Comprehensive Overview of Reflow Soldering Process

[Applause] [Music] welcome to part 1 of the reflow

soldering video training Series in this video and in tape number 2 we'll discuss what reflow soldering is what it does

and how it's done we'll also discuss what you as an operator need to know to eliminate the problems that lead to

defective reflow solder connections reflow is the process of applying heat to a group of components that are

positioned on a solid solder or solder in a paste form on a printed board the heat causes the solder to melt or flow

and since the solder may have been melted before the term reflow describes the action taking place there are

several processes used in the reflow soldering of electronic assemblies there is vapor phase or condensation heating

infrared or radiation heating air or nitrogen convection heating both forced and natural and also there is a

combination of IR and convection heating this video and tape 2 will explain the various processes and the details that

must be controlled in order to get a good solder joint to start we will look at the nature of the reflow solder

connections for our examples we will use surface mounted electronic components on a double-sided printed wiring board

there are two reasons for soldering the component to the surface of the printed wiring board one is to create a physical

or mechanical connection that will sturdy and reliable the second reason is to provide an electrical path between

the electronic component and the printed board conductors if both of these criteria are met there will be an

acceptable solder joint the process that takes place to provide this properly soldered connection is called wedding

there are several steps that take place in a reflow soldering system to achieve wedding when the assembly enters the

oven the components are positioned in their proper location on top of a mixture of flux and solder called solder

paste how this mixture is applied and how the components are placed will be discussed in some detail in the next

video of this series oxides are formed simply by contact of the metals with oxygen in the air oxides are a

combination of oxygen and the surface metal of the leads terminations lands and solder during the preheat stage many

fluxes become more active as their temperature increases as the flux touches the surface metal of the lead

and the land its purpose is to remove the oxides that have formed on those metals when these metals the solder also

is a metal are heated to proper temperature wetting can take place the solder is pulled or wicks on to the

surface metal of the lead or termination and land on the printed board at this point the solder combines with the

surface metal of the lead or determination and the land to form what is called the inter metallic compound

this thin layer of inter metallic is a combination of the surface metal of the lead or land and the tin portion of the

solder this is an example of wetting which indicates that there is a solder bond notice the concave surface of the

solder fillit that tapers to a thin edge these are signs of a properly wedded and therefore acceptable soldered connection

next we will provide an overview of a simple convection reflow soldering system the first stage in the machine is

the preheat area in this section the Assembly's temperature is slowly raised there are three things that are expected

to happen in the preheat section first some fluxes are activated by heat second the chemicals that hold the constituents

of the flux evaporate these volatiles must be driven off and third the parts to be soldered and the solder itself

must be closer to the soldering temperature the second stage inside the machine is the soldering area in this

part of the process reflow soldering takes place the parts to be soldered and the solder reach soldering temperature

the amount of time that the parts remain at this temperature is important too short a time will mean that the solder

will not flow properly too long a time will create an improper joint or may damage the parts being soldered the

final stage is the cool-down area in this area the assembly cools gradually and the solder joints are allowed to

solidify the heating that takes place in a reflow soldering oven is controllable moreso in some systems than others we

need a way to measure the temperature and heat transfer in all the stages so that any necessary adjustments can be

made this measure is called a thermal profile each oven has a generic profile each assembly has a unique and specific

profile if an assembly is heated too quickly or a too high a temperature there could be damage to the printed

board or some of the electronic components if an assembly is not heated enough proper soldering will not take

place the temperature profile is shown as a graph of temperature versus the time that the assembly takes

to pass through each stage in an ideal profile the rise in the Assembly's temperature during the preheat stage

will be slow and gradual the temperature should also be essentially uniform for the whole assembly in the soldering

stage the temperature of the assembly and the solder must reach the reflow temperature range each part to be

connected that is the lead or termination of the components and the land to which they are being attached

must reach soldering temperature at the same time for instance if the lead gets to soldering temperature first the

solder wicks up that lead by the time the land reaches the proper temperature there is not enough solder left to form

the connection the final stage of reflow soldering is the cool-down where the assembly slowly approaches room

temperature this is recorded as the end of the temperature profile the cool-down stage must also be controlled so that

the temperature change is not too quick there are many ways to check the heat transfer in a reflow soldering system in

some facilities waxes that melt at specific temperatures or stickers that change color at certain temperature are

applied to the printed board surface these devices will indicate that a specific temperature has been exceeded

but will not tell the actual upper limit of the temperature of the board surface another device used to record the

temperature profile is built into some machines as the assembly on the conveyor travels through the reflow soldering

machine thermocouples take readings at predetermined locations in the oven information from the thermocouples

inside the oven is passed to an on-board computer that will display the profile on a screen another method that is used

to determine the thermal profile is a device that actually travels with the assembly through the reflow soldering

