Additional Information
LOCTITE ABLESTIK ABP 8068TB can be used for Screen printing. But what is the process and what should you be careful of?
What stencil material should we use?
We have found that it is easier to achieve good results with a metal stencil (50um thickness seems ideal), compared with screen printing with a mesh (which tends to give a lower thickness and a rougher surface to the adhesive). Laser cut stainless is OK for a quick feasibility study, but I would recommend electroformed Nickel for good quality , high volume manufacturing.
Bleeding under the stencil?
It might be caused by too hard stencil unable to make a perfect fit with the substrate, due to the surface morphology. Using a mesh screen with an emulsion backing would probably solve this, but bring other processing issues.
Are there dimples?
Those can be caused as the stencil separates from the substrate, pulling up the edges of the printed adhesive. This can be improved by polishing the stencil apertures. Lower viscosity ABP 8068TA may be also worth testing in parallel if you observe dimple effects (although 8068TA also has a high thixotropic index of 6 for dispensing purposes)
Recommendations for aperture reduction and post cure cooling rate?
Aperture reduction needs to be established by DOE trials, as it depends on component size, stencil thickness and customers inspection requirements. Regarding cooling rate, we do not believe this is critical, but thermal shock should be avoided. Normal practice is to allow parts to cool fairly slowly in the oven until below about 70C (when they are safe to handle).
What are the optimal Printing parameters?
Those will depend on the design of the print pattern and the equipment used, but the following will be a good starting point:
- Print speed: 30 mm/s
- Force: 12 kg
- Print gap: The Print Gap should be set to Zero mm (stencil is in contact with substrate during print). And the Separation Speed (the rate at which the palette drops away from the stencil) should be set to 2 or 3 mm/sec. The Separation distance should be at least 2mm.
- Squeegee material: Stainless is good. Rubber will deform into stencil opening & reduce the glue thickness. Rubber is good with a mesh screen, but not usually suitable for metal stencil.
- Squeegee angle: Not really critical. Use what is available – typically 45 or 60 degrees.
- Die placement force: This depends on the component size. Suggest to start with parameters for conventional silver filled die attach paste. Aim for BLT = 25 um after cure (and a minimum of 10um).
Cure Schedule
For die size
- 20 minutes ramp from 25°C to 130°C, hold for 30 to 60 minutes; 15 minutes ramp to 200°C, hold for 120 minutes in N2 or air oven
For die size >5 x 5 mm
- 20 minutes ramp from 25°C to 130°C, hold for 120 minutes; 15 minutes ramp to 200°C, hold for 120 minutes in N2 or air oven
Alternate Cure Schedule
Suitable for Ag, Au and PPF substrates
- 20 minutes ramp from 25°C to 130°C, hold for 30 minutes; 10 minutes ramp to 175°C, hold for 60 minutes in N2 or air oven