SMT process for PCB manufacturing
PCB assembly process on SMT line
After manufacturing of Printed circuit board, now we have to assemble electronic components on that board using surface mount technology (SMT).
So far now we have only bare PCB (The PCB without SMT components). This bare PCB pass through various processes on SMT line.
Bare PCB don’t have any assemble component, it has printed circuit as per Gerber data. Location of SMD component, vias and copper traces are beneath green solder masking. Next is assembly of SMD component over that bare PCB. Here in this article we are going to learn about SMT line process. How PCB assembly are done. Only basic details are covered for understanding of surface mount technology. There are lot of books and websites having information related to SMT, but you may get confused if you go thought very details of this technology. You have background related to non electronic field, this article may help you to understand basic of SMT process of PCB making.
Manufacturing of PCB with an automated surface mount assembly (SMA) line consist following sequence:-
- Solder paste application
- Component placement
- Reflow soldering
- Cleaning
- Electrical Testing
PCB Loading in Magazines
The PCB magazine loader is used for PCB production on SMT production line. It provides secure and care to the handling of printed circuit
board. PCB magazine loader feeds PCBs into the Process Line from Stacks or Magazines. Later Magazine loading in loader.
Bare PCB surface cleaning
After loading PCB in magazine, loader move PCB for cleaning, where cleaning is done through PCB cleaner- Example- Unitech machine. This cleaner is Able to clean top surface of PCB even chip components attached on the bottom side also it can clean bare PCB. It removed the dust particles by brush units (Anti static brushes) & Vacuum unit. Machine Conveyor speed, Adhesive roller direction are control check points for PCB cleaning.
Stencil Loading process in SMT
Here we have to understand what is PCB stencil? PCB Stencil is a sheet of stainless steel with laser-cut openings used to place solders paste on a PCB board for surface mount component placement. Stencil design is prepared according to circuit requirement on PCB. Bare PCB contains holes and location for SMD component. Stencil design is done by CAD. Latter stencil manufacturing is done.
How stencil is used in SMT line?
Once the PCB is loaded into the machine, the stencil is positioned over the panel, that is called the stencil loading. Positioning of stencil is properly align with bare PCB. The gap in stencil is called stencil aperture. Below the stencil aperture there will be location for SMD (Surface mount device).
Paste Stirring or Paste preparation
Paste stirring here is solder paste making. Example of Solder Paste- F640SA, HERAEUS 500gm JAR Lead Free Solder Paste. For more details about solder paste read – SMT process and SMT soldering Solder paste is alloy mixture in molten state.
F645:- Sn95.5/Ag4/Cu0.5 (Tin-Silver-Copper alloy), mean- tin 95.5%, silver-4% and copper-0.5%. See no lead is added in this paste.
F640SA– This is lead free solder paste. This is Sn/Ag/Cu alloy soldering. Sn (Tin-95.5%) Ag- 4% and Cu-0.5%. Type 3 is particle size for this solder paste,
size of type 3 come- 25 ~40 micron. Viscosity of this solder paste is 130±40 Pascal second. Solder paste stirring is done using machine and it is stored in Jar.
Specific storage conditions to be maintained for solder paste. For new jar of solder paste cooling temperature to maintain at 5~10˚ C in refrigerator and to be consumed before expiry date. For opened jar of solder paste to be maintain at 15 ~20 degree of temperature and to be consumed within 24 hours.
Solder Paste Printing
After solder paste stirring. Solder paste is then added in front of the squeegee (Made of smooth rubber blades). The squeegee lower onto the stencil such that is applying a very specific amount of pressure. The Squeegee move along the length of stencil while solder paste is deposited onto the stencil apertures. At the end of process squeegee raised up and stencil is separated from the panel.
Solder paste Inspection
3D inspection machine- KOH Yong, model-KY3020T can be used for inspection of solder paste. In 3D inspection, solder paste location and other solder related defect are checked.
Chip Mounting in SMT process
Chip Mounting machine (Yamaha-Model- YS12), Program Based Machine
Placement sequence of component on PCB:-
- Board indexing
- Board registration
- Fiducial vision alignment
- Component pick up
- Component centering / vision inspection
- Component placement and board indexing
- AOI- Automatic optical inspection
In 3D optical inspection or component placement at right place using SMT placement equipment is done with help of fiducial marks.
What are fiducial marks?
Fiducial are reference marks or reference point with respect to which the measurement or tracing of an object can be done using X, Y & Z
coordinates.
If we understand it in details, we come to know about augmented reality (AR), in which we learn components can be traced only if it has absolute position, means- position with respect to reference point or mark. So here those reference marks are fiducial marks. These fiducial marks also
used in PCB known as circuit pattern recognition marks which allow SMT placement equipment accurately locate and place part on board.
Here board indexing, PCB is loaded to appropriate position. Fidualcial marks presence in PCB provide common measurable point for all steps in assembly process.
There are 3 types of fiducial marks in PCB :-
- Panel Fiducial
- Global Fiducial
- Local Fiducial
Component placement is done using Chip Mounting machine (Yamaha-Model- YS12). These are robot machine which pick and place the SMD on
PCB with high precision and accuracy. These chip mounting machines are also called pick and placement machines (P&Ps). Generally chip mounting machine work on specific program to create PCB assembly. Usually uses work on air pressure and placement is done through pneumatic suction cups.
Reflow Soldering process in SMT
Reflow machine- Heller, model- 1809 MK111, Program Based machine.
- It is a Process of attaching surface mounted components to printed circuit boards
- The solder paste melts during this process and cools down again to create a good solder joint.
- Conveyor is used to move the PCB with a constant speed.
- There is reflow oven-which has multiple heating zone & Cooling zone
AOI- Automatic optical inspection in SMT
Machine-Amron,program based machine
After reflow soldering- PCB Is unloaded and again loaded in magazine for AOI- Automatic optical inspection. AOI of a PCB is done program based machine. This machine capture images or scan the PCB, compare the picture with added master sample. OK/NG judgment is done as per input added in program-Like OK soldering condition and NG soldering condition.
With AOI we try to arrest catastrophic failure- like component missing, quality defect, soldering defect, illumination defect of LEDs. Also component offset, component skew, excess solder joint, height defect, insufficient solder joint and other soldering defect is checked with AOI-Automated optical inspection machine. After AOI, OK PCB is unloaded and moved for depanelling.
PCB Depenelling in SMT PCB assembly
It is the process of removing numerous smaller, individual PCBs from a larger multi-PCB panel produced during manufacturing. PCB cutting is done with cutting robots which work on program. This is also precision work. Here special care to be done to avoid chip off, burrs and component damage. For different PCB router bit size changes and Bit life is also monitored.
MDA testing in SMT PCB assembly Process
Manufacturing Defect Analyzer (MDA) is a tool that uses in-circuit test techniques to enable the detection of manufacturing defects within a printed circuit board assembly. Testers can often detect the presence of resistors, capacitors, and transistors.
This is also done as per program. PCB circuit is check before final packing of part. MDA is last checking station to ensure product we manufacture is OK or not. If OK move to Packing.
Packing – PCB packing is done in specific ESD trays.
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