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Tiny components on circuit boards, how assembled?

  1. Jan 3, 2010 #1
    Out of curiosity I took a dead cell phone apart and can't help being puzzled at how the multitude of tiny components get onto the circuit board of the phone. A typical component size is 1mm X .5mm X .5mm. (smaller components were .5mm X .25mm X .25mm in size). Placement accuracy looked to be of order .1mm. Humans with really steady hands might do this but I'm guessing it is done by "robots"?

    Are the components greasy with some type of flux that makes them stick to the circuit board Or is the board covered in a thin layer of greasy flux to accomplish the same thing?

    How such tiny parts are made must be an interesting story.

    Thanks for any help!
     
  2. jcsd
  3. Jan 3, 2010 #2

    sophiecentaur

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    The actual soldering is probably done by 'wave' soldering. The board (held horizontally) is lowered over a bath of melted solder and a wave is send across the bottom, briefly immersing the underside of the board. The components are all put in 'by robot' and have slightly sprung leads which keep them in place until soldered.
    Needless to say, this only works with very clean / new components and under strict conditions. The 'flux' you have seen may well be sprayed on first. Have a look at links to "wave soldering".
     
  4. Jan 3, 2010 #3

    Averagesupernova

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    Components with leads in a cell phone? Damn few if any. Surface mount components are put onto a circuit board with what is called a pick-and-place machine. The components come on plastic tape wound on a reel or in plastic tubes. The machine picks the parts off of the tape or plastic tube with a vacuum to hold onto the parts. Ahead of this process solder paste may be applied with a screen printer, or glue applied to hold the components in place also using a screen printer. In either case, the board will be run through an oven to cure the glue or melt the solder paste. If no solder paste was applied, the board can be run through a wave solder machine. I'm sure wiki could tell you alot about modern electronic assembly.
     
  5. Jan 3, 2010 #4
    Last edited by a moderator: Sep 25, 2014
  6. Jan 8, 2010 #5
    A group of circuit boards are made together on one panel, so that they fit the handling machinery and make best use of the board material.

    The most predominate process uses a thin stainless steel and a screen to determine where a solder paste is placed. The solder paste is a mixture of microscopic solder balls in a flux paste. The paste is tacky enough to hold the small components in place though some care has to be given to the design of the landings for the parts. Otherwise, some tend to slip out of position during soldering.

    After the paste is applied to the top of the panel, it is usually moved to another line have components placed on the paste. This is the job of the pick and place machine.

    The pick and place machine brings the board into an assembly area, holds it there, and finds tiny round features, termed feducials, so that it knows precisely where the components are to be placed. Then, the pick and place head gathers components from trays, reels, or tubes and places them on the board. Older pick and place heads simply had chucks that grabbed components, but new ones have built in force sensors, alignment cameras, and component value testers.

    Subsequent to placement of components on the top side of the panel, the panel is sent to the reflow oven. The reflow oven heats the board in a controlled fashion as it travels down a conveyor through varying temperature zones. Temperature and time are critical to getting the solder paste to function properly. Different panels have different rates of heating because of their construction and components, so it's not unusual for the panels to have each zone programmed for the properties of the solder paste and the panel type.

    As the panels exit the reflow machine, they are allowed to cool in a controlled fashion, gathered, and returned to have solder paste deposited on the back side. Once again, components are placed and the assembly goes through reflow. The topside components are now hanging freely from the bottom of the board and are typically in place by the capillary action of the solder.

    Most designers attempt to keep their large components on one side of the board. Otherwise, an additional step, glue dotting, is required. Glue dotting places tiny spots of glue to the board that hold large parts in place while they're hanging upside down during soldering.

    Afterwords, the circuit boards are separated from the panel. The method of separation varies. Some boards have score lines engraved around them to allow them to be cracked or sheared from one another. This inexpensive, but hard on the components. A more costly, but safer approach uses a router machine, which cuts around each board, separating it from the others.

    Depending on the customer, the boards are typically checked to ensure they function, and may be tested with either a bed of nails or a flying nail tester. The nail testers are essentially spring loaded, gold coated nails that make connections with test points within the circuitry in an effort to confirm that the various V/I characteristics or voltages of almost every point in the circuit have been confirmed.

    The entire process is highly automated and given that components are in stock and no test fixtures are needed, it's not unusual to go from schematic to delivered boards in as little as four to six weeks.
     
  7. Jan 8, 2010 #6

    sophiecentaur

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    So is wave soldering old fashioned, now?
     
  8. Jan 8, 2010 #7

    berkeman

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    Mixed wave/reflow or reflow/reflow are definitely the most common processes now. You use wave soldering for through-hole components. On some boards with just a couple through-hole components, it is often more cost effective to just hand solder them after the reflow/reflow process is done.
     
  9. Jan 9, 2010 #8
    I don't see wave machines much anymore. Many facilities use paste dispensers to apply solder to the through hole areas and pass the board through reflow, through hole parts and all.

    There's also a growing trend to use "no-clean" paste. Thus the flux residue is left in place. This is not a good thing for the precision analog guys as the new fluxes still need to be removed, and they're very difficult to remove.

    - Mike

    - Mike
     
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