|Ask the Experts|
April 23, 2021
Solder Paste Thickness Measurements
On average how many locations should we measure on our PCBs to confirm our solder paste thickness? Our PCBs are approximately 10" x 10".
|Expert Panel Responses|
I never used locations but I learned that the best way to measure solder paste thickness is to measure the deposition on your fine-pitch IC's and/or micro-bga components. If the solder thickness is ok on those then you may conclude that is ok throughout the assembly.
Always verify your results before carrying-on. Pick a sampling pattern across your assembly. The number of locations will vary from PCB to PCB.
Senior Manufacturing Engineer
Ideally every paste deposit should be measured regardless of the size of the size of the board.
If there is some sort of constraint that prevents that, at an absolute minimum the locations on the board that are especially challenging to print should be measured.
Technical Support Engineer
Since your PCBs are approximately 10" x 10", I would measure three on each side, then I would check the middle of the board, which equates at least 10 measurement per side.
Vice President, Technical Director
The most accurate way to answer this question is to first look at the overall complexity of the board to determine the potential areas of most concern. When using a manual bench top SPI system, it is best to create a sample measurement routine that focuses on the 4 corners and center areas of the PCB to determine overall print support and then incorporate the finer pitch devices as time permits. Run this same routine over your first handful of boards and after all adjustments have been set to create a stable process, monitor the print every 10-20 boards to maintain traceability.
If you are using an automated offline SPI system, then I would recommend creating a script that allows for 100% measurement of the first few boards in the production run. This will provide you with the most accurate indication as to where the "Hot Spots" are being found on the PCB. Through proper corrective actions, these areas can be stabilized and a sample measurement routine can be implemented as a way of monitoring the print process over the entire production run. If there are other critical devices that require extra attention, selective 100% inspection of those devices can be included into the routine as the much higher speed of inspection allows for the increased coverage.
For inline SPI, a 100% inspection routine is what the system best serves. Similar to the automated offline solution, areas of concern can quickly and easily be recognized. Implementing proper corrective actions will bring the process into control and monitoring can be accomplished by measuring every board in the production run or at a specific board intervals to satisfy both your QA and overall traceability requirements.
Bottom line - I applaud you for being proactive in using SPI to help improve the quality of your boards . Whether you decide on a very limited sample based inspection routine or a full blown 100% solution, SPI at any of these levels will introduce valuable feedback that will assist you in creating the best recipe to create the best board possible.
National Business Manager
In general, 30 measurements typically gives good statistical data for most parameters. Most in-line solder paste inspection systems can measure every solder paste deposit within seconds. If your SPI system is an in-line model, I would recommend measuring as many spots as you can.
It's a conundrum, to be sure. In an ideal world, we want to be certain that the volume of paste on every land is within tolerance (3D 100% inspection). We often don't have that luxury. If we aren't doing 100% volumetric inspection, then what gives us the most bang for the buck?
I'd suggest that using thickness measurement primarily as a set-up tool, to ensure that the board supports, squeegee pressure, etc. are what they should be, is the most important thing. I'd make measurements near both rails and near the center, using moderate size pads (neither very small nor very large). On the set-up piece, I'd also consider making measurements of the largest apertures to ensure that excessive scooping is not present. After set-up is confirmed, I'd measure the a sample of the production boards near both rails and the center; the sampling rate required would be dependent on your process.
Even this strategy will be a bit time consuming, and it's relatively easy to point out scenarios that it will not catch. A skilled operator can probably tell as much or more about print quality by a visual check under a ring magnifier, but again, there are scenarios that visual inspection will almost always miss. An example would be partial release from small BGA apertures, which may result in volumes below 30% of nominal, but which can appear visually good. Only a good 3D volumetric scan is going to catch that.
A big chunk of print-related problems can be avoided by good process design, including ensuring that you are not pushing the envelope with area ratios, ensuring that you have good, even board support, and ensuring that you purchase a quality stencil and keep it clean and in good condition, and use good squeegee blades that are monitored for wear and replaced as needed.
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