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Comparison of Hot Air Rework Systems with IR
As rework becomes more complex, companies exam-ine alternative heating techniques to see if they are more efficient. This document outlines some of the consider-ations that must be taken into account when specifying
rework applications for present and future electronic assemblies.
Generally speaking the rework process should be assimilar as possible to the original manufacturing process,
but applied to the single component. The latest manufactur-ing techniques still adopt forced air convection for
reflowing components and there has been a significant shift away from IR.
Solder Paste Application
It is also important to apply the correct amount ofsolder paste to the pads/components to ensure a reliable
solder connection. This is particularly relevant to large BGA components and assemblies that are subjected to
heat and severe environments. Component standoff must be suffi-cient to allow the processor components to
keep cool whenthey are working. Sufficient standoff is also vital to ensure maximum fatigue strength is given
to the solder joints.Theapplication of solder paste as wellas flux is the only way ofensuring that correct stand
off heights are achieved.
MLF and leadless components now have large internalground pads underneath the component. The correct
amount of solder paste must be applied to this pad to ensure good grounding and heat dissipation.
If excessive solder paste isapplied then the component will “float” and open jointswill be observed around the
perimeter of the device.It is essential that any rework system used can deliverthese important process
Rework should not“intrude”on the integrity of sur-rounding components. Any un-necessary heating applied
tocapacitors, resistors and transistors located close to the reworked component will significantly increase the
chanceof producing brittle joints. While it is claimed that IRheating is localised, it is difficult to focus an IR
beam on thecomponent alone. There is always some IR energy absorbedby the surrounding areas. Nozzles,
used in hot air rework areexcellent at shielding surrounding components from exces-sive temperatures,and the
ability to adjust air flow rates canalso contribute to safe rework practices. (See attached pagefrom Ersa web
site showing circular heat zone applied torectangular/square component)
Nozzles are relatively cost effective and only represent a small percentage of the total cost of a
rework system. SomeIR systems do claim that there are no additional costs withtheir technology, however
some do require the purchase oflenses to focus the IR beam.
The materials used in manufacturing array packagesdo vary. PBGA’s are manufactured from materials
thatreadily absorb IR radiation. CSP and micro SMD compo-nents tend to have bodies manufactured from
metal. Metalreadily reflects IR like a mirror. This can make removal ofsuch components difficult and may need
excessive amounts of heat.
The heat transfer associated with hot air convectionrework is independent of the material the
component is manufactured from, allowing the safe rework of nearly alltypes of component.
Unified Preheat for complete PCB
IR is focused heating and it is difficult to heat up thewhole PCB evenly, especially. For the PCB more than
10layers such as Telecom boards,Because components arealways applied higher Temperature than PCBs, IR
preheat-ing technique will easily cause the board wrapped And leadsto bad solder joint connection.
Ease of profiling is essential. All parts of the reflowprofile are equally important. The cooling phase of
theprofile is becoming significant when reflowing and rework-ing components.Rapid cooling when reworking
with leadfree alloys is essential to prevent fillet tearing. Time inreflow should be no greater than 30 seconds
because leadfree alloys have higher melting points,long reflow timeswill damage pcb’s and components.
Therefore, IR ma-chines need to turn on the cooling fan after reflow, And PACE machines need to have
operator move the boardsfrom the heat source after reflow zone. The cooling zone isnot under machine's
Control of ramp rates is also important to ensurecorrect flux activation and solder flow. This can only
beachieved in a repeatable fashion if heat transfer rates arefully adjustable and under-board heat can in
some instancesbe set at a higher level than nozzle temperatures. Program-mable airflow rates deliver this
requirement. IR tends toheat components too quickly resulting in incorrect fluxactivation and thermal shock
The use of nitrogen is only possible with convectionsystems and is now considered useful when reworking
arraypackages used in lead free assemblies.
There are a variety of rework systems available on themarket employing different heating techniques.
In general terms the following points are important forsuccessful rework of present day components
1. Use the same heating technology used in manufac-turing the assembly in the first place....usually
2. You must be able to accurately print solder pasteonto the component/p.c.b.,
3. Reflow of adjacent components is not desirable.
4. A rework system must be able to rework ALL types of components, including MLF, leadless and
5. Adjustable flow rates and rapid cooling are definiterequirements for successful rework of
conventional and lead free alloys.