How EMS (Electronic Manufacturing Services) fabricate a PCB (Printed Circuit Board) Assembly?
This article is writing for the people who still not yet really under what is the PCB Assembly. Because WorkingBear found there are many people just come to this EMS area and still not yet well understand what is SMT (Surface Mount Technology) and what is Wave Soldering.
The PCB Assembly is a very popular technology in modem industry. For example, the cell phone, Tablet, Computer, Television, Remote toy car, etc. all have the PCB Assembly module inside. The invention of PCB Assembly technology is really help the electric products development trend to small size and multi-function.
So, how EMS fabricate a PCB Assembly? It is almost automatic production line now through SMT or wave soldering process to solder the electronic components on the board to instead of hand soldering by iron. Of course, you still can hand solder all electronic components on the board as manual by yourself but you must take high risk for the poor solderability quality and can’t do it mass production. Besides, the 0201 small chip are hard to be hand soldered on board and some of the package that design the soldering point under the package body can’t be hand soldered. For example BGA, QFN, LGA package.
This article will introduce the SMT (Surface Mount Technology) process first. Later on WorkingBear will introduce the Wave soldering process. Below is the SMT process flow:
Step. 1. Bare Board Loading
Loading the bare board into the SMT production line is the first step to fabricate the PCB assembly. Now almost of the SMT line has the automatic bare board loading system. Operator only need to remove the packing from the bare board and put them overlap together into the auto-loading system then machine will push the bare board into the product line one by one.
Step 2. Solder paste printing
Once the PCB go into the SMT production line then the first thing will be print the solder paste on the board. I must say it is very similar that girl do the face makeup.
The solder paste will be printed on the soldering pad of PCB through stencil. Later on the electronic components will be placed upon the solder paste. The solder paste will be melted and solders the electronic components on board through high temperature reflow oven. Of course, the peak temperature of reflow oven must be little higher than the melting pint of solder paste.
Step 3. SPI, Solder Paste Inspector (option)
Since the solder paste volume and printing location are really critical and relates to solderability, so many of the SMT production line will setup a Solder paste inspector stage behind the solder paste printing to check printing quality by laser inspection to measure its thickness and optical inspection to printing location and size.
It will do repair to remove the solder paste volume for the over size of solder paste printing if screen out by the SPI machine. It will need to be clear the solder paste and re-print the solder paste if the volume is insufficient or location, size can’t meet requirement.
Step 4. Pick and Place Speed machine
There are two kinds of place & place machine for SMT. One is high speed and one is low speed. High speed machine will pick small chip, for example resister, capacitor, inductor…etc., and place them on the PC board first. The placement speed just like machine gun and you may also hear the da da da voice from the machine.
No need to worry the component will be moved out while the board was moving fast. This is what the solder make as paste purpose to stick these small components in their position. The big components can’t do this way since heavy body has bigger centrifugal force. Fast moving board will make big component can’t keep in its position.
Step 5. Pick and Place General machine (low speed)
The lower speed pick & place machine also call “General Machine”. This kind of machine will pick the bigger component, for example BGA, IC, connector…etc., and place them on the PC board. One reason is their heavy body can’t move the board fast. The another reason is the bigger package always have multi-leads need to be soldered on the board and need more accurate position to prevent any solder short happening.
You may find almost the pick & place machine use the vacuum nozzle to pick up the component. We call the component can be picked & placed by SMT machine and survival after reflow high temperature as SMD (Surface Mount Device). Below is the requirements to meet SMD:
- A flat surface on the top of package. If the top surface has any gap or hole there then the nozzle will had to use vacuum to pick up the component. You may also find some component has a tape stick on it or use a cap on its top to make a flat top surface.
- Tape & Reel packing. This is for the SMD material auto loading purpose to reduce the frequency material loading. A hard tray packing is 2nd choice since some SMD hard to design the tape & reel packing. This will require another extra SMT machine since general pick & place machine can’t feed by tray.
Step 6. Hand place component and visual inspection
There usually setup a checkpoint before sent the board go into the reflow oven once every component are placed by the SMT machine. Because reflow process will solder each components on the board. This stage is in order to screen out any placement not good or missed components then correct the error to prevent using iron to touch-up the board.
So you may think about why not setup a AOI (Auto Optical Inspector) here to instead of human checkpoint. It is possible to setup a AOI here, especially for some cell phone board that design a shielding frame and must be soldered on board.
Sometimes you will find there still have some components need to be placed by manual. Maybe components no tape & reel packing or may be bulk packing components. By the way even there is AOI here, still need a operator to deal with board that screen out by the AOI, right?
