1.Solder paste

1).Description

①. A thixotropic suspension composed of flux and metal particles

2). Purpose

① Provide solder intermediates during the reflow process to form solder joints with sufficient mechanical and electrical strength

3). Good solder paste has

①. Solderability

②. Printability

③. Stable quality

4). Review and Outlook

①. CFC cleaning-water cleaning-no cleaning

②. Solder paste suitable for ultra-small pitch assembly process

Selection of solder paste

The selection of solder paste in solder paste printing usually follows the following rules.The vertical thickness of the steel template should be at least three times the maximum solder ball diameter in the solder paste (H> = 3Dmax). Three times the maximum solder ball diameter (W> = 3Dmax).

2.Solder paste composition

Chemical composition of solder paste

⊙ Solder paste particles

⊙ Flux system

· Flux

— Remove the metal oxide on the surface of the pad and the surface of the component pin

— Prevent surface reoxidation

· Activator

— Activate the flux to remove metal oxides

· Solvent

— Dissolved components

— Provide liquidity

· Thixotropic agent

— Prevent the solder paste from agglomerating

— Improve printing characteristics

3. Classification of solder paste

4.Characteristics of solder paste

⊙ Solder paste particles

·Alloy composition

Influence — welding characteristics

Normally — 63Sn / 37Pb, 62Sn / 36Pb / 2Ag

· Particle size

Too small-easily oxidized

Too big-difficult to get into template holes

Choice — The width of the hole must be greater than 3 times the diameter of the metal particles

Usually — -200 / + 325mesh (45-75 micron) for printing with a pitch of 50mil or more -325 / + 500mesh (20-45 micron) Micron) for printing under 20mil pitch

· Particle shape

influences

Spherical — reduces the oxidized area to reduce solder balls and easily enter the template pores

Usually — spherical

· Metal content

Usually — mass percentage 90-90.5%; volume percentage is about 50%

⊙rheological properties

· Thixotropic and pseudoplastic rheology

—- Shear force increases and viscosity decreases

—- Under a constant shear force, the liquid flow characteristic is a hysteresis loop.

Three rheological indicators

Viscosity-defined as the resistance to flow deformation (η = τ / D); greatly affects the rolling properties of the solder paste on the printing template

The thixotropic index is defined as the change in viscosity ratio as the shear rate increases {TI = [Log (η1 / η2) −Log (D2)] / Log (D2 / D1)}. Affects the solder paste and template pore Away.

Thixotropic value — is defined as the area of the hysteresis loop.

Thixotropic fluids: Dynamic viscosity decreases with time under shear pressure.

Rheopectic fluids: dynamic viscosity increases with time under shear pressure 

⊙ Viscosity: reflects the self-flow ability of the substance

·influences

Fix the component .

— Insufficient viscosity will cause the component to move on the PCB, and the solder paste is separated from the stencil.Affected byTime, temperature & humidity

⊙ Collapse

·influences

Solder ball — The solder paste falling outside the pad is not pulled back by the molten solder during the reflow process, so a solder ball is formed next to the pad.

⊙Flux activity

Impact: solder balls — the higher the activity, the less corrosion the solder balls — usually the stronger the activity, the greater the corrosion. Determine whether the board needs to be cleaned. Nitrogen — When the flux activity is low, nitrogen is required for soldering Protective gases are classified according to their activity — R, RMA, RA

⊙Other properties

SIR — PerIPC-SF-818,> 108

· Electrical migration

· Bronze mirror

· Silver chromate test paper

·colour

· Flux residue

⊙Storage

Usually — why it can be stored for three months under the sealed condition of 2-8 degrees Celsius and 30-60% RH-to prevent oxidation of metal particles, moisture absorption and volatilization of the flux system.