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Since SMT has gradually become mature, with the rapid development of electronic products in the direction of portable/miniaturization, networking and multimedia, higher requirements are put forward for electronic assembly technology, and new high-density assembly technologies keep emerging, among which BGA(Ball Grid Array packaging) is a high-density assembly technology that has entered the practical stage. This article on the BGA welding process need to consider several points to share:
1. The welding point is disconnected or not fast
When BGA is connected to the plate, the main factors affecting the welding point break or weak are the following:
(1) excessive warping of the plate
Most BGA designs allow for a larger local plate warpage from center to assembly edge up to 0.005 in. When warping exceeds the desired tolerance level, the weld may be broken, unstable, or deformed.
(2) coplanar tolerance
The coplanicity required for the carrier solder ball is not as strict as for the fine-spaced leads. But better coplanar properties reduce the welding point disconnection or weak. Specifying coplanicity as the distance between the higher and lower solder balls, a coplanicity of 7.8 mil (200μm) is achievable for PBGA. JEDEC set the coplanar standard at 5.9 mils (150μm). It should be noted that cohedral is directly related to the warpage degree of the plate.
(3) It is related to wetting
(4) It is related to the formation of excessive solder in reflow welding
2. Connecting Bridges
Finely spaced BGA components that are 0.060 inches (1.50mm) or 0.050 inches (1.0mm) apart do not form Bridges between adjacent interconnection positions. In addition to the spacing size, there are two other factors affecting the bridge problem.
(1) Excessive amount of solder related to pad size
Due to the mutual affinity of the molten solder between two adjacent locations, a bridge may occur when there is too much solder. The characteristics of each BGA vary depending on the alloy composition used, the melting temperature of the carrier solder ball, the pad design associated with the carrier solder ball, and the weight of the carrier. For example, ball carriers containing high temperature solder (which do not melt during plate assembly) do not easily form Bridges, other things being equal.
(2) solder paste collapse
When solder paste is used as interconnecting material, the solder paste collapse during printing and reflow welding plays an important role in connecting bridge. The required anti-collapse characteristics greatly affect the thermal dynamic characteristics of the flux/excipient system. Therefore, it is important to design a flux/excipient system that provides sufficient surface tension for the welding powder in a chemical system binder that can both uniformly wet the surface of the welding powder and give high adhesion.
3. Lump the solder
After rewelding, if the loose solder mass on the plate is not removed, it can lead to electrical short circuit when working, and also can make the weld can not get enough solder. There are several reasons for the formation of solder blobs:
· No effective melting of welding powder, substrate or reflow welding presets, resulting in uncondensed discrete particles.
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