Editing Surface-mount technology (section)

== Rework ==
[[File:Soldering a 0805.jpg|thumb|Removal of surface-mount device using [[Soldering iron#Soldering tweezers|soldering tweezers]]]]
{{main | Rework (electronics) }}

Defective surface-mount components can be repaired by using [[soldering iron]]s (for some connections) or a non-contact rework system. In most cases, a rework system is the better choice because SMD work with a soldering iron requires considerable skill and is not always feasible.

Reworking usually corrects some type of error, either human- or machine-generated, and includes the following steps:

* Melt solder and remove component(s)
* Remove residual solder (may be not required for some components)
* Print solder paste on PCB, directly or by dispensing or dipping
* Place new component and reflow.

Sometimes hundreds or thousands of the same part need to be repaired. If due to assembly, such errors are often caught during the process. However, a whole new level of rework arises when component failure is discovered too late, and perhaps unnoticed until the end user of the device being manufactured experiences it. Rework can also be used if products of sufficient value to justify it require revision or re-engineering, perhaps to change a single firmware-based component. Reworking in large volumes requires an operation designed for that purpose.

There are essentially two non-contact soldering/desoldering methods: infrared soldering and soldering with hot gas.<ref>{{Cite web|url=http://smt.iconnect007.com/index.php/article/107339/two-prevalent-rework-heating-methods-which-one-is-best/107342/|title=Two Prevalent Rework Heating Methods--Which One is Best?|website=smt.iconnect007.com|language=en|access-date=2018-07-27}}</ref>

===Infrared===
With infrared soldering, the energy for heating up the solder joint is transmitted by long-, medium- or short-wave infrared electromagnetic radiation.

Advantages:

* Easy setup
* No compressed air required for the heating process (some systems use compressed air for cooling)
* No requirement for different nozzles for many component shapes and sizes, reducing cost and the need to change nozzles
* Very uniform heating possible, assuming high-quality IR heating systems
* Gentle reflow process with low surface temperatures, assuming correct profile settings
* Fast reaction of infrared source (depends on the system used)
* Closed loop temperature control directly on the component is possible by applying a thermocouple or pyrometric measurement. This allows compensation for varying environmental influences and temperature losses. Enables use of the same temperature profile on slightly different assemblies, as the heating process adapts itself automatically. Enables (re)entry into the profile even on hot assemblies
* Direct setting of target profile temperatures and gradients possible through direct control of component temperature in each individual soldering process.
* No increased oxidation due to strong blowing of the solder joints with hot air, reduces flux wear or flux blowing away
* Documentation of the temperature elapsed on the component for each individual rework process possible

Disadvantages:

* Temperature-sensitive nearby components must be shielded from heat to prevent damage, which requires additional time for every board
* On short wavelength IR only: Surface temperature depends on the component's [[albedo]]: dark surfaces will be heated more than lighter surfaces
* Convective loss of energy at the component possible
* No reflow atmosphere possible (but also not required)

===Hot gas===
During hot gas soldering, the energy for heating up the solder joint is transmitted by a hot gas. This can be air or inert gas ([[nitrogen]]).

Advantages:

* Some systems allow switching between hot air and nitrogen
* Standard and component-specific nozzles allow high reliability and faster processing
* Allow reproducible soldering profiles (depending on the system used)
* Efficient heating, large amounts of heat can be transferred
* Even heating of the affected board area (depends on system/nozzle quality used)
* Temperature of the component will never exceed the adjusted gas temperature
* Rapid cooling after reflow, resulting in small-grained solder joints (depending on the system used)

Disadvantages:

* Thermal capacity of the heat generator results in a slow reaction whereby thermal profiles can be distorted (depending on the system used)
* Precise, sometimes very complex, component-specific hot gas nozzles are needed to direct the hot gas to the target component. These can be very expensive.
* Today, nozzles can often no longer be deposited on the PCB by neighboring components, which means there is no longer a closed process chamber and adjacent components can be blown on strongly from the side. This can lead to the blowing of adjacent components and even to thermal damage. In this case, adjacent components must be protected from airflow, e.g. by covering them with polyimide tape.
* Local turbulence of the hot gas can create hot and cold spots on the heated surfaces, resulting in uneven heating. Therefore, perfectly designed, high-quality nozzles are a must!
* Swirls at component edges, especially at bases and connectors, can heat these edges significantly more than other surfaces. Overheating can occur (burns, melting of plastics)
* Losses due to environmental influences are not compensated for since the component temperature is not measured in the production process
* Creation of a suitable reflow profile requires an adjustment and test phase, in some cases involving several stages
* Direct temperature control of the component is not possible because measuring the actual component temperature is difficult due to the high gas velocity (measurement failure!)

===Hybrid technology===

Hybrid rework systems combine medium-wave infrared radiation with hot air

Advantages:

* Easy setup
* The low flow velocity hot air supporting the IR radiation improves heat transfer but cannot blow away components
* Heat transfer does not depend entirely on the flow velocity of hot gas at the component/assembly surface (see hot gas)
* No requirement for different nozzles for many component shapes and sizes, reducing cost and the need to change nozzles
* Adjustment of the heating surface is possible through various attachments if required
* Heating even very large/long and exotically shaped components possible, depending on the type of top heater
* Very uniform heating possible, assuming high-quality hybrid heating systems
* Gentle reflow process with low surface temperatures, assuming correct profile settings
* No compressed air is required for the heating process (some systems use compressed air for cooling)
* Closed loop temperature control directly on the component is possible by applying a thermocouple or pyrometric measurement. This allows compensation for varying environmental influences and temperature losses. Enables use of the same temperature profile on slightly different assemblies, as the heating process adapts itself automatically. Enables (re)entry into the profile even on hot assemblies
* Direct setting of target profile temperatures and gradients possible through direct control of component temperature in each individual soldering process.
* No increased oxidation due to strong blowing of the solder joints with hot air, reduces flux wear or flux blowing away
* Documentation of the temperature elapsed on the component for each individual rework process possible

Disadvantages

* Temperature-sensitive nearby components must be shielded from heat to prevent damage, which requires additional time for every board. Shield must also cover from gas flow
* Convective loss of energy at the component possible