Full PCB Assembly Process in MADPCB Facility

At MADPCB, we pride ourselves on our turnkey PCB assembly service for the purpose of high quality and efficiency. We employ a number of experienced employees in process control and quality control to ensure that every PCB assembly order is completed right from the first time. For achieving the fastest possible turnaround of PCB assembly projects, we continuously strive to improve our services and to make each step in PCB assembly process as efficient as possible.

Here we show you a step-by-step overview of our standard turnkey PCB assembly process, providing vital information at each stage that might relate to your interest. While, this is a brief overview only, and for those who have interest in a more detailed elaboration around MAPCB’s capacities, we recommend visiting and auditing us at our PCB assembly house.

1. Material Preparation

  • PCB Fabrication

  • SMD Stencil

  • Components Souring, IPQC, Counting, (Programming), Designator & Feeder Marking, Uploading

2. Solder Paste Printing

Solder paste (or solder cream) is a material used in the printed circuit board (PCB) assembly to connect SMDs (solder paste device) components to PADs on the board. Solder paste printing is to apply solder paste onto the SMD PADs of a bare board using a printer through a stainless-steel stencil in which there are opening windows of those PADs. It’s said that 60% - 90% of SMD assembly soldering defects are related to solder paste printing. So, solder paste printing process is one of the most important process of the SMT assembly, and the purpose is to accurately deposit correct amount of solder paste to the PADs for soldering components.

The solder pastes used in the electronic industry are rapidly converting from Tin/lead (Sn/Pb) solders to Lead-free (Ph-free) solders to meet new environmental and Green requirements. At MADPCB, we use adopt lead-free and RoHS complaint solder pastes.

Our auto solder paste printer supports and features:

  • Board Size: min 48mm x 48mm (1.89’’ x 1.89’’), max 250mm x 610mm (9.84’’ x 24’’)

  • Applicable for all kind of fine pitch ICs

  • Solder paste height monitor

  • Repeat Alignment: +/-12.7μm(0.05mil) @ 6sigma, Cpk>2

  • Printing Precision: +/-20μm(0.79mil)

Note: In solder paste printing on flexible circuit and rigid-flex PCB, it requires a SMT pallet to support the bendable PCB and make sure they get flat surface on the printer, then the technical can print the solder paste on the PADs.

3. Solder Paste Inspection (SPI)

The solder paste deposited on the bare PCB then need to inspect through solder paste inspection (SPI) optical system machine before SMD pick-and-place process. It’s observed that most of the solder joint defects in PCB assembly prototypes are because of improper solder paste printing. When deliver the solder paste printed boards through the rail to SPI machine, it will automatically conduct solder paste inspection. The CCD camera in SPI machine takes pictures, which are used for measuring the volume, square, height, location, shape an alignment of the solder paste. Through auto analysis, it identifies all solder paste problems, then solve them before SMD assembly process, and we can reduce or eliminate almost all solder joint defects to improve the assembly quality, production yield, printing quality and PCBA performance.

Our auto solder paste inspection supports and features:

  • Board Size: min 50mm x 50mm (1.96’’ x 1.96’), max 250mm x 610mm (9.84’’ x 24’’)

  • PCB Thickness: 0.4mm (16mil) – 5.0mm (196mil)

  • < 10% GR&R at 6μm (0.24mil)

  • Volume Repeatability<1% at 3μm (0.12mil) on calibration target

  • Defect all defects: insufficient & excessive deposits, shape deformation, missing paste, paste offset, smearing, bridging …

4. Pick-and Place (SMT Assembly)

Once the blank PCB board has solder paste applied checked ok, the components are placed on top by hand or by a machine. At MADPCB, we use SMT (surface mount technology) component placement system, commonly called pick-and-place machine or PNP, is a robotic machine which is used to pick SMDs from the components feeder or tray, and place them onto the PADs of a printed circuit board. The pick-and-place PCB assembly, also called SMT assembly. Before SMT assembly, the PNP machines require to be program set-up according to the PCB size and coordinates file (or pick-and place file). Then the placement machines carry out the specific programmed steps to build PCB assemblies. SMDs placement in this process features high speed and high precision placing of various ranges of electronic components, like capacitors, resistors, ICs onto the PCB boards.

Surface mount components are placed along the front (and often back) faces of the machine. Most components are supplied on plastic tape, in tape reels that are loaded onto feeders mounted to the machine. Larger integrated circuits (ICs) are sometimes supplied arranged in trays which are stacked in a compartment. More commonly ICs will be provided in tapes rather than tray or sticks. Improvements in feeder technology mean that tape format is becoming the preferred method of presenting parts on a pick-and-place SMT machine.

Through the middle of the machine there is a conveyor belt, along which bare PCBs travel, sometimes the bare PCBs need to be hold and supported by a SMT pallet (/barrier) if the PCB size is smaller than the width of the conveyor belt. The nozzles pick up individual components from the feeders or trays, rotate them to the correct orientation and then place them on the appropriate PADs on the PCB with high precision.

