Generally speaking, the curve can be broadly categorized into (1) RSS type with a saddle, (2) L type without a saddle (RTS type Ramp to Spike), and (3) long saddle type (LSP type Low Long Spike), explained as follows:
(1) Saddle type:
Starting from room temperature, gradually heat the board to the peak of the saddle at a rate of 1-1.5°C/sec. Then, employ a slow rise or constant temperature approach to reach the end of the saddle at 150-170°C within 60 to 90 seconds. This phase’s primary function is to allow the PCB and components to absorb sufficient heat, ensuring internal and external temperatures equalize, preparing for the peak temperature. The peak temperature for this profile is approximately 240±5°C, with TAL (time above the melting point) lasting about 50-80 seconds, and a cooling rate of 3-4°C/sec, totaling around 3-4 minutes.
(2) Saddleless type:
The temperature increases linearly throughout the entire process, with a rate maintained between 0.8-0.9°C/sec, peaking at 240±5°C. The L-shaped curve should ideally be straight or slightly concave, avoiding any bulging to prevent surface blistering on thick boards due to overheating. Additionally, the first 2/3 of the heating phase should not exceed 150°C, with other parameters remaining consistent with those mentioned above.
(3) Long saddle type:
1. When the PCB circuit board requires soldering multiple BGAs, reducing voiding in the solder balls and supplying sufficient heat to the ball’s base is essential. A saddle profile can be gently extended to help expel volatile content from the inner ball solder paste. This process begins with a heating rate of 1.25°C/sec, reaching the first saddle at 120°C, and maintaining that temperature for 120-180 seconds before the peak temperature sharply rises. All other parameters remain consistent with the aforementioned settings.
2. The quality and technique of the mobile profiler are crucial. The number of thermocouples that such thermometers can utilize ranges from 4 to 36, with significant variations in brand and price (NT. 100,000 to 300,000). A high-quality thermometer should record essential data, including the heating rate of the initial section, PCB temperature differential, heat absorption time, peak temperature increase rate, peak temperature readings, and key parameters such as the duration of the molten solder paste (TAL) and cooling rate at the final stage. For optimal performance, the thermometer’s main body and internal battery must withstand heat, its design should be flat enough to avoid furnace obstructions, and the thermal error of the thermocouple wire should not exceed ±1 degree Celsius. Temperature measurement frequency should not exceed once per second, and the memory capacity must be adequate. Additionally, data output should support statistical process control (SPC), with straightforward software upgrades.
3. Generally, in the reflow of larger PCB circuit boards, the leading edge must warm up and cool down faster than the middle or trailing edge. Moreover, the heat absorption and temperature rise at the four corners or edges must occur more quickly and reach higher temperatures than the center. Thus, thermocouple wires should cover these areas. Additionally, larger components absorb heat differently, causing their pins to heat more slowly than smaller passive components. The heat penetration of the bottom area of a large BGA can also be challenging. Therefore, it may be necessary to drill through the PCB and weld temperature sensing wires from the front of the board for accurate readings in those blind spots. Components sensitive to high heat should also have thermocouple wires attached to help define the reflow curve.
4. To prevent damage to heat-sensitive components and thick boards, it’s vital to use a thermometer to identify the “hottest” and “coldest” spots on the assembly board. This can be achieved by testing another board with printed solder paste, first running it through the reflow oven at high speed (e.g., 2m/min) and checking if the double pads of small passive components, like capacitors, are properly soldered. Subsequently, reduce the speed (e.g., 1.5m/min) to identify the first soldering point, indicating the hottest area of the board. Continue reducing speed (increasing heat) until the large component’s soldering is complete, marking the coldest point. By targeting a predetermined spike temperature (e.g., 240°C), the ideal conveyor speed for the assembled board can be established, allowing for TAL measurements of the solder paste. This approach ensures that heat-sensitive components and thick plates remain safe, preventing damage during reflow.
5. In terms of solder paste specifications for various brands like SAC305 or SAC3807, the heat absorption time for the saddle typically spans 60-120 seconds, with a temperature rise from 110-130°C on the front saddle to 165-190°C on the hot saddle. For thick multilayer PCBs, particularly those carrying large components, achieving proper heat absorption across all sections is critical. Rapid heating to peak temperatures must occur without causing thermal stress due to temperature discrepancies between the inner and outer layers. Thus, the profile should include a saddle or hat brim reflow curve.
6. However, for smaller boards or single/double panels with mostly small components, the temperature differential is less significant. To optimize production speed, the heat absorption phase can be accelerated to less than 1°C/sec, eliminating the saddle and resulting in an L-shaped profile resembling a roof. At this point, the quality of the reflow oven is paramount, as efficient and uniform heat transfer across sections is required. The board’s entry and temperature loss must be managed rapidly and accurately to prevent excessive local temperature drops (not exceeding 4°C on any surface area).
7. In general solder paste formulations, 90% by weight consists of powdered (small ball) metal solder, while 10% comprises organic additives. Despite similar primary components (like SAC305), differences in tin particle size and quantity significantly affect fluidity and healing properties. The liquid duration (TAL) of molten solder paste also depends on the type and thickness of the surface treatment on the PCB. Typically, TAL for boards is around 60-100 seconds to minimize intense heat exposure to the soldering board and flux. However, sufficient healing of solder paste and proper soldering or intermetallic compound formation is critical. An overly long TAL can damage components and the board, and even cause flux to carbonize (charring). Conversely, a short TAL may lead to insufficient melting, poor tin absorption, or inadequate solder paste healing, compromising granular appearance. For highly heat-sensitive boards, initiating with the shortest TAL during test welding can help determine the most suitable welding conditions through fine-tuning travel speed without altering the hot air conditions in each section.
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