machine this device is wired to thermal couples that are attached to various locations on the assembly the readings

are sent to a receiver via radio signals or are stored in the profiling device and unloaded to a computer after exiting

the reflow soldering machine the information from the device is used to form the graph of the temperature at the

assembly versus the time of travel through the various areas of the machine some systems employ a series of enfermo

meters which are like small TV cameras as each board passes the meters read the board surface temperature and convey

that information to the system computer if preset temperature limits are exceeded this machine notifies the

operator any of these methods can be used to simulate or give a picture of the Assembly's temperature rise at given

locations on the board or components surface as the assembly travels through the reflow soldering process

a typical temperature profile might look like this these colored lines indicate the journey of three specific locations

on the assembly through the preheat section of the reflow soldering machine this particular profile shows a

temperature gain of about two degrees Celsius per second during the preheating after preheat the Assemblies pass into

the reflow soldering area in this section they will be heated to the proper soldering temperature for a

solder alloy that melts at 230 degrees Celsius or greater the upper heating temperature that the assembly must reach

needs to be at least 20 degrees Celsius above that melting point in some situations that upper limit may be even

greater if the melting point of the alloy is less than 230 degrees Celsius the upper limit of the temperature

profile during soldering needs to be at least thirty degrees Celsius above that temperature exposure of the assembly to

this upper temperature is only required for a few seconds however the various parts of the joints to be soldered may

reach soldering temperature at different times depending on location component type and size some joints will be at

soldering temperature for only about 10 seconds while others may be there for 40 or 50 seconds then the assembly exits

the soldering section of the machine and goes into cool-down the information in this video is intended to give you an

idea of what happens inside a reflow soldering machine in free flow soldering part 2 we will

discuss the mechanisms used to affect that heat transfer [Music]

in the first video of this reflow soldering training series we looked at the reasons for the heat transfer that

takes place within the reflow soldering system we also looked at what happens to the components and the printed wiring

board when that heat transfer takes place properly now if you don't understand any part of this reflow

process ask your supervisor or trainer for help before a discussion of the various really you the techniques for

applying the correct amount of solder to the printed board this is done before the components are positioned the lands

to which the parts are to be attached will be covered with an application of solder paste which is a mixture of

solder particles and flux these compounds have various other ingredients but they all have solder flux and

binders that determine the physical properties of the particular paste solid solder may also be used for reef llowing

a solder addition during the printed board manufacturing process is sometimes used these are known as solder bumps in

our examples however we will use solder paste which is added just prior to component placement most Sauter's that

are used in electrical soldering are an alloy of metals most often tin and lead in the paste form the solder is a powder

in this extreme close up the solder looks like tiny balls of metal the flux which you learned about in reflow

soldering part 1 is a chemical compound designed to remove oxides from the surface of the parts to be soldered the

type of flux is decided on the basis of what fluxing action is required and what type of post soldering cleaning machine

will be used the binder that is used will determine the consistency and maintain the shape of the applied paste

on the land to than slumping there are two basic ways of applying the pace that we will look

at here one printing pattern is made of metal usually brass and is called a stencil another method is called screen

printing it uses a plastic fiber or stainless steel mesh that has an image added to it to define the openings

through which the paste is forced the solder paste is wiped across the stencil or screen with a squeegee which pushes

the pace through the openings depositing it on the lens in only the right places the thickness of the stencil or screen

the type of solder paste the kind of material the squeegee is made of and the pressure that is applied by that

squeegee will determine the amount of paste that is deposited the amount of paste deposited on the lands will be

critical to the quality of the reflow solder connection after the paste has been applied the components are placed

on the printed board there are many machines that are available for the accurate placement of surface mount

components on printed boards we will not go into any detail here concerning these machines it is important however that

you understand that if a component is not properly placed or is moved out of place before it enters the soldering

machine the required connection may not be made another situation that may exist in your facility is the need to solder

components on both sides of a board in one pass through the machine in some cases the components that will travel

through the reflow soldering machine on the underside of the board are glued in place this glue known as adhesive can be

applied by injection when all the required components are in place the unsoldered assembly is ready

to enter the reflow soldering machine now let's take a look at some of the ways of applying heat for reflow

soldering the first method is called the vapor phase or the condensation reflow soldering process the heat comes from

the vapor provided by a liquid that boils at a specific temperature the liquid is heated in a tank some of it

vaporizes into gas and Rises upward in effect carrying the heat to the assembly above as the vapor condenses on the

assembly its heat is transferred to the assembly and the solder paste with further heating the solder and the paste

melts and wedding can take place no part of the assembly can be heated above the boiling temperature of the liquid with

an upper temperature that cannot be exceeded components that are sensitive to high temperatures are safe and places

that are hard to contact are easily reached by the heat of the condensing vapor there are a variety of chemically

