Tin ball formation principle
Formation of tin balls is made through a standard SMT manufacturing process.
It can be separated in the following steps:
- First, the bare PCB is received
- Solder paste is applied to all tin balls using a stencil
- The contact plate then goes through a reflow oven, to turn the paste into a tin ball
- The flux residing in the paste is then washed from the tin balls
Solder paste application
The solder paste application process, for tin balls of up to 3mm diameter, is a standard SMT solder paste application.
A stencil is first placed on the PCB. After that, a squeegee spreads solder paste inside the stencil holes. Finally, the stencil is removed from the PCB.
This process allows for very precise amounts of solder paste to be deposited.
The design has been well-tried using SAC305 paste, both in laboratory & real-life use.
Such paste is RoHS & REACH compliant, meaning that it can be used everywhere in the world.
Because the mechanical properties of solder paste changes with its composition, it is advised not to change the paste type.
It is advised to mix the paste before use to ensure that the tin-to-flux proportions are correct.
The target height of tin balls is 0.6mm. To achieve this, footprints in this document are designed for a 120µm stencil thickness, with a solder paste containing 88.5% of metal content.
Depending on the presence of other components in the board, such as thermistors, or the manufacturing process, such parameters may need to be adjusted. In this case, the following calculator is provided:
Reflow
Once the paste has been applied, the board goes through a reflow oven, to convert the paste to a tin ball.
The reflow step can be visualised hereunder:
It is important that the oven is well calibrated for the paste used and, depending on the oven technology, PCB thermal mass & color. Note that all this is standard in SMT manufacturing.
Cleaning
The PCB reflow process tends to leave flux residues, which may lead to degraded performance of the tin balls, or bad/non reliable electrical contact. It is thus necessary to clean the solder flux, even for non-clean solder pastes.
This is a standard but optional process in SMTA manufacturing. Typically, this is done using a flux dissolvant product.
For ideal results, it is recommended that PCBs are cleaned soon after the reflow process.
PCB manufacturing
The contact plates PCBs can be panelized to have the left and right contact plates on the same panel.
In this case, it is recommended to use mouse-bites to detach the boards easily once the panel has been reflowed. V-cuts can also be employed, if the boards geometry allows it.
During manufacturing, the contact plates are assembled and reflowed before cutting off the mouse-bites to separate the two boards:
Critical PCB parameters:
Field | Choice | Remark |
Material | FR-4 | Any TG should work well. Gouach commonly uses TG 130-140. |
Thickness | 1mm | Critical parameter, as it impacts PCB rigidity. |
Layers | 2 | |
Surface finish | ENIG / HASL | Gouach typically uses ENIG, as the contact plate incorporate a fine pitch connector. HASL should not cause issues and is lower cost. |
Outer Copper Weight | - 1oz can be used for low-power application.
- 2oz often has the best cost-performance tradeoff.
- >3oz could be used if power capability is not high enough with 2oz, but costs more as it is less common. | Impacts the thermal performance. |
Critical Stencil parameters:
Field | Choice | Remark |
Stencil Side | Top | Tin balls & components usually are only present on top side of contact plates |
Stencil thickness | 120µm | If using other stencil thickness, tin ball paste volume must be re-calculated |
Stencil Remark | “Please do NOT close the holes, please produce stencil exactly according to files with etch though hole, no modification.” | Important to avoid production issues |