Lead frame soldering method

Abstract

The invention provides a lead frame soldering method which is characterized in that a nickel base layer is plated on the surface of a lead frame before soldering, a tin surface layer is plated on the surface of the nickel base layer, and then follow-up operation of soldering paste coating and reflow soldering is conducted. By plating the tin surface layer as a protection layer of the nickel base layer, oxidization of the nickel base layer can be prevented, and solderability of the lead frame is improved. Black pads and brittle failure are prevented from happening, interface cavity rate is reduced, and soldering spot reliability is improved. On the basis of the existing lead frame soldering, soldering spot requirements of surface pasting components can be met by changing the plated layers of the lead frame and the content of soldering flux lead (Pb), and other procedures are not required. The whole method is simple and easy to implement and popularize.

Description

technical field [0001] The invention relates to a welding method for a lead frame, which belongs to the field of SMD surface mount devices. Background technique [0002] In the field of microelectronics manufacturing, surface mount SMD packaging devices generally use lead frame plastic packaging technology. The lead frame used for soldering is generally made of copper or phosphor bronze. In order to make SMD surface mount devices meet the requirements of 260°C reflow soldering, Sn10Pb88Ag02, a high-melting-point high-lead solder, is generally used to prevent solder joints from being heated and melted. The melting point of Sn10Pb88Ag02 is: 268°C (solidus) to 290°C (liquidus). The solidus line is greater than 260°C, and the Pb content is greater than 85wt%, which meets the RoHS exemption requirements. Due to the high melting point of this alloy, the lead frame base material will be oxidized during high temperature brazing, so the surface treatment of the lead frame is neces...

AI Technical Summary

Problems solved by technology

However, since SMD surface-mount devices use Sn10Pb88Ag02 solder with a Pb content of only 88wt%, tinning the lead frame will cause the following problems: Since the melting point of Sn is 231.9 ° C, the tin in the lead frame coating will Remelt and remelt with Sn10Pb88Ag02 solder, so that the Sn content in the solder joint increases and the Pb content decreases; the change of the alloy composition of the solder joint will cause the melting point of the solder joint to change. Once the solidus line of the melting point is lower than 260 ° C, SMD SMD surface mount devices will not meet the requirements of 260°C reflow soldering; in addition, when the Pb content drops from 88wt% to below 85wt%, SMD surface mount devices will not meet the requirements of RoHS exemption

In this way, the Ni plating layer brings a relatively big problem in welding: if the Ni plating layer of the base material is passivated, poor wetting is prone to occur during welding, causing problems of virtual soldering and false soldering; moreover, it is easy to form interface voids , eventually causing solder joint reliability issues

[0018] ② When the Ni barrier layer is removed, Cu atoms will continue to diffuse into the solder, which will easily lead to excessive IMC formation and ε-Cu 3 Sn malignant IMC

Thinner gold plating creates a second problem: the black plate (black nickel) phenomenon

This will bring a third problem in addition to increasing the cost: golden brittleness

But despite this, the high cost of Pd has prevented it from being greatly promoted in brazing