Precautions for flux and solder paste for the hott

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Notes on flux and solder paste for lead-free welding

make lead-free welding a reality

solder paste, wave soldering liquid flux, flux gel and lead-free flux used in solder wire have become a reality today. These flux combinations can strengthen the welding process, and their formulations can provide excellent wetting properties and improve the chemical and thermal stability required for lead-free assembly

the traditional flux used with tin lead alloy may not be suitable for preventing the slow wetting speed of lead-free alloy and the high temperature usually associated with lead-free solder. The flux matching system specially used for lead-free welding requires the use of new activator kits and gel and wetting agents with thermal stability to avoid solder defects

due to the slow wetting speed and high surface tension of many lead-free alloys, choosing the correct flux suitable for lead-free welding can prevent the increase of solder defects and help maintain production output

the general defects that may increase when switching to lead-free assembly will be described in detail below. These defects can be eliminated by selecting the correct flux and process control

potential defect growth - lead-free surface mounting and assembly

· bridging - poor thermal collapse performance of solder paste

· solder ball - poor thermal collapse characteristics of solder paste

· tombstone effect - thermal difference on circuit board

· non wetting - excessive preheating or insufficient flux activity

· poor wetting performance - poor flux activity or excessive preheating

· solder porosity - thermal characteristic curve is too low, Or the chemical properties of the flux are not enough

· solder beads - poor thermal collapse performance of solder paste or excessive preheating

· potential defect growth - lead-free wave soldering

· bridging - solder passivation during preheating or solder contact

· condensation vertical column - flux activity is too low or preheating temperature is too high

· solder balls - preheating is not enough or flux solder mask is incompatible

· incomplete pore filling - flux activity is too low, solid content is too low, Or the preheating temperature is too high or the contact time with molten solder is too short

requirements for lead-free flux:

· low activation temperature

· sufficient shelf life

· high activity level

· high reliability

· the residue is benign, or if the solder paste rubber fatigue testing machine is the same kind, it will not affect the operation of the equipment due to the influence of external forces, and it is washable, Other precautions for easy removal of

lead-free flux:

· is the solder paste used for dispensing or printing

· please note that the manufacturer uses different types of activators for different alloys

· the flux should be carefully selected to make a balance between the activation temperature and the thermal characteristic curve

· what is the compatibility between the flux and the selected alloy

· reliability attributes (Sir, electromigration, corrosion)

precautions for lead-free solder paste

important attributes that should be considered when selecting:

· solder ball formation test activity

· wetting test, specific finishing and solder gas (air or nitrogen)

· possible degree of pore formation, Lead free alloy is easier to form solder pores

change of adhesion life with time

· template life and waste time

· cold collapse

· hot collapse test temperature can reach a higher C

· shelf life test

characteristics that should be evaluated during processing:

· printability

- attenuation/recovery

- printing speed

- durability

· component placement

- retrograde viscosity

· reflow

- Inspection of solder joint formation on a variety of pins and printed circuit board finishing materials

characteristics that should be evaluated after reflow

· thermal shock

· thermal cycle

· impact resistance

· Reliability (SIR)

technical precautions for wave soldering flux used in lead-free assembly

the ability to achieve uniform coating through spray, wave crest or foam coating

activator kit capable of withstanding high preheating temperature

can be used on a variety of lead-free finishes, bare copper organic solderability protective coatings (OSP), gold nickel, tin, silver immersion, tin 1-copper materials

continuously maintain activity, The flux should remain active during the contact time with the molten solder to ensure good stripping of the solder

low residue capacity. The flux must not react excessively with the molten solder to generate a large amount of residue

the flux must not fade or scorch under the higher welding temperature used in lead-free wave welding

the flux should not decompose under the higher welding temperature

if it is a cleaning free flux, the flux residue must be benign, Moreover, if the flux is water washable flux, it should also be easy to remove with hot water

technical precautions for lead-free cored solder wire

the flux should not splash or excessive smoke under slightly higher welding temperature of lead-free welding

the flux should have an activator system that can weld a variety of lead-free circuit boards and component finishes

the flux must have sufficient activity and maintain sufficient activity when the soldering iron head contacts, To make up for the reduced wetting capacity of lead-free alloy

if it is a non cleaning flux, the flux residue must be benign. Whether the technical parameters meet the requirements of the national standard can be referred to 1 or 2: and if the flux is a water washable cored solder, it should also be easy to remove with hot water

when using a slightly higher temperature of the soldering iron head, the residue should not be scorched or darkened (e fixture is an indispensable part nd in the hydraulic universal testing machine)

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