What environmental conditions need to be strictly adhered to in order to use laser solder paste?

The process application of laser solder paste needs to be based on environmental control as the cornerstone, through the construction of "temperature - humidity - cleanliness" trinity of precision management system, combined with dynamic parameter adjustment and process adaptation, in order to achieve the quality of welding from "qualified" to "excellent" leap. "Excellence". The following is an in-depth analysis of the four dimensions of technical principles, implementation points, risk avoidance, case evidence:

I. Temperature control: precise management of the whole chain from "storage - rewarming - use".

1. Storage temperature: "double insurance" of refrigeration and sealing.

Unopened solder paste: need to be refrigerated in 0~10℃ special refrigerator, avoid direct sunlight and high temperature environment. If the temperature has been completed but not used, need to be refrigerated again to prevent flux activity decay (every 10 ℃, the activity of the rate of decline accelerated by 2 times).

Opened solder paste: the unused portion should be sealed and stored (vacuum-sealed jars are recommended), and follow the "24-hour use principle". Beyond this period, the flux volatilisation will cause the viscosity of the paste to rise by 15%~20%, leading to problems such as printing burrs and insufficient tin.

2. Use of environmental temperature: 20 ~ 26 ℃ "golden zone".

High temperature (> 28 ℃): flux evaporation accelerated, the paste viscosity increased by more than 20%, printing prone to "pulling" "bridging" defects. For example, in the 0402 size components (pin spacing 0.4mm) in the printing, the temperature rises 1 ℃, the bridging rate increased by 0.5%.

Too low a temperature (<18 ℃): paste thickening, fluidity decreased by 40%, resulting in printing pattern edge blurring, uneven thickness. Experimental data show that in the 15 ℃ environment, 0.12mm line width printing deviation up to ± 0.03mm, far beyond the allowable range (± 0.01mm).

Temperature fluctuation control: need to avoid fluctuations of ± 2 ℃ or more, otherwise the internal stress changes in the solder paste will trigger "performance drift". It is recommended to use a constant temperature workshop with PID control (such as Panasonic environmental control system), the fluctuation range is compressed to ± 0.5 ℃.

3. Re-warming requirements: 2~5 hours of "patient waiting".

After taking out of cold storage: need to be left at room temperature (20~26℃) for 2~5 hours, so that the temperature difference between the solder paste and room temperature is ≤2℃. If used directly, the condensation of water vapour will lead to the phenomenon of "fried tin" - when soldering, water vapour evaporates to form solder beads with a diameter of 0.05~0.2mm, which increases the risk of short-circuit by 3 times.

Temperature verification: Use an infrared temperature gun (e.g. Fluke 62 MAX+) to check the surface temperature of the solder paste to ensure that there is no local temperature difference (e.g. a temperature difference between the centre and edge of the can of >1℃ requires an extended temperature return time).

II. humidity control: 40% ~ 60% RH "dynamic equilibrium"

1. Humidity risk: high humidity and low humidity "double trap"

High humidity (>60% RH): solder paste absorbs moisture, the soldering of water vaporization to form air holes (diameter of 0.02 ~ 0.1mm), resulting in a decrease in the shear strength of the solder joints by 20% ~ 30%. For example, in the soldering of NASA's Mars Rover sensor module, the porosity rate soars from 0.5% to 3% at humidity >60% RH, directly threatening reliability in extreme environments from -120°C to 150°C.

Low humidity (<40% RH): solvent evaporation is accelerated, solder paste viscosity rises 10% to 15%, and printing requires increased squeegee pressure (leading to pattern distortion) or shorter printing cycles (reducing productivity).

Humidity fluctuation control: need to avoid ± 10% RH above the drastic changes, otherwise the surface of the solder paste will form "micro-cracks", triggering the subsequent expansion of solder cracks. 2.

2. Extreme humidity response: dehumidification and humidification of the "precision control"

High humidity environment: use rotary dehumidifier (e.g. Munters DF300) to stabilize the humidity at 50%~60% RH. If the humidity is >80% RH, production should be suspended and emergency dehumidification should be activated (the humidity will be reduced to less than 60% RH within 1 hour).

Low humidity environment: Supplement humidity by ultrasonic humidifier (e.g. Honeywell HUL535W), or shorten solder paste exposure time (e.g. from 1 hour to 30 minutes). For example, in the soldering of battery modules for new energy vehicles, the use of "solder paste pre-storage + fast soldering" mode in a low humidity environment resulted in a 12% increase in yield.

