Ideally, the solder paste should be a homogeneous mixture of flux and solder powder. However, solder paste sometimes shows flux separation when the container is opened. There us a typical symptom that yellow flux layer on top of the off-white solder paste, either in the can or in the injection container. If the injection container is laid flat during storage, the flux separation will show flux streaks along the top of the injection container. Slight flux separation is acceptable, and severe flux separation can cause stains, collapse, and uneven solder coating, which must be corrected.
Possible causes of flux separation are as follows:
1 The temperature of shipping and storage is too high;
2 Solder paste storage time is too long;
3 The viscosity of the solder paste is too low;
4 The solder paste has too low thixotropic properties.
Solutions to eliminate flux separation can be divided into two categories: process and material .
2.Process solutions :
1 Use solder paste within the recommended storage life;
2 Store solder paste on a rotating shelf;
3 Low temperature storage solder paste, usually considered 5 ~ 10 ° C is appropriate, although lower temperatures are more beneficial;
4 Stir the solder paste before usage and mix by hand or equipment. However, it should be noted that excessive mixing of the solder paste can cause the solder paste to harden due to cold soldering and should be avoided.
1 Use solder paste with high viscosity; 2 Use solder paste with high enough thixotropic properties. For printing and dispensing distributions, a thixotropic index between -0.5 and -0.8 is appropriate.
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