The production cost will be final concern include the quality and repair cost.
Step 7. Reflow
PCB and component go reflow oven is in order to melt the solder paste and become liquid then wet component solder leads and PCB pad. The solder will grow IMC (intermetallic compound) between component and PCB as solid joint.
The reflow temperature profile is critical. It will directly affect to soldering quality if the profile setup is not suitable. We usually make 4 zones design for the reflow temperature setup as pre-heating zone, soak zone, reflow zone, and cooling zone. If we choice the regular lead-free solder paste of SAC305 as example then its melt point is 217°C and reflow peak temperature shall be higher than this melt point but no recommend it over 250°C. Higher peak temperature or put parts inside reflow too much time will damage the PCB and components. You may find PCB warpage issue after reflow is kind of PCB can’t sustain so high temperature or so much thermal energy.
迴流焊的溫度曲線 Reflow Profile
Step 8. AOI, Auto Optical Inspection (Option)
Actually, AOI are not the must for SMT line to inspect PCB Assembly soldering quality. Some low cost of SMT still use human to do visual inspection 100% (Step 10). One reason to setup AOI after reflow is in order to find out SMT placement or solder paste printing quality and feedback the issue immediately to correct system error in time. The another reason is in order to replace headcount and reduce human error.
Since AOI use the optical image picture to compare good and poor PCBA, so AOI still can’t do 100% coverage to instead of ICT or FVT. AOI only can screen out below defect:
- Missed parts
- Component shift
- Wrong polarity
- Solder bridge
- Solder empty
AOI can’t detect below defect:
- BGA or LGA solderability. (The solder is under body can’t be checked by image)
- Component value. (Resister, Capacitor, Inductor, Diode)
Step 9. Unloading PCBA
Once the board had finish the reflow soldering process then the PCBA will be automatic unloaded and put into the Magazine. Left drawing show the actual Magazine structure. There are many slots inside the Magazine to save the PCBA.
There is one thing need to be taken care is the space between board to board. Because of there already had components soldered on the board , we must make sure the components will not hit each others and bring damage while put the PCBA into Magazine.
Step 10. Visual Inspection
No mother there is AOI setup behind reflow process. Regular SMT will setup a operator to do the visual inspection for the PCB Assembly finished board. The operator here will do double check the board that screen out by the AOI machine or do 100% visual inspection on the board by inspection template.
Step 11. Touch-up
Even the SMD parts are so popular but sometimes we only can find DIP parts. For example the coin battery still can’t go reflow high temperature. Sometimes RD maybe have cost concern then select DIP parts to instead of SMD parts. The DIP parts always use lower cost material and can’t sustain reflow high temperature. So TOUCH-UP usually be defined as hand solder component on PCB or repair the component by iron and solder wire.
Step 12. ICT, In-Circuit Test
The major purpose to do ICT (In-circuit Test) is in order to pre-detect any circuit open/short to prevent directly power on PCB Assembly then damage or burn out the PC board permanently.
The ICT also can measure the value of resister, capacitor, inductor, diode and triode. This will help to find out any electronic components be damaged after high temperature reflow, wrong components or components missed.
There is another lower level of ICT machine call as MDA, Manufacturing Defect Analyzer, can measure the value of mentioned passive components and check circuit open and short function.
The high level of ICT not only can test every items what MDA can do but also can do lower level diag test as board level function simulation test. If the coverage can do 100% test then next step of board level function test can be eliminated to save the labor hour, fixture cost. But the high level of ICT machine and fixture is more expensive than MDA. The cost of high level ICT test fixture is about 15~25 times of MDA fixture. So it better to phase in this high level ICT after mass production with volume.
Step 13. Board level Function Test (FVT)
The board level functional test, also call FVT (Functional Verification Tester) , is in order to cover the items that ICT can’t test it. The almost all ICT/MDA only test the circuit open/short and measure the value of resister, capacitor, inductor, diode and triode. Most of the ICT/MDA didn’t do the well function test for BGA and IC package components.
Step 14. Assembly board de-panel
Almost of the PCB will do panelization to put two single boards as one panel (call 2 in 1) or 4 single boards as one panel. The benefit to do panelization will get better production line efficiency and increase the hourly output.
After all PCB assembly process has been done then we need to de-panel it as single board and remove unnecessary break-away, otherwise the panel board can’t be installed into the final product box.
There are two proposal methods to do de-panel as single board:
- Scoring machine to cut off the V-cut.
- Routing machine to cut off the ribs junction between boards and break-away.
No prefer directly bend the V-cut by hand to to de-panel. The bending stress will damage the component or its soldering.