There are many reasons why the board manufacturer will choose to fill Via holes (the plated through holes which interconnect the multiple layers of the PWB). Generally, the fill material will be either conductive or insulating and may additionally help moisture resistance and add to the board’s thermal conductive properties.

An increasingly important strategy in supporting fine pitch devices and heavily populated boards is the via-in-pad technique. Here, the vias are part of the pads on which the fine pitch device i.e. BGA is mounted.

Challenges:

• The aspect ratio of board thickness to hole diameter can be very challenging
• To date we have achieved excellent results with aspect ratios up to 8:1
• BGA type packages with a pitch of 0.5 mm (0.020”) are common
• Fine pitch components present today's PWB designers with challenges to increase routing channels, reduce layer count and try to stay within conventional lamination standards

ProFlow® Process: ProFlow is the recommended method of material transfer since it is capable of filling the holes most effectively for 100% fill without voids and with minimal surface residue. Special considerations relating to via fill include the need for relatively slow speed, multiple strokes and higher pressures due to the large aspect ratio of the via depth to its diameter. Since the aim is to push material through the board, provision must be made to prevent spreading of the material on the board underside. In the example above this is achieved using blotting paper on the tooling.

Higher throughput – more than five times higher than squeegees
Enhanced process control - leading to greater repeatability
Reduced material waste
Reduced defects - less voiding and insufficients
Less operator intervention
Eliminates need for vacuum tooling

Tooling: A tooling block manufactured to the standard tolerance is required. If squeegees are used there may be a requirement for vacuum tooling. Provision must be made for the extrusion of material on the underside of the board. This can be absorbed using a sheet of blotting paper underneath the board and on top of the tooling. Alternatively, use standoffs or milled out areas between the board and tooling.

Material Selection: Many materials can be used to fill Vias including insulating polymers and resins, conductive metals (gold, silver etc) and thick film materials. The material should be suitable for screen printing i.e exhibit appropriate viscosity and rheology. Contact DEK’s applications department to discuss material compatibility.

Stencil Design Rules: The stencil should be a maximum 100 microns (0.004”) thick. Tests suggest that laser cut stainless steel is the preferred technique of manufacture. Aperture dimensions are nominally on a 1:1 ratio with the Via hole. However, this dimension may increase or decrease depending on the exact process.

Parameter Settings:

Speed – keep low to maximise fill potential
Squeegee / Piston Pressure – high to push material through board
Print Strokes - two strokes for ProFlow®
Multiple strokes (five+) for squeegees

Under Screen Cleaner: Standard blue cleaner optimised as normal. A dry or wet/dry stroke will normally suffice.

Software: From Version 7 Service Pack 03 (for ELA / Horizon iX / Infinity) the print speed and paste pressure can be individually ramped during the print stroke, resulting in improved hole fill. A fully optimised stencil will aid the improvements seen with this software.

The following is an example of a typical Site Implementation Plan:

Foundation
Present the product
Feasibility study
Benchmark current process
Set goals and objectives
Establish project team
Milestone review

Preparation
Critical success factors
Lab test
Define induction and education plan
Milestone review

Implementation
Functionality
Robustness
Pre-production
Milestone review

Introduction to high volume
Roll out
Site-wide induction and education
Sign off

Audit
Three months
Six months
One year

Contact DEK for more information or to discuss your requirement

Press Release:

ProFlow® Via Fill plugs the gap


See also:

ProFlow® DirEKt Imaging

Enclosed Head Printing