Method for protecting exposed part of lead notch of plastic package QFN (Quad Flat No-lead) package side copper frame

Abstract

The invention belongs to the technical field of electronic component packaging, and particularly relates to a plastic package QFN packaging side face copper frame lead notch exposed part protection method. The method comprises the pretreatment steps of oil removal, activation and water washing, the chemical tinning step and the cleaning and air drying steps. According to the invention, a layer of tin is uniformly plated at the lead notch at the side edge around the plastic package QFN package by adopting a chemical tin plating method, so that not only can the notch be protected, but also the welding reliability of the element in the use process can be improved. The method has the advantages of being stable and reliable in process, good in uniformity of plated products, high in production efficiency, low in cost and the like, and is particularly suitable for production and supply of large-batch plastic package QFN package elements.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of electronic component packaging, and particularly relates to a protection method for exposed parts of cutouts of copper frame leads on the side of plastic package QFN packaging. BACKGROUND

[0002] The plastic package QFN packaging is a typical surface mount electronic component without leads, and a large-area exposed pad is designed at the central position of the bottom of the packaging for heat conduction. The pad has a direct heat dissipation channel for releasing heat generated by the internal circuit during operation. The periphery of the packaging around the large pad is designed with conductive pads for electrical connection, and the structure has the characteristics of small size and light weight. Due to the excellent electrical and thermal performance of the plastic package QFN packaging, it is widely used in high-reliability application scenarios with strict requirements on size and weight, such as consumer electronics, communications, automotive electronics, medical devices, industrial control, etc.

[0003] Due to the structural characteristics of the plastic package QFN packaging, the metal frame leads on the periphery of the side will leave exposed cutouts after cutting. If no protective measures are taken, the metal at the cutout will easily oxidize and contaminate over time, thereby seriously affecting the soldering reliability during use.

[0004] At present, for the problem of metal exposure at the cutout of the metal frame lead, the general method for protection is manual tin dipping, that is, the packaging element is manually or clamped by a tool and immersed in a tin pot to allow the surface of the cutout to be coated with a layer of tin. This method has obvious disadvantages, specifically: on the one hand, the tin dipping depth is relatively random and difficult to control accurately, which may result in different tin dipping depths, poor uniformity and consistency of the products, and if the element enters the tin pot too shallowly, there is a risk of too thin tin coating or no tin coating, thereby affecting the soldering reliability during use; on the other hand, the tin dipping efficiency is low and the cost is high, which seriously restricts the mass production of the element.

[0005] The metal exposed part at the cutout of the metal frame lead can in principle use chemical tin plating, but in existing chemical tin plating schemes, silver ion additives are generally used in the