Best Practice Guide Battery bonding
There is an increasing need in the industry for a reliable method for securely connecting lithium battery cells instead of spot welding. Due to the different geometrical shapes of the cells and the additionally required battery management system (BMS), a high degree of flexibility in the cell connection is required. High quality requirements of the manufacturers complete the requirement profile. Through our product portfolio and know-how in bonding and testing, F&S BONDTEC offers everything that is necessary for a qualitative and durable wire connection between the cells. In order to achieve the highest connection quality, we identified several parameters which must be taken in consideration when producing a battery pack. With our range of bond testers, quality standards can be checked at the same time.
Preparation
- Mounting
Depending on the shape of the battery pack or the arrangement of the cells, various mechanical fastening forms can be chosen. All that is important for the bonding process, is the firm attachment of the cells during the bonding process. 3D printing (see figure 1) is often used in prototype construction, although other holders are possible as well. For series production often milled cases or plastic injection molding are used.
When it comes to wire bonding, two different connections are currently used when connecting cylindrical cells. Both methods have different characteristics.
| Connection from cell to cell | Connection from cell to busbar |
|---|---|
| With wire or ribbon | With wire or ribbon |
| Limitation of the current capacity of the wire or ribbon | Current limited by wire size, multiple wires possible |
| Currents up to 20 A per wire (see wire data sheets for exact specification) | Busbar made suitable for currents |
| Connections also possible at the cell end for negative pole | Connections also possible at the cell end for negative pole |
| Higher currents possible through multiple wires |
Table 1: Comparison of wire connections from cell to cell and from cell to busbar
- Quality standards
In cooperation with our partner BOND IQ GmbH, we identified the following quality standards:
- DVS-2811
- MIL-883
- AEC-Q100/006
- ASTM
- JEDEC
- IEC
- IPC
As stated in the DVS-2811 [DVS-2811 p. 2-15] it is highly recommended to use automated bond testing with automated angle correction of the wire bonds, directly on the fully bonded battery packs. F&S BONDTEC will publish a fully Best-Practice-Guide on Testing battery packs in the future soon.
- Cleanness
It has been found out, that the battery contacts can be contaminated even when new. Cleaning with acetone is not practical in an industrial setting. In various tests, we have tested different methods that give good results on bondability. The best results we got are the following two methods of cleaning:
- CO2 Cleaning
CO2 cleaning is an inexpensive method for cleaning surfaces. The intensity can be selected so that the surface is cleaned but not attacked. Compared to laser cleaning, it is easier and cheaper to implement.
| Advantages | Disadvantages |
|---|---|
| + Easy to bond | - Lärm |
| + No change in the surface | - Static charge |
Table 2: Advantages and disadvantages of CO2 cleaning
- Laser Cleaning
Before the bonding, the surface is scanned with a laser. In order to find a reasonable power range, the laser power is gradually reduced, starting with 50%. The laser beam could also be slightly defocused, which leads to an effective enlargement of the area hit per pulse and thus a lowering of the energy density.
| Advantages | Disadvantages |
|---|---|
| + Easy to bond | - Stress cracks in the surface |
| + Cleaning can be carried out selectively |
Table 3: Advantages and disadvantages of Laser cleaning
BONDING
An accurate preparation of the cells is essential for a quality bond result. However, the actual bond process needs to be configured correctly too.
- Bond wire
The main wire suppliers all recommend less ductile wire. You don’t want to use a too soft bond wire with respect to the harder electrolytic nickel surface of the cell.
- Bond parameters
Since a battery pad behaves fundamentally different from a printed circuit board, the bonding parameters need to be adjusted. Battery pads behave differently from nickel-plated steel surface in both, the bonding and the BAMFIT test. The generator power and the US power had to be increased significantly. Example:
-
- 18650-Cells
- Bond wire: 375 µm H14
- Bondtime: 410
- US Power: 190
- Force: 550
- Ramp Time: 250
- Ramp End Force: 1200
- Burst Time: 50
- Burst US: 133
- TD Force: 550
- BAMFIT
- Loops
After the first bond is made, the bonding tool travels a defined pattern to form a loop of the disered height and length. The flexibility in shape of the loop is an additional advantage, especially when the battery housing is designed accordingly you can have benefits in durability of the battery pack. Looped wires are also flexible in many directions and can therefore handle mismatches among different thermal expansion coefficients better than other joining technologies.
- Pattern Recognition
A modern Pattern Recognition Unit is required to have the machine placed on the top of the bead of the battery. This allows to accurately identify the bonding area, which is required to get a good result (see figure 9).
- Batterie Management System
Another important component is the Battery Management System (BMS). There are various ways to position the BMS on a battery pack (E.g. it can be placed directly on the cells). Using wire bonding as form of connection makes balancing the cells way more flexible because you have more options to connect the BMS with the cells.
Machines
The wire bonders from F&S BONDTEC are flexible in their applications and offer the highest quality. Thanks to years of experience and proven technologies, the bonders of F&S BONDTEC are always able to meet the high demands of its customers. Depending on the size of the battery pack, you need a machine that covers the necessary travel distances for the battery back. In figure 11 you see the different travel distances of our machines:
Conclusion
Currently there are several methods of interconnecting cylindrical cells together to a battery pack. Spot welding, laser welding and wire bonding are the most common interconnection methods in the market. However, we believe ultrasonic wire bonding is the most favorable technology due to its flexibility and high connection quality. Process support: We support you in the development and implementation of your individual process requirements to connect your batteries. Find the right wire bonder for your battery connections with us! Call us at +43 (0) 7722 67052 8270 or send us your request under info@fsbondtec.at.
