Tips on design for manufacturing and BGA reballing.

A customer requested the development and documentation of a robust manufacturing process for a high-reliability PCB assembly. We also conducted a design for manufacturability review of the assembly, noting any issues affecting its producibility.

The goal was to solder both ceramic and metallized BGA packages simultaneously to this substrate at high yield and good reliability of the final assembly. The project addressed 10 different processes used to manufacture and test the circuit assemblies. The three following processes are addressed:

• Bare board fabrication.

• Component assembly.

• BGA reballing.

The customer provided a test vehicle of the prototype product. The 12-layer bare board was a high-temperature substrate material that included blind and buried vias.

The bare board design was compared to IPC design guidelines for high-reliability design. Many of these design rules can be found in the IPC-2220 documents. Several suggestions were provided for improving manufacturability and reliability. ACI insisted that the line spacing should not reduce the manufacturability of the substrate. As a rule, the most economical PWB fabrication features minimum lines and spaces of 0.005", usually available for no price premium over wider line boards. (Lines and spaces down to 0.0015" usually cost more.) The line spacing to adjacent copper features should not be less than 0.005" to guarantee suitable insulation resistance between conductors and other features. Long parallel traces are difficult to produce and subject to crosstalk, and should be avoided.

ACI also made suggestions about the copper used for internal and external traces. Copper weight and density should be balanced on all layers (assuming 0.5 oz. copper cladding of the starting laminate) to avoid excessive warp and twist of the finished multilayer board post-lamination. Heavier copper, usually used for higher current carrying capacity, reduces etch accuracy because copper etching is isotropic. This narrows the imaged lines as it etches through the thickness of the foil cladding. Also, copper planes should not be exposed on PWB edges when routed from the substrate.

For high reliability surface-mount assemblies, land patterns must conform to IPC-SM-782 design rules for Class 3 solder fillets. BGA land patterns should be equal in size. For wave-soldered plated-through hole components, general requirements for PTHs and annular rings must conform to IPC-2221 and IPC-2222, Class 3.

Assembly Manufacturability

A small R&D build of the prototype was performed to gather information regarding manufacturability. A small set of test boards were printed, placed, reflowed, inspected and reworked using automated equipment. Notes obtained about every aspect of the assembly process resulted in a number of recommendations.

As test is an important part of manufacturing, the customer was advised to consider in-circuit test by designing in nodes and test points for access where possible. Also, silkscreen part outlines, polarity, pin 1 indicators and reference designators (where possible) can improve assembly test, inspection and repair.

Proper design for reflow calls for placing heavy components on the primary side of the board so they will not drop off when the secondary side is soldered. Remove vias from under BGA components to be underfilled so that the underfill material will not run down the vias. Location and size of tooling holes should be appropriate for the equipment requirements.

Fiducial marks should not be obscured by equipment rails (placing them a minimum of 0.100" from edges is adequate in most cases). Add local fiducial marks to BGA components to improve placement accuracy.

Soldermask-defined lands for BGA components to be underfilled are recommended, to keep the underfill from encapsulating the pad edges. Provide keep-out distance of a 0.100" around BGAs for rework and underfilling.

For handling, all components should be placed a minimum of 0.050" from board edges. Land patterns must be isolated from adjacent features and lines by soldermask over bare copper. Exposed copper should not be closer than 0.050" to the features to be soldered, to prevent inadvertent shorting.

If possible, all active components should be placed on the same side of the assembly; avoid placing BGA components directly behind each other on opposite sides.

BGA reballing. This process was developed by the EMPF using customer-supplied ceramic components. Fixturing occurs by applying a 0.004" deposit of solder paste to the ceramic component prior to application of high temperature balls. Once the balls are affixed to the component via solder paste, the component is subjected to reflow, where the paste melts and permanently attaches the balls to the component.

The BGAs supplied had incorrect solder alloy ball contacts. It was necessary to reball them before assembly. First, water-soluble solder paste was applied to the component substrate using the designated "paste on part" fixture (Figure 1). With the component still on the fixture, the fixture and component were installed on the rework station. Using the vacuum pickup nozzle of the rework station, the pasted component was slowly separated from the fixture. The solder preform with the correct alloy was inserted into the reballing frame and the frame was accurately positioned on the rework station platform under the pasted component.

The component and solder preform were then reflowed together using the specified reflow profile. After reflow, the paper backing was removed from the reflowed component and visually inspected for missing balls. Finally, the component was cleaned using DI water. We recommend baking components at 125°C for 24 hrs. prior to use.

By following these basic DfM principles and instituting the reballing procedure developed at the EMPF, the customer was able to achieve a higher yield and higher volume manufacturing.

 

The American Competitiveness Institute (aciusa.org) is a scientific research corporation dedicated to the advancement of electronics manufacturing processes and materials for the Department of Defense and industry. This column appears monthly.