Challenges of the pin-in-paste printing stage
Embedding THT in the pin-in-paste process can save a lot of time, but getting enough solder to fill the hole can be very difficult.
The first question to consider when planning the pin-in-paste process is how well the hole is to be filled. If the goal is to achieve 100 percent filler hole filler volume, the printing of the solder paste must provide enough solder paste to fill the hole minus the lead volume of the component. A very important aspect is the fact that the metal balls themselves constitute only 50% of the paste volume and the remaining 50% of the volume is flux, so to obtain 100% filling, twice as much paste should be provided. Moreover, if the goal is to achieve full fill in one printing cycle, the aperture must be completely contaminant-free so that they do not 'take up' valuable volume. Next, consider whether there is enough space on the PCB surface to print a large amount of paste around the holes.
It is possible to fill the holes with solder paste by performing multiple printing cycles. Ideally, this would be done with a template designed exclusively for printing paste over the holes. To determine how many cycles should be used to fill the holes so that the paste starts to drain from the bottom of the holes, it will always be a trial and error process.
To maximize paste transfer, Tony Lentz, Application Engineer at FCT Assembly recommends the following process parameters:
- Instead of the standard 60 ° tilt, the doctor blade should be angled at 45 °.
- The printing speed should be reduced to 10-20mm / s to maximize the injection time of the paste into the holes.
- To optimize the release of the paste from the sidewalls of the aperture, a ceramic-type nano-coated template would be used.
- Type 4 or 4.5 solder paste is best suited for the described operations.
Similarly, Fritz Byle, a process engineer at Astronautics, begins his statement on the difficulty of the pin-in-paste process, noting that in terms of volume, the paste contains only about 50% of metal, even if it is even 90% by weight. This means that even if the entire hole is filled with paste, the reflow process will result in a much smaller volume of solder than the planned 100% fill.
This problem can be partially avoided by printing a larger area on the upper surface of the PCB, but the author emphasizes that the possibilities of such an action are limited. In addition, the ability to fully fill the hole depends on the hole diameter. Holes below 0.040 "in diameter are very difficult to fill. Or, for holes larger than 0.070", there is so much paste that it can flow out of the hole. Finally, the shape and cross-section of the pins themselves have a big influence.
Fritz, I propose to take the following approach:
- In the first step, calculate the volume of the hole minus the volume of the leads
- Then, the resulting volume should be multiplied by two, obtaining the required volume of paste for a 100% solder fill.
- Then, by repeated trials, estimate the percentage of the hole that can be filled with paste in one cycle.
- Finally, calculate the fill volume and determine if additional paste volume is needed, and if so check if it can be printed on the surface and let it drain into the hole.
Achieving the fill required to meet IPC Class 3 standards can be very difficult (classes 1 and 2 tend to be much easier in this regard). If your application is in class 3, you may need to consider developing evidence to prove that you do not actually need a 75% filler. For example, using a 93-mile-thick PCB it usually doesn't take as much as 75 percent fill for strength or reliability, but as always, it all depends on the specific component and application.
Not all THT components are suitable for the pin-in-paste process
Always remember that the peak temperature of a standard lead-free reflow process can be as high as 250 ° C, so THT components must be able to withstand this temperature. If this is not the case, you can also use a low melting point solder paste, lowering the top of the profile to 180-200 ° C. However, it should be considered very individually, because not every application allows for soldering at lower temperatures.
In addition, it is wise not to introduce too much heat into the system during assembly. Due to excessive heating, the viscosity of the solder paste will decrease quickly, so there is a risk that some of the solder paste will drip into the oven. The result is irregular solder volume and fouling of the reflow oven.
Besides, it is important that the PIP component has a certain distance (i.e. the distance between the PCB and the bottom of the component body). This distance must ensure that there is sufficient space between the PCB and the component for the application of the response
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