1. Concerning the unique requirements of HotBar, this issue has consistently troubled PCB companies. Specifically, RD mandates the use of a three-layer FPC to facilitate HotBar, where the FPC can only have pads on one side. This means that the heat from the hot press head cannot directly contact the solder pads of the FPC and PCB for effective heat transfer and solder melting. Instead, the thermal energy from the hot head must traverse the three-layer FPC to reach the solder surface.
2. In fact, while this HotBar process is challenging, it is not unfeasible. The primary concern is the risk of burning the FPC if subjected to excessive heat or prolonged exposure, which could lead to reliability issues in subsequent quality assessments.
3. After careful consideration, alongside the traditional use of the HotBar machine to complete the process, we devised two alternative methods to meet this requirement:
4. **1. Utilize low-temperature solder paste and employ the HotBar machine for FPC welding.**
The downside to this approach is that general low-temperature solder paste typically exhibits lower reliability, making it prone to brittleness and unable to endure significant pulling forces. Consequently, this method is not advisable for printing low-temperature solder paste on the PCB side. If the PCB only accommodates HotBar connections and a few small resistors or capacitors, direct printing of low-temperature solder paste may be feasible. Otherwise, it is advisable to apply the low-temperature solder paste on the FPC using a reflow oven before proceeding with the HotBar process.
2. The FPC is directly soldered using SMT through a furnace.
One drawback is that the FPC may require manual adjustments, necessitating the creation of a furnace fixture to secure and press the FPC. While I haven’t personally executed this process, it appears viable, as I’ve observed products made this way by others.
Additionally, some have proposed whether ACF could replace the HotBar process. In reality, ACF is primarily utilized in the COG process. Although many LCM FPCs currently employ ACF as the solder medium, its bonding strength is insufficient. Under an ACF area of 10mmx3mm, the peel resistance in the X-direction is approximately 500 grams, while in the Y-direction, it’s around 200 grams. It can be easily detached with a simple pull, so additional protective materials are often necessary to enhance its peel strength. Currently, it’s common to see silicone used as a cover for COG and FPC. Furthermore, ACF has two significant drawbacks. First, its reliability is low; over time, it tends to peel off, especially in high temperature and humidity environments. Second, the storage conditions for ACF raw materials are critical; they can undergo degradation in such conditions, leading to poor adhesion.
Low temperature solder paste
To address this, we opted for Indium 5.7LT 58Bi/42Sn (bismuth tin) low temperature solder paste, which has a melting point of only 138°C, with a recommended peak temperature of 175°C. After completing the HotBar hot pressing, the tested peel force was 1.5 Kgf, which is lower than I anticipated. Additionally, we are promoting LED components that also use low-temperature solder paste, yielding a thrust of 4.0 Kgf.
Overall, this result is barely acceptable. If a better PB process isn’t identified, this PCB process condition will be prioritized.
