Lead-Free Selective Solder Guidelines

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Lead-Free Selective Solder Guidelines

A study is conducted with a thick, thermally challenging test vehicle wave soldered using a wide range of selective pallet opening sizes.
Production Floor

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Authored By:

Ramon Mendez, Ismael Marin
Celestica Monterrey, Mexico

Helen Lowe
Celestica Toronto, Canada

Transcript

As the use of lead-free alloys has increased in electronic assemblies, much work has been done to develop Design for Manufacturability guidelines for the new materials.

However, there are still some challenges remaining with wave solder, which is a complex process with many interacting factors. One such challenge is achieving good pin-through-hole barrel fill on thicker PCBs, particularly for power and ground pins connected to multiple plane layers.

One important factor in the selective wave solder process is the size of the selective pallet opening around the PTH pins. It has been observed that larger pallet openings generally provide better barrel fill than smaller ones, but further research is needed to determine the recommended pallet opening for more thermally challenging product designs.

The recommended pallet opening can then be used to determine Design for Manufacturability guidelines for the component keep out from the PTH pins on the solder side of the board.

This paper presents the outcome of a study done with a thick, thermally challenging test vehicle wave soldered using a wide range of selective pallet opening sizes.

In the first part of the investigation, a Design of Experiment was performed to optimize the wave solder process parameters and in the second phase, the optimized process parameters were held constant to focus on varying the pallet opening size only.

The results for the various pallet opening sizes and their interaction with the other design factors is discussed.

Summary

As the use of lead-free alloys has increased in electronic assemblies, much work has been done to develop Design for Manufacturability (DFM) guidelines for the new materials. However, there are still some challenges remaining with wave solder, which is a complex process with many interacting factors. One such challenge is achieving good Pin Through Hole (PTH) barrel fill on thicker PCBs, particularly for power/ground pins connected to multiple plane layers. One important factor in the selective wave solder process is the size of the selective pallet opening around the PTH pins. It has been observed that larger pallet openings generally provide better barrel fill than smaller ones, but further research is needed to determine the recommended pallet opening for more thermally challenging product designs. The recommended pallet opening can then be used to determine DFM guidelines for the component keep out from the PTH pins on the solder side of the board.

This paper presents the outcome of a study done with a thick, thermally challenging test vehicle wave soldered using a wide range of selective pallet opening sizes. The test vehicle is 3.05mm (0.120") thick with twenty copper layers, including ten plane layers, and is populated with several PTH component types. Other design variables include pin to hole clearance, and quantity of plane layers connected to each pin. The PCBs were assembled with a Pb-free alloy (Sn-Ag-Cu) and also SnPb as a baseline. In the first part of the investigation, a Design of Experiment was performed to optimize the wave solder process parameters and in the second phase, the optimized process parameters were held constant to focus on varying the pallet opening size only. The results for the various pallet opening sizes and their interaction with the other design factors will be discussed.

Conclusions

The results from this study confirmed the conclusions from prior work that Pb-free selective wave soldering is much less robust than with SnPb solder, and more sensitive to variations in the product design. Much greater attention must be paid todesign features, such as the component orientation, quantity of plane connections, PTH hole sizes, and SMT keepouts around the PTH pins, in order to achieve acceptable yields on thick, thermally challenging PCBs.

Key findings related to the selective pallet opening for Pb-free wave solder on a thick, complex board:

In general, increasing the selective pallet opening around the PTH pins is very beneficial with respect to defect reduction.

The degree of benefit provided by increasing pallet openings is highly dependent on other conditions present, and was seen to vary based on the component type, pin to hole ratio, component orientation, and the quantity of planes connected.

A selective pallet opening of 7.62 mm (0.300") can produce acceptable assembly results for most components, providing DFM guidelines are being followed with respect to other key factors, e.g. the quantity of plane connections, pin to hole ratio, and use of thermal reliefs. Electrolytic capacitors may require somewhat larger pallet openings to achieve acceptable results.

If the DFM guidelines for other important factors like plane connections and pin to hole ratio are not followed, simply increasing the pallet opening may not be sufficient to offset the negative effects and achieve acceptable assembly results. This is particularly true for more thermally challenging components like electrolytic capacitors. In order for an increase in pallet opening to be effective in these cases, other changes must also be implemented, for example reducing the quantity of plane connections.

Some interesting observations in addition to the selective pallet opening results:

Under at least some conditions, heat transfer from a leading component to a trailing neighbor component can be very beneficial to the trailing neighbor. Additional experiments would be needed to better understand this phenomenon and how it might be advantageously applied in production board designs.

For the PCI connector, the optimal results with larger pallet openings were attained for holes in the middle of the range studied. The reasons for this were not immediately evident, and further study is needed to better understand why larger hole sizes appear to produce more defects than the mid-range ones under this set of conditions.

Initially Published in the IPC Proceedings