Dedicated Saw Singulation Offers CSP Assembly Advantages

With CSPs moving into volume production at an increasingly rapid rate, saw singulation is emerging as an efficient method of separating densely populated packages.

Chip-Scale Packaging technology development has been driven by the demand for smaller footprints in complex consumer electronic products. The reported 100+ types of CSP designs have typically attempted to utilize some combination of silicon wafer, PC board or other substrate with existing packaging equipment.

For the singulation process, this has yielded designs which use slicing/dicing saws, PC board routers or existing punches. The use of non-specific singulating equipment with certain CSP designs may not offer the production capability needed to make the singulation process a cost-effective one.

CSP designs are chosen to put more silicon in a smaller package, and often require a higher cost interconnect substrate material, such as FR-4, BT, polyimide, etc. To offset the increased raw material costs, these substrates must be used in a highly efficient manner.

Additionally, to produce packages under cost-effective conditions, the assembly format must be in the largest possible substrate format, such as "strip level," "wafer level" or "panel level."

Singulation Overview

Densification, by definition, places chips in close proximity and minimizes wasted space and costly materials. To singulate these densely populated packages with minimal clearance requires the use of precision sawing or slicing/dicing equipment. MTI has developed the NSX-250 singulation system, shown in Figure 1, as well as several other machines suitable for use in singulating CSPs.

In general, saw singulation provides the following advantages:

• Highest chip density/lowest material loss production with common blade widths from under 0.100 to over 0.650 mm
• Maximum flexibility
  - Can also perform most routing and punch singulation duties
  - Can accommodate widest parts variety
  - Flex, rigid, and leadframe packages; wafer, strip, and panel substrates
  - Polyimide, FR-4, BT, copper, ceramic, glass, composites, etc.
• Low production costs
  - High UPH
  - Quick setup changes
  - Low consumables costs
  - Low operating costs
• Factory/automation integration
  - Pre/post-process integration
  - In-line pick-and-place, JEDEC tray output
  - Vision, O/S testing
  - Clean/dry
  - Reject sort/mark

Figure 1. The MTI NSX-250 dual spindle, CSP strip and wafer-level singulation system offers advanced controls.

Pre-production development efforts with saw singulation primarily used existing silicon dicing equipment. Limited in their capability because they have often been designed for thin, light-cutting silicon, single purpose silicon dicing saws are not usually capable of volume production.

Dedicated Singulation

With a production legacy in the demanding hard-disk drive, read/write head production environment, gang wheel slicing has been proven worldwide for precision, high volume electronics production with materials far more difficult to slice than simple silicon. Fabrication of these ceramic-based heads requires intensive slicing.

The proven technology of gang slicing is directly applicable to BGA and CSP singulation, and offers the flexiblity of a basic silicon saw, an emphasis on production volume and low production costs. Equipment designed specifically for the needs of CSP singulation, based on head production experience, offers a dramatic production capability. For production volumes, multiple "gang-wheel" assemblies are capable of single pass, full substrate cutting. (Figure 2 shows a typical gang hub assembly with reusable, precision ceramic spacers and gang quality diamond blades.)

On a typical rigid, FR-4-based, overmolded CSP strip, the gang wheel CSP singulation system is capable of almost a 2x UPH improvement on a 15 mm x 15 mm part and a 4x UPH improvement over a basic silicon dicer on a 3.5 mm x 5.5 mm part.

In addition to critical throughput power, dedicated CSP singulation equipment offers more advanced controls and integrated vision systems. MTI offers the MTIvision™ NT II system, which aligns and inspects solder balls to customer-trained patterns and/or edges.

The advantages of read/write-head-based CSP singulation machines over silicon dicers include the following:

• Blade Utilization
  The consumables are a major consideration in cost-effective production. This is a key difference between silicon dicers and CSP singulation equipment. Silicon dicers have been designed for the smaller format ~55 mm diameter blades. Silicon dicing equipment blades are typically 0.100 mm width and less, and generate a very light cutting forces, especially in silicon; hence, the design of the lightweight machines. Larger format 115 mm diameter blades are required for efficient CSP loads, single wheel or gang wheel.
• Automation
  Singulation process automation can take many facets and requires the flexibility to meet individual customer requirements. In the development stages of chip-scale packaging, typical silicon saws with tape frame handlers met small lot production needs. The expense of tape, however, with the additional costs of mounting/demounting on tape, secondary cleaning, etc., add significant per unit costs, which are not viable for volume production.
• Systems Approach
  Volume BGA/CSP singulation requires full consideration of all factors: equipment, reliability, up-time, ease of operation, process optimization flexibility, consumables costs, throughput, factory integration, etc. By adding available assembly modules, gang wheel CSP singulation saws become highly automated.

Figure 2. Gang hub assembly with precision ceramic spacers