III. environmental cleanliness: dust-free and pollution-free "micro-defense".

1. Workshop cleanliness requirements: ISO Class 5 "nanoscale guarding".

Air cleanliness: ISO Class 5 (Class 100) standards, that is, the number of particles in the air ≥ 0.5μm ≤ 3520 / m³. If particles are contaminated, particles with a diameter of >3μm will directly lead to open solder joints, while submicron particles of 0.1~1μm will be embedded in the interface of the solder joints, forming Kirkendall cavities (which triggers early failure in thermal cycling tests).

Corrosive gas control: Chlorine, sulfide and other gases need to be <0.1ppm, otherwise it will react with the metal components in the solder paste to generate corrosion products (such as Cu2S, AgCl), resulting in the resistance of the solder joints increased by more than 50%.

2. Personnel protection and operational standards: from the "source" to block pollution

Protective equipment: operators need to wear dust-free clothing (such as DuPont Tyvek 1422A), headgear, masks and gloves (such as Ansell Microtouch Nitrile), to prevent sweat (containing Cl-, Na ⁺) contamination of the circuit board surface. Experiments have shown that solder joint corrosion rates are accelerated by a factor of 3 when gloves are not worn.

Solder paste exposure time control: solder paste after printing or dispensing should be soldered as soon as possible (ideal time: 30 minutes ~ 1 hour). Beyond this time, flux volatilization will lead to a 15% decrease in wettability, thickening of the oxide layer (from 0.01μm to 0.03μm), the need to increase the laser power (about 5%) to compensate for this, but it will expand the heat-affected zone (radius increased by 0.02mm).

IV. welding parameters and process matching: from "universal" to "customized" upgrade

1. Welding temperature and time: the material characteristics of the "precise fit"

Optimal soldering temperature: 200 ~ 250 ℃, to ensure that the solder paste fully wet the soldering surface (contact angle <20 °), the formation of uniform solder layer. For example, SnAgCu alloy has the fastest wetting at 230°C (within 0.5 seconds), while SnBi alloy needs 210°C (due to the lower melting point of the Bi element).

Welding time adjustment: thin materials (such as 0.1mm thick PCB) need to be welded for a short period of time (0.2 ~ 0.5 seconds), to avoid thermal deformation; thick materials (such as 2mm thick heat sink) need to be extended to 1 ~ 2 seconds, to ensure that the depth of penetration ≥ 0.5mm.

2. equipment and process matching: laser parameters of the "millimeter-level control".

Heat-affected zone control: laser welding needs to precisely control the energy input (e.g., Tesla 4680 battery module welding, the use of pulsed laser (pulse width of 500μs, frequency of 1kHz), the heat-affected zone radius compressed to 0.1mm, to avoid damage to the surrounding polyimide film (260 ℃).

Alloy formulation adaptation: different alloys (such as SnAgCu, SnBi, SnSb10) need to match the corresponding process parameters. For example, SnBi alloys require a 20% reduction in laser power (due to the low melting point) and a 30% reduction in welding time (to prevent segregation of Bi elements).

V. Typical Cases and Data Verification: From "Theory" to "Practice" Verification

1. NASA Mars Rover Case: Reliability Breakthrough in Extreme Environment

Environment control: Humidity <60% RH, temperature 22±1℃, cleanliness ISO Class 5, to ensure stable operation of solder joints in extreme environments from -120℃ to 150℃.

Performance data: Shear strength of solder joints up to 35MPa (40% higher than traditional solder joints), porosity <0.5%, resistance change <2% after thermal cycle test (1000 times, -55℃~125℃).

2. An enterprise humidity exceeds the standard case: from 75% to 98% of the yield leap

Root cause of the problem: Workshop humidity >60% RH led to solder joint porosity rose to 3%, yield fell to 75%.

Improvement Measures: Deploy rotor dehumidifier to stabilize humidity at 50%~60% RH; optimize solder paste exposure time (shorten from 1 hour to 30 minutes).

Effectiveness Verification: After 3 months, the porosity rate was reduced to below 0.5%, the yield rate was increased to 98%, and the annual savings in rework costs exceeded 2 million RMB.

Summarize: the laser solder paste process "quality formula"

Laser solder paste solder quality can be quantitatively expressed as:

Q = f(T, H, C, P, M)

Where:

T: Temperature control (storage/use/return)

H: Humidity control (20%~60% RH dynamic balance)

C: Cleanliness control (ISO Class 5 + personnel protection)

P: Welding parameters (temperature/time/energy)

M: Process matching (equipment / alloy / process)

In actual production, it is necessary to continuously optimize the conditions through environmental monitoring equipment (such as temperature and humidity meters, particle counters) and process validation (such as trial welding tests, X-ray inspection), to build a closed-loop quality system of "prevention-monitoring-improvement", and ultimately to achieve high-precision (linewidth deviation of ±0.01mm), high-efficiency (UPH>5000), high-reliability (MMI), and high reliability (MLR>1000). ) and high reliability (MTBF>100000 hours).