Before you go and work on your PCB assembly, you have to keep in mind that there are certain guidelines that you must adhere to in order for it to work properly.
Here are those guidelines:
Pad Design and Solder Masking
The PCB Assembly’s Solder Pad should by no means be larger than the solder mask so that optimal strength can be expected. There should be a 1:1 ratio for the pad and the mask.
You will need non-solder masks because it has been determined that these masks have copper-etching capabilities that make the PCB even more effective. They are also able to make the solder pad more reliable and are able to give it a longer life.
Guidelines for Board Mounting
As this process deals with stencil design of the PCB Assembly, it is important that you give utmost care and attention to it. Use only stainless steel stencils that are metal squeegees and with a thickness of 0.10 to 0.20 millimeters. However, if you are going to use polymer squeegees, make sure that you use at least 90 durometer minimum.
Electro-polished and laser-cut stencils are a must. This is because polishing the stencils makes them smoother and therefore, make them easier to attach. It is also recommended that you use tapers with bottoms of 25 to 50 microns so that paste can be easily released. Do not exceed an aspect of 1.5 and a ratio of 0.66 to maintain proper stencil design.
Use type 3 or 4 solder paste in mounting the assembly. It would also be helpful if you use Nitrogen Purge so that gasses would easily reflow around the system and so that there would be less risk of voiding the solder joints. You should also ensure that there is enough time for gasses to escape from the molten solder.
Peak temperature and profile of reflow also have strong influences on void formation. It would be better if you follow the recommendation of paste makers on the packages.
Design and Trace
You have to use the dog-bone style pad with a pitch of 0.8 or 0.5 millimeters so the pitch could easily land on the pad. Take note that the holes need tenting so you have to leave a space with a width of 0.075 millimeters in order for dimensions to be detailed and so that there would be proper PCB breakout.
Guidelines for Rework
Retouch is limited because the solder joints are not fully exposed. If you see any defects, you have to remove the whole package and assemble it again. This might be a challenge as PCB assembly parts are quite small but with the right guidelines in mind, you will be able to do a fine job.
In order to make sure that you get to place those parts the right way, you have to check if the PCB is dry. If it is, you are on the right track. Next, you should work on the solder joints and place it on the PCB. Heat the board from the side convector to easily remove the parts and be able to place them again.
Clean the site so it will be easier for you to work. Use a soldering blade and a conductive tool to do the cleaning. You should now be able to do solder-pasting again by using a miniature stencil. You can also use the mini stencil to print the paste.
Attachment and Placement
Lastly, perform alignment with the help of 50x by 100x magnification so the placement machine would easily understand what you are doing. Use the reflow profile to easily attach the PCB assembly parts and so that the need for thermocouple feedback will be lessened or eliminated. This will also help reduce dependency on operators.
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