What Does PCBA Too Hot Mean? Top Reasons

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PCB heating

What does pcba too hot mean? Voltage, current, electrical resistance, thermal conductivity, convection, temperature, PCB current distribution, PCB temperature distribution, electrical resistance calculation, thermal conductivity calculation

An example prepared

Voltage is applied to the sides of a conductive plate that is vertical and surrounded by air on the outside. The passing current heats the plate due to resistive losses. The front and back sides of the plate are cooled by air under natural convection conditions.

Problem type: Plane-parallel multiphasic problem of DC electric field and stationary heat transfer .

Geometry :

10 A

Given

Sheet thickness d = 0.035 mm;
Material resistance ρ = 2·10 -8 Ohm·m;
Current I = 10 A;
Thermal conductivity coefficient λ = 380 W/K m;
Convection coefficient α = 10 W/K m²;

Task

Calculate the distribution of temperature and potential in a conductive sheet.

What does pcba too hot mean? Solution

Resistive losses are calculated in the DC problem. These losses are then transferred to the associated heat transfer problem.
The plate is cooled by convection. The heat flow is equal to:
F ( T ) = -α·( T - T 0 ),
where α is the convection coefficient,
T 0 is the external temperature (assumed to be 0).

Assuming that the plate is thin, and its temperature does not change along the thickness. Convection from the front and back side can be modeled by volumetric heat sink:
Q ( T ) = k · T , where k = 2α/ d .
PCB

Results:

Current distribution in the PCB

Current distribution in the PCB

Temperature distribution in the printed circuit board (temperature rise)
Temperature distribution in the printed circuit board

The task pcb_current.pbm refers to the calculation of the current distribution, and pcb_heat.pbm to the analysis of the temperature field.

Through high temperature solder curing, SMT PCB and SMT components are connected together to form an electrical circuit.

 Reflow

1. Soldering principle

After the solder paste is applied to the printed circuit board, after the parts are assembled, the solder paste is heated and melted, and after cooling, the printed circuit board and the parts are connected into one. To achieve the specified mechanical properties and electrical characteristics.

2. Three soldering

Welding consumables - PCB parts - PCB parts

Welding environment-----welding material: solder paste

A certain temperature ----- heating equipment

Craft Zone

Main process:

In the process of hot air reflow soldering, the solder paste must go through the following stages: the solvent evaporates, the flux removes oxides on the surface of the solder, the solder paste melts, reflows, and the solder paste cools and solidifies. 

1. preheating zone

focus: Preheat slope, preheat temperature

Purpose: Preheat the PCB and components to reach equilibrium, and remove the water and solvent from the solder paste at the same time to prevent the solder paste from breaking down and solder spattering. Make sure the temperature rises slowly and the solvent evaporates. 

Mild, thermal shock to components is as small as possible, too high a temperature will damage components such as multilayer ceramic capacitors, crack, and also cause solder spatter, so that solder is formed in the entire non-soldering area of ​​the PCB. Solder the joint with insufficient solder balls and balls.

Function and Features ﹕ Used for heating circuit boards and parts: the slope is 1-3 per second, which is about 30% of the total time, and the maximum temperature is controlled below 140 to reduce thermal shock.

2. SOAK constant temperature area

Focus: Time for temperature equalization, temperature for temperature equalization

Purpose: Ensure that the solder can be completely dried to reach the reflow temperature, and it also plays the role of a flux activator that can remove metal oxides from components, gaskets and solder powder. The time is about 60 ~ 120 seconds (depending on the nature of the solder and the PCB).

Function and Features ﹕ ensure uniform heating of parts and PCB and eliminate local temperature difference, remove the electrode from parts and PCB surface oxide, PAD and solder through the solvent in the solder paste component, reduce surface tension, and prepare for redissolution. The time is about 45% and the temperature is between 140-183.

