Clive AshmoreAdding just enough material for a set amount of prints can ensure good outcomes.

Although no-clean solder pastes are the most prevalent materials used in electronics assembly today, water-soluble pastes are still in the game. In market sectors like aerospace, military, automotive and industrial, water-soluble materials are frequently the specified-in, legacy product – often because of the reliability requirements to remove flux residues. Printing water-soluble solder pastes, however, is quite a different process than printing no-clean materials. Assembly specialists take note!

Back in the day, no-clean pastes were the more fickle materials, with delicate operating windows and strict storage requirements. Over the years, massive amounts of development and a focus on maximizing process efficiency (i.e., eliminating an unnecessary cleaning step) put no-clean in the processability fast lane, while water-soluble material R&D got lapped. Although new water-soluble pastes have been released in recent years, they are still generally more difficult to print than no-clean pastes, and the finesse required to successfully print them isn’t always well understood. Put simply, the primary challenge with water-soluble pastes is they are hydroscopic (absorb water) in their function, making them a bit sponge-like.

Humid environments wreak havoc on water-soluble paste’s stability on the printer and in the reflow oven, as the material’s chemistry changes. Installing temperature and humidity controllers on the printer is the obvious answer to managing these challenges, though that’s a bit like using a sledgehammer to crack a nut; it’s a lot of investment. Manufacturers already working regularly with water-soluble materials are certainly aware of the water absorption characteristic, and their factory environments are adjusted to cope. However, the EMS shop that just got the contract with water-soluble paste specified for the assembly may not be as up-to-speed, and some simple process tweaks can make a big difference for anyone engaged in assembling boards using water-soluble pastes.

Adjusting the amount of material put on the screen prior to a production run is probably the most effective way to manage printing with water-soluble pastes. It’s certainly not uncommon for an operator to overload the stencil with solder paste. With no-clean, this is usually no problem. With water-soluble materials in an uncontrolled environment, it’s not ideal. The better technique in this situation is to add material more often. Begin with less solder paste to start and add in fresh material throughout the shift – roughly every 40 to 50 prints. With this approach, the paste is more stable, and you won’t have an entire shift’s worth of material taking on water and adding variation to the process. In addition, because water-soluble pastes are a lot stickier than no-clean materials, aperture filling can be more challenging. To encourage better filling, I suggest trying a 45° squeegee angle instead of a standard 60° angle. This will push material into the apertures more efficiently and completely to permit greater process bandwidth.

While adding material more often is easily managed manually in low- to medium-volume operations, doing so in higher-volume manufacturing isn’t practical. The principle, however, still applies. Maintaining material stability on the stencil by adding just enough for a set amount of prints is the best way to ensure good outcomes with water-soluble pastes. So, for high-volume manufacturers, an automatic paste management system should be considered. Automatic paste management generally consists of two components: a paste deposition tool and a roll height monitor. The paste roll height monitor uses laser technology to sense paste presence and roll height. When the paste roll falls below a certain preset threshold, it alerts for either manual or automatic paste replenishment, ensuring a constant supply of fresh paste on the stencil for maximum productivity. Combining the paste roll sensing function with automatic paste dispensing solves the high-volume challenge for water-soluble paste printing, allowing operators to focus on other tasks and improving material stability for a more robust, high-yield process.
In a nutshell, following these rules for printing with water-soluble pastes will raise the odds of a good process outcome considerably:

  • Less is more. Avoid putting on a shift’s worth of paste; instead, add material more frequently.
  • Change your angle. Modify the squeegee blade angle from the conventional 60° to 45° for better aperture filling.
  • Let it go. Employ automatic paste management technology to ensure proper amounts of paste on the squeegee throughout the entire shift.

Water-soluble materials are here to stay, and by applying these techniques to your process, you can take on printing these pastes and achieve excellent results.

Clive Ashmore is global applied process engineering manager at ASM Assembly Systems, Printing Solutions Division (asmpt.com); clive.ashmore@asmpt.com. His column appears bimonthly.

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