inert non flammable liquids that are used to vary the boiling point although these liquids are quite

expensive they are recycled and reused most machines have cooling coils that cause the vapor that rises past the

assembly to be condensed and returned to the heating pot another advantage of vapor phase reflow soldering is that the

density of the vapor will keep oxygen out of the system this will keep the metals from reoxidized

during any point of the operation but vapor phase does have a disadvantage although the upper temperature that the

assembly can reach is controllable the amount of time it takes to reach that temperature is not some components can

be damaged if their temperature goes up too quickly this problem can be dealt with by having

a preheat area before the vapor chamber anyone working around the vapor phase reflow soldering machine should be aware

of what chemicals are being used and refer to the material safety data sheets for safe handling of those chemicals the

material safety data sheets or the MSDS should be made available in the work area where the chemicals are being used

it is your responsibility to take whatever precautions are necessary the two types of systems used to vapor phase

solder are the batch type and inline systems in a batch system the assemblies are lowered in a basket into the

soldering section of the machine after a predetermined amount of time the basket is raised up and the soldered assemblies

are removed batch equipment is used for small and prototype groups of assemblies in the inline system the assembly rides

on a conveyor through the vapor chamber with some inline systems the conveyor is inclined towards and then away from the

vapor chamber this will help the condensed liquid to flow off the assembly to be recycled and reused

now we'll have a study of radiation and convection heat transfer with radiation rays of energy will heat any surface

that absorbs them the color and texture of the surface to be heated can affect how much heat is absorbed depending on

the wavelength of the IR energy and any area that is not in a straight unobstructed line from the energy

emitting source will not receive direct heating here's a simple example on a cold day if you stand in the Sun you can

feel quite warm if you reach your hand into a shaded area it will feel much colder it's not

that the air in the shade is that much colder it's simply that you're not receiving radiated heat from direct

contact with sun rays with convection there must be direct contact with a heating medium usually a gas or liquid

for instance burning natural gas in an oven heats the air and the hot air would then heat the food there are two basic

kinds of infrared reflow soldering the first is called near IR radiation soldering in these systems almost all

the heat comes from a source that radiates infrared energy in the short wavelength range typically the source is

a group of IR lamps about 5% of the heat in this type of system comes from convection that is from the heated air

in the oven one problem with this method is that the IR energy is of a wavelength that is much more readily absorbed by

some colors than by others this means that some parts of the assembly will get hot faster than others depending on

their color if the leads and lands don't reach soldering temperature at the same time all of the solder may wick to the

hottest part this would leave insufficient solder to bridge the gap necessary to make the

connection the second infrared reflow soldering system is called convection IR and uses

the middle range of the infrared energy wavelength about 60% of the heat transfer comes from convection through

the hot air in the oven the remaining 40% of the heat transfer comes from infrared radiation in both of these IR

reflow soldering methods the assembly travels through the machine on a belt or conveyor the first area inside the

machine transfers a low amount of heat to allow for a gradual temperature rise in this preheating area of the system

the volatile x' are driven off and the cleaning action starts if these liquids do not evaporate slowly and before

soldering they may be turned abruptly to gas at soldering temperatures and explode out through the solder this will

cause blow holes and solder balls as the assembly travels on through the machine the next heating zones are set at higher

temperatures these are the zones where reflow of the solder and wedding will take place the amount of heat in the

reflow area and the speed of the conveyor or belt will decide how much heat is transferred

another type of reflow soldering machine transfers energy with 80 to 95% convection heating in these forced

convection systems air or another gas such as nitrogen is heated it is then blown over and around the Assemblies

through many holes above and below the assembly conveyor an advantage of the forced convection system is its ability

to gradually and uniformly heat all of the assembly both the maximum temperature and the rate at which that

temperature is reached can be controlled this machine will also heat the parts to be soldered very evenly component leads

terminals and printed wiring board lands will reach soldering temperature at the same time one problem that has been

observed is that sometimes the blowing hot air will dry out the surface of the solder paste and form a skin of resin

the paste may not be able to vent its volatile is completely by the time the soldering zone is reached again this

situation can create blow holes and solder balls present developments indicate that the combination of

radiation and convection heating will be the system of choice in the future radiation to add the correct amount of

heat quickly and convection to provide uniformity of heating to the surfaces an adaption of all the IR and convection

systems is the nitrogen blanket system or equipment this modification helps reduce the amount of oxide formation on

the surface of the parts to be soldered this is accomplished by substituting nitrogen for the oxygen normally present

in the heating areas of the machine if there is no oxygen to touch the metals no oxides will form

it's important that you have an overall understanding of the reflow soldering process ask questions in this

competitive environment that exists in our industry today your company's ability to compete will

depend on each employee doing the best quality job possible [Music]

you [Music]