3. REFLOW reflow zone

Focus: Maximum reflow temperature, reflow time

Purpose: The solder in the solder paste causes the gold powder to melt and flow again, replacing the liquid flux to wet pads and components. This wetting action causes the solder to expand further, and the wetting time for most solders is between 60 and 90 seconds. 

The temperature of reflow soldering is higher than the melting point temperature of solder paste, and generally it should exceed the melting point by 20-40 degrees to ensure the quality of reflow soldering. Sometimes this zone is also divided into two zones, namely the melting zone and the reflow zone.

Full function and characteristics This is a full-scale thermal remelting, the temperature will reach the peak temperature, the peak temperature is usually adjusted between 205-230, the peak temperature is too high will cause the PCB deformation, parts cracking and secondary reflow phenomena.

4. COOLING of the cooling zone

Focus: Cooling slope

Purpose: The solder solidifies when the temperature is lowered, so that the components make good electrical contact with the solder paste, and the cooling rate is equal to the preheating rate. Slow cooling will cause more PAD decomposition products to enter the tin, resulting in gray and rough solder joints, and even lead to poor tin immersion and poor adhesion to the solder joint.

Function and Features ﹕ Cooling to evenly cool the PCB and parts, there are cooling air blower motors at the top and bottom of the reflow oven, and the PCB temperature is generally controlled below 120 (75). The cooling rate is usually within -4 / sec. SESC standard: Tilt ›-3 / sec.

General welding defects and countermeasures

1. Short circuit between solder balls and solder balls

Cause countermeasure

Too much solder paste (1mg/mm). Use a thinner steel plate (150 µm) to reduce the hole (85%).

The printing is inaccurate and the steel sheet is slightly adjusted

The solder paste is destroyed, the reflow profile curve is corrected, and the reflow profile curve is corrected.

The squeegee pressure is too high. Reduce squeegee pressure.

The gap between the steel plate and the circuit board is too large. Use a thin solder mask.

Incorrectly designed pads, same lines and spacing, same lines and spacing

2. Air welding SMD parts with feet

Cause countermeasure

Uneven leg parts or solder balls Check the flatness of the leg parts or solder balls

The amount of solder paste is too small. Increase the thickness of the steel plate and use a smaller hole to increase the thickness of the steel plate.

Lamp effect: solder paste is baked first after solder paste is applied

Do not eat tin in parts and legs. The parts must meet the food tin requirements.

3. Airless welding of SMD parts without legs

Cause countermeasure

Incorrect designs of solder pads Solder pads are separated to prevent solder film, and the size is suitable

Uneven heating at both ends of the same parts according to the size of the gasket should be the same

Too little solder paste. Increase the amount of solder paste.

Parts have a poor ability to use tin. Parts must meet the requirements for the use of tin

4. SMD parts float (drift)

Cause countermeasure

Uneven heating at both ends of parts, separation of tin pads

One part has poor tin properties. Use parts with the best properties of tin.

Reflow Method: Preheat to 170 before reflow

5. Tombstone effect

Gravity of the part lowers the part

Molten tin under the part will also destroy the part

Molten tin on the outside of the part on the tin pad will make the part up

Cause Countermeasures

Incorrectly designed soldering boards optimal soldering board design

The two parts have different properties of tin, and both parts have the best properties of tin.

Uneven heating of the two ends of the part, slowing down the heating rate of the temperature curve

Temperature curve heats up too fast, preheated to 170 before reflow

6. Cold solder joints

Cause countermeasure

Reflow temperature too low Minimum reflow temperature 215

Reflow time too short, solder paste at least 10 seconds above melt temperature

Trouble of a pin in a tin meal Checking the behavior of a pin in a tin

The problem of eating tin is to inspect the pad for tin consumption

7. Granular solder joints

Cause countermeasure

Reflow temperature too low High reflow temperature (215)

Reflow time is too short. Longer reflow time (> 183 or more for at least 10 seconds)

Solder Paste Pollution New and fresh solder paste

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