest spot to tape or clamp the sensor is still near a cold spot which would beon a ground trace/zone near a large component Densely populated areas on the PCBare also colder than unpopulated areas Common solder is an alloy from metals tin silver lead and copper Themelting points of these metals are The alloys however have lower melting temperatures than the pure metals For soldering with lead the Sn63Pb37 alloy is almost universally used Forleadfree soldering the alloy Sn965Ag3Cu05 is the most common The alloy isalso known as the abbreviation SAC305 Pure metals have a sharp transition from solid to liquid below the melting pointthe material is solid above the melting point it is liquid An alloy may havea “plastic range” where the material is between solid and liquidphases Alloys with a melting point instead of a melting rangeare called eutectic alloys The Sn63Pb37 alloy is an eutectic alloy The “shelf life” of solder paste is generally 6 to 9 months Potsof solder paste should be kept in a cool storage (1 °C to 6 °C) butallowed to acclimate to room temperature before use Solder paste is usually applied via a stencil —typically from lasercutstainless The standard reflow profile has four zones preheat soak reflow and coolingThe profile describes the ideal temperature curve of the top layer of the PCB The preheating zone should increase the temperature at a maximumrate of3 °C/s The purpose of preheating is to allow the solvents to evaporateand to activate the flux The soak zone brings the temperature of all components and board areas to anequal level Due to differences in thermal inertia components do not heat up atthe same speed —this is especially the case with infrared heating dueto uneven absorption of infrared energy by components (depending on colour andreflectivity) In the reflow zone the temperature rises at a rate of approximately 2 °Cto a temperature above the melting point (the temperature above liquidusor TAL) The peak temperature for the reflow zone is at least 25 °Cabove TAL because solder both coalesces better with the copper and wetsthe pads and component pins better when it is hotter —thereby crea The table below is a starting point for a profile for a batch oven The optimalprofile also depends on the PCB and therefore the suitability of a profile mustbe verified with actual measurements It should be verified (by measurements) that the peak temperature does not exceedthe specifications of the components (in both temperature and the period at thistemperature) and that the duration of temperature above liquidusis ina 40 s to 80 s window To give an indication of a temperature profile in practice here is a screen shotof the software for the eCreflowmate with the prescribed profile superimposedIn this graphic the red curve represents a sensor that floats above the PCB theyellow curve is for the sensor the floats below the PCB the green curve representa sensor that touches the PCB and the blue curve is the prescribed profile Clearly visible is the effect of thermal inertia and how the eCreflowmatecompensates for it by overshooting the target curve (see for example the yellowcurve near 130 from the start) Also visible is how the PCB is stillheating up while the oven itself is already cooling down (see the green curve near415 from the start) The PCB cools down slowly until the door opens —whichhappens when the green curve reaches 200 °C The curves correspond to a PCB with a size of 100×150mm that was put onPCB spacersin the middle of the oven (see the image atthe right) The bottom heating is set to foll Different kinds of PCBs need different profiles This application note assumesPCBs from common FR4 material For power electronics and notably power LEDsPCBs based on “Insulated Metal Substrate” (IMS) are often usedIMS consists of a metal carrier (usually aluminium) with a thin epoxybasedlayer glued on top of it and an etched copper layer for the circuit on the epoxylayer By necessity IMS PCBs are singlesided circuits with only traces on thetop layer (the “component” layer) and they are only suitable forsurfacemount components (SMDs) For the solder profile the main characteristicof IMS is that aluminium has low thermal inertia and good thermal conductivityThe temperature differences are minimal across the board (no hot spots versuscold spots) The soak zone can therefore be shortened (or be removed altogether) Most reflow ovens can run as an independent selfcontained unit All reflowovens that we are aware of can be monitored with a computer We advise tomonitor the Lee Dr NingChen Optimizing Reflow Progile via Defect Mechanisms AnalysisSoldering and Surface Mount Technology vol 11 nr 1 1999 1 A survey of the effects of temperature and duration of various zones on thesolderability and the quality of solder joints The conclusions are onlypartially relevant for the eCreflowmate because the paper assumes the useof forced convection ovens (Although the eCreflowmate has a ventilator to createair flow it behaves primarily as an infrared oven) Peo Marc How Challenging Conventional Wisdom Can Optimize Solder ReflowSurface Mount Technology Magazine (Pennwell Corp) May 1998 1 This article focuses on six conventional wisdoms It discusses thenew perspectives brought by modern reflow ovens and modern solder pastes JSTD020D1 JEDEC March 2008 1 A joint IPC/JEDEC standard on the moisture/reflow classification of SMD devicesThough oft cited note that the standard prescribes the profile in section 56for the purpose of classification of components It is not necessarilysuitable as a production profile In particular the recommended holdtime of the temperature above liquidusof 60 to 150 seconds is farlonger than other recommendations.

Reflow Soldering Profile, Per JSTD020D, Table 52, PbFree

Recommended Reflow Soldering Profile Limiting Values* The below temperature profile for moisture File Size 33KBPage Count 1.