As the electronic component is carried from the part feeders or trays on either side of the conveyor belt to the PCB, it is photographed from below. Its silhouette is inspected to check if it is damaged or missing (was not picked up), and the inevitable registration errors in pickup are measured and compensated for when the part is placed. For example, if the part was shifted 0.25mm (10mil) and the rotated 10o when picked up, the pickup head will adjust the placement position to place the part in the correct location. The SMT machines have these optical systems on the robot arm and can carry out the optical calculation without losing time, thereby achieving a lower derating factor. The high-end optical systems mounted on the heads can also be used to capture details of the non-standard type components and save time to a database for future use. A separate camera on the pick-and-place head photographs fiducial marks on the PCB to measure its position on the conveyor belt accurately. Two fiducial marks, measured in two dimensions each, usually placed diagonally, let the PCB’s orientation and thermal expansion be measured and compensated for as well. 

5. Reflow Soldering

Reflow soldering is the most widely used soldering technology used for PCB assembly. Reflow soldering a process in which a solder paste is used to temporarily attach the electronic components required in the BOM to their PADs, after which the entire assembly is subjected to controlled heat. The solder paste reflows in a molten state in an infrared reflow oven, creating permanent solder joints.

The reflow process itself consists of a number of individual processes. These are required to ensure that the PCB board is brought up to the correct temperature for reflow soldering without applying any unacceptable levels of thermal shock. Correctly profiling the temperature of the reflow tunnel or chamber ensures that the resulting solder joints are of the highest quality. The four stages normally used as follows:

  • Preheat: The PCB boards need to be brought steadily up to the required temperature. If the rate is too high, then the board or the components attached may be damaged by the thermal stress. Additionally, if the printed circuit board is brought up to temperature too quickly then areas may not reach the required temperature because of the thermal mass. If the board is brought you to temperature too slowly then the PCB may not reach the required temperature. The temperature rise rate is often for infrared soldering is between 2 and 3 oC per second, although rise rates down to 1oC per second may be used on some occasions.

  • Thermal Soak: Having brought the PCB board up to temperature it next enters what is often termed a thermal soak area. Here the card is maintained at temperature for two reasons. One is to ensure that any areas that are not adequately heated because of shadowing effects come up to the required temperature. The other is to remove the solder paste solvents or volatiles and to activate the solder paste.

  • Reflow: The reflow area is the area of the soldering process where the highest temperature is reached. It is here that the solder paste is caused to melt and create the required solder joints. The actual reflow process involves the solder paste reducing the surface tension at the junction of the metals to accomplish metallurgical bonding, allowing the individual solder powder spheres to combine and melt.

  • Cooling: The cooling for the PCB boards with components after reflowing must be accomplished in a way that does not cause stress to the components. Proper cooling inhibits excess intermetallic formation or thermal shock to the components. Typical temperatures in the cooling zone range from 30 – 100oC (86- 212 oF). The temperature in this zone creates a relatively fast cooling rate and this is chosen to create a fine grain structure in the solder paste to provide the most mechanically sound joint.

Reflow soldering is the most widely used form of soldering for PCB assembly both in mass production and for prototype PCB assembly. Reflow soldering enables is reliable and effective for all pitches of leads and pads, enabling very high-quality circuit boards to be manufactured and assembled.

5. First Article Inspection

Improper setups cause a great deal of rework due to incorrectly loaded, reversed, or skewed parts after SMT reflowing process. In electronic manufacturing, the first article is built in PCB assembly house and sent to the inspection department to check the PCB assembled against its BOM, ensuring that the programs contained in the pick-and-place machine are correct, and that parts are placed into the feeders in the correct locations.

Customer documentation would include, but is not limited to BOM, loading diagram with designators included, PCB files and/or Gerber files used to generate a pick-and-place program and solder paste stencil. The PCB assembly production engineer of a PCB assembly manufacturer would input the BOM into the system, and place additional documents on file for use within the manufacturing area. He would ensure that a stencil is customized and would convert the BOM and PCB files into the CAD files required for SMT assembly loaders. The program that designates which feeder part is required, as well as its location of the PCB (X/Y/Z), is constructed.

If changes have been made to the parts on the PCBs that are not reflected in the CAD data, then the BOM must show current part numbers associated with the particular reference designators. Full feeders are then loaded onto pick-and-place machines in designated, pre-established slots.

Once this is complete, the first PCB (first article) is screen-printed and populated. Prior to reflow, operators inspect the PCB. This minor inspection is limited to ICs and polarized parts, and checks for correct values and parts direction for expensive or difficult-to-rework electronic components. The PCB is then reflowed. This first article is then handled to the inspection. To perform a full first article inspection, quality assurance (QA) is required to search the PCB using the BOM with reference to the loading diagram. This occurs because most PCBs do not have reference designators marked on them. Each part is inspected to ensure correct setup of the SMT machine. The QA must cross off the parts one-by-one until all parts are found and inspected. The pick-and-place machine program also is crossed off to ensure that parts not contained in the BOM are not loaded.

