Circuit board welding process

Technology principle

BGA welding using the principle of re-flow soldering.Let’s introduce tin ball in the back-flow mechanism in the process of welding.

Circuit board welding preheating:First of all, First of all, used to achieve the required viscosity and printing properties of the solvent evaporated,Temperature rise must slow (about 5 ° C per second),to limit the boiling and splashing,to prevent the formation of small beads.Some components of internal stress is more sensitive.If the external temperature rising too fast,it can lead to fracture.

Active flux (cream),Chemical cleaning action began.Water-soluble flux (cream) and disposable type flux (cream) will occur the same cleaning action.Temperature is a little different.Will some pollution from the combination of metal oxides and metal removed and solder particles.Good metallurgy tin solder joint requirements on the surface of the "clean".
When the temperature continues to rise.Solder particles separate melt in the first place.Liquefaction and surface tin absorption and start the process of "rush".So in all possible covering on the surface, and began to form tin solder joints.

Circuit board welding back-flow:The most important at this stage.When all individual solder particles melt, combine together to form a liquid tin,the surface tension then began to form surface welding feet.If the clearance between the PCB welding plate and pin over 4 mil,Is most likely due to divide the pins and welding plate surface tension, namely tin caused by open circuit.

Circuit board welding cooling:If cooling fast, tin some strength slightly larger.But not too fast otherwise it will cause the temperature stress inside the element.

Circuit board welding temperature area:BGA solder ball, divided into lead-free and has two kinds of lead.Have lead solder ball in 183 ℃ ~ 220 ℃, melting point of lead-free solder ball melting point at 235 ℃ ~ 245 ℃.
No matter how lead welding or lead-free soldering.Tin ball melting stage are in the re-circulation region.Only the temperature is different.Before re-flow curve can be viewed as a slow process of heating and heat preservation.

Circuit board welding preheating zone:Is used to set the temperature of the PCB from ambient temperature to the activity of required temperature.Circuit boards and components of the heat capacity is different.Their actual temperature increase rate is different.Pcb and the temperature of the components shall be no more than 2 ~ 5 ℃ per second speed continuous rise.If too fast, can produce thermal shock.Circuit boards and components may be damaged.Such as ceramic capacitor of tiny cracks.And the slow temperature rise, solder paste will be excessive heat.Inadequate solvent evaporation, affects the welding quality.Furnace preheating zone of 15 to 25% of the entire heating zone length.

Circuit board welding heat preservation area:In this area generally account for heating area 30 ~ 50%,The active area of each element on the main purpose is to make the PCB temperature to stabilize.To minimize the temperature difference.In this area to give enough time to make the heat capacity of the temperature of the components to catch up with smaller components.To the end of the activity area, solder, solder ball and element oxide on the pin be removed, the whole circuit board temperature equilibrium.Attention should be paid to the PCB all components at the end of this area should have the same temperature.Or into the back-flow zone will be because of uneven temperature of each part to produce all kinds of bad welding phenomenon.General common active temperature range is 120 ~ 150 ℃.If active zone temperature setting is too high.Flux (cream) don't have enough time to activity.Increasing temperature curve slope is an upward slope.Although some solder paste manufacturers allow some increase in temperature during activation.But the ideal temperature curve should be a steady temperature.

Circuit board welding back-flow zone:The role of the ward is will increase the temperature of the PCB from the active temperature to the recommended peak temperature.Activity is always lower than the alloy melting point temperature, temperature.And the peak temperature is always on the melting point.Is the scope of a typical peak temperature solder paste alloy melting point temperature of 40 ℃ or so.Re-circulation region scope of working time is 20 to 50 s.The zone temperature setting is too high will make its temperature rising slope over 2 ~ 5 ℃ per second.Or make re-flow peak temperature is higher than recommended.Or work time is too long can lead to excessive crimp PCB, take off a layer or damage, and damage the integrity of the component.Re-flow peak temperature is lower than that of the recommended, work time is too short possible defects such as cold welding.

Circuit board welding cooling zone:This area of the solder paste tin alloy powder is melted, and fully connected surface wetting.Should be cooled quickly as possible.This will help the alloy crystal formation to get the bright spot.And has good appearance and low contact angles.Slow cooling leads to more decomposition of impurities into the tin of circuit board, resulting in a rough welds.In extreme cases, it may cause poor wetting and weaken the solder joint binding force.Cooling section of the cooling rate is commonly 3 ~ 10 ℃ / S.