Editing Surface-mount technology (section)

== Assembly techniques ==
[[File:Juki KE-2080L by Megger.jpg|thumb|Assembly line with [[pick-and-place machine]]s]]
[[File:SMT-Bestueckung.jpg|thumb|PCB assembly line: pick-and-place machine followed by an SMT soldering oven]]

Where components are to be placed, the [[printed circuit board]] normally has flat, usually [[tin]]-lead, silver, or gold plated [[copper]] pads without holes, called ''solder pads''.  [[Solder paste]], a sticky mixture of [[flux (metallurgy)|flux]] and tiny solder particles, is first applied to all the solder pads with a stainless steel or nickel stencil using a [[screen printing]] process. It can also be applied by a jet-printing mechanism, similar to an [[inkjet printer]]. After pasting, the boards proceed to the [[SMT placement equipment|pick-and-place machines]], where they are placed on a conveyor belt. The components to be placed on the boards are usually delivered to the production line in either paper/plastic tapes wound on reels or plastic tubes. Some large integrated circuits are delivered in static-free trays.  [[Numerical control]] pick-and-place machines remove the parts from the tapes, tubes or trays and place them on the PCB.<ref>{{cite web|last1=Jena|first1=Hanings|title=PCB Assembly - Description|url=https://www.ourpcb.com/pcb-assembly|website=www.ourpcb.com|access-date=7 February 2018|date=4 January 2016}}</ref>

The boards are then conveyed into the [[reflow soldering]] oven. They first enter a pre-heat zone, where the temperature of the board and all the components is gradually, uniformly raised to prevent thermal shock. The boards then enter a zone where the temperature is high enough to melt the solder particles in the solder paste, bonding the component leads to the pads on the circuit board. The [[surface tension]] of the molten solder helps keep the components in place. If the solder pad geometries are correctly designed, surface tension automatically aligns the components on their pads.

There are a number of ''techniques'' for reflowing solder. One is to use [[infrared]] lamps; this is called infrared reflow. Another is to use a hot gas [[convection]]. Another technology that is becoming popular again is special [[fluorocarbon]] liquids with high boiling points which use a method called vapor phase reflow. Due to environmental concerns, this method was falling out of favor until lead-free legislation was introduced which requires tighter controls on soldering. At the end of 2008, convection soldering was the most popular reflow technology using either standard air or nitrogen gas. Each method has its advantages and disadvantages. With infrared reflow, the board designer must lay the board out so that short components do not fall into the shadows of tall components. Component location is less restricted if the designer knows that vapor phase reflow or convection soldering will be used in production. Following reflow soldering, certain irregular or heat-sensitive components may be installed and soldered by hand, or in large-scale automation, by focused infrared beam (FIB) or localized convection equipment.

If the circuit board is double-sided then this printing, placement, reflow process may be repeated using either solder paste or glue to hold the components in place. If a [[wave soldering]] process is used, then the parts must be [[glue]]d to the board before processing to prevent them from floating off when the solder paste holding them in place is melted.

After soldering, the boards may be washed to remove flux residues and any stray solder balls that could short out closely spaced component leads. [[Rosin]] flux is removed with fluorocarbon solvents, high [[flash point]] [[hydrocarbon]] solvents, or low flash solvents e.g. [[limonene]] (derived from orange peels) which require extra rinsing or drying cycles. Water-soluble fluxes are removed with [[deionized water]] and detergent, followed by an air blast to quickly remove residual water.  However, most electronic assemblies are made using a "No-Clean" process where the flux residues are designed to be left on the circuit board, since they are considered harmless. This saves the cost of cleaning, speeds up the manufacturing process, and reduces waste. However, it is generally suggested to wash the assembly, even when a "No-Clean" process is used, when the application uses very high frequency clock signals (in excess of 1 GHz). Another reason to remove no-clean residues is to improve adhesion of [[conformal coating]]s and underfill materials.<ref>{{Cite news|url=http://www.assemblymag.com/articles/84286-why-clean-no-clean|title=Why Clean No-Clean?|work=Assembly Magazine|access-date=2017-10-03|language=en}}</ref> Regardless of whether cleaning or not those PCBs, the current industry trend suggests carefully reviewing a PCB assembly process where "No-Clean" is applied, since flux residues trapped under components and RF shields may affect surface insulation resistance (SIR), especially on high component density boards.<ref>{{Cite web|url=http://www.ipc.org/feature-article.aspx?aid=To-clean-or-not-to-clean|title=No-clean is a process, not a product|website=www.ipc.org|access-date=2017-10-03}}</ref>

Certain manufacturing standards, such as those written by the [[IPC (electronics)|IPC – Association Connecting Electronics Industries]] require cleaning regardless of the solder flux type used to ensure a thoroughly clean board. Proper cleaning removes all traces of solder flux, as well as dirt and other contaminants that may be invisible to the naked eye. No-Clean or other soldering processes may leave "white residues" that, according to IPC, are acceptable "provided that these residues have been qualified and documented as benign".<ref>IPC-A-610E, paragraph 10.6.3.</ref> However, while shops conforming to IPC standards are expected to adhere to the Association's rules on board condition, not all manufacturing facilities apply IPC standards, nor are they required to do so. Additionally, in some applications, such as low-end electronics, such stringent manufacturing methods are excessive both in expense and time required.

Finally, the boards are visually inspected for missing or misaligned components and solder bridging.<ref>{{Cite web |last=Ayodele |first=Abiola |title=SMT Manufacturing: Everything You Need to Know |url=https://www.wevolver.com/article/smt-manufacturing-everything-you-need-to-know |access-date=2022-09-30 |website=Wevolver}}</ref><ref>{{Cite web |title=PCB Manufacturer with HDI Circuit Board fabrication for PCB and PCBA |url=https://www.hemeixinpcb.com/ |access-date=2022-09-30 |website=www.hemeixinpcb.com}}</ref> If needed, they are sent to a [[#Rework|rework]] station where a human operator repairs any errors. They are then usually sent to the testing stations ([[in-circuit test]]ing and/or functional testing) to verify that they operate correctly.

[[Automated optical inspection]] (AOI) systems are commonly used in [[PCB manufacturing]]. This technology has proven highly efficient for process improvements and quality achievements.<ref>{{cite magazine|url=https://www.academia.edu/25794300|title=3D Solder Joint Reconstruction on SMD based on 2D Images|first=Pedro|last=Vitoriano|date=June 2016|magazine=SMT Magazine|pages=82–93}}</ref>