At MADPCB, the automated first article inspection system aids in the accurate setup of a pick-and-place machines. It can reduce 2-3 hours to perform a manual complete SMT first article inspection of all parts on a PCB containing 300 – 400 SMD parts, speed up the production, and eliminate errors in PCB assembly manufacturing. PCB first article inspection during SMT assembly will be long to allow the pick-and-place machines to lay idle, but it’s necessary to ensure the high-quality PCBA output in volume assembly production.

6. AOI

AOI, is the abbreviation of Automated or Automatic Optical Inspection, is an automated visual inspection of PCB assembly where a high definition (HD) camera autonomously scans the Device Under Test (DUT) for surface mounting quality defects. AOI is commonly used in the SMT assembly process because it is a non-contact test method, and it is implemented at two main stages through the PCB assembly process in pre-reflow and post-reflow. SMT Inspection with AOI can detect following defects.

Component offset

Component polarity

Component presence or absence

Component Skew

Excessive Solder Joints

Flipped component

Height Defects

Insufficient Paste around Leads

Insufficient Solder Joints

Lifted Leads

Solder Paste Registration

Severely Damaged Components

Tombstoning

Volume Defects

Wrong Part

Solder Bridging

Presence of Foreign Material

7. X-ray Inspection

AOI can only detect the surface quality defects, while AXI (Automated X-ray Inspection) can detect the component connection defects under the parts soldered onto the PCBs. AXI is a technology based on the same principles as AOI, but it uses X-ray as its source, instead of visible light, to automatically inspect features, including opens, shorts, insufficient solder, excessive solder, missing parts and mis-alignment components, which are hidden from view. In SMT assembly manufacturing, the connections are underneath the chip package (like BGA, QFN…), there is a greater need to ensure that the manufacturing process is able to solder these chips correctly.

8. Wave Soldering

Wave soldering is a method of PCB assembly that involves conveying boards through a “wave” of molten solder. The solder bonds together exposed components PADs and leads while wicking off of the body of components and the solder masked bulk of the board. Wave soldering has fallen somewhat of favor in the PCB assembly industry in recent years, at least in comparison to its past prevalence. The growing popularity of surface mount devices and high-density PCB layouts make reflow soldering the method of choice for a majority of projects, and manual soldering can be used to take care of the typically few components not suitable for reflow. That being said, wave soldering certainly still has its place. For legacy PCBs requiring high proportions of Thru-hole assembly, as well as PCBs incorporating large connectors with very high pin counts, wave soldering is often still the most efficient method for PCB assembly.

At MADPCB, we recommend that PCB designers design for reflow soldering as often as possible since wave soldering does require more strict controls in design than reflow soldering. Our process engineering team performs a through analysis of the PCB design before any wave soldering job, and will bring up any potential complications with you in advance, but this does require additional time and cost compared to a reflow soldering project.

9. Hand Soldering

Hand or Manual Soldering is generally used when a PCB design incorporates some parts that are not suitable for either reflow or wave soldering. For example, a majority surface mount PCB might include a few through-hole components, and wave soldering these few parts would unnecessarily drive up the cost of production. The skilled soldering technicians can take care of any such requirements, who provide consistent and reliable workmanship with strict soldering standards.

While soldering technicians’ manual soldering are incredibly proficient at their tasks, they are still subject to certain human limitations that do not apply to automated assembly methods. The most common manual assembly restrictions are listed below:

  • BGA, QFN and other lead-less packages cannot be manually soldered with soldering iron

  • Part-to-Part Spacing and Part-to-hole Spacing requirements must be carefully observed, and an additional margin of 0.13mm (5mil) to 0.25mm (10mil) is recommended around any manual assembly parts.

  • Pad and Hole Size requirements must be strictly followed, and an additional margin of 10% is strongly recommended for the SMT Pad-to-lead ratio of manual assembly parts.

10. Final Inspection

As noted above, inspections are carried out after every individual stage of the PCB assembly process to better localize the source of any error. We view these intermediary inspections necessary but not sufficient to fully guarantee the highest in quality of workmanship, as is our mission, and as such we always perform a thorough final inspection on the fully finished PCB assemblies at the end of the process.

These final inspections always include visual inspection by our highly experienced quality assurance team, and AOI for complex or high-volume PCB assembly projects. We can also provide additional service such as Functional Circuit Testing (FCT) upon your request according to the testing procedures. The more involved testing method FCT will requires some additional lead time and labor cost upon the specific requirements for a given PCBA project, but can relieve the need for involved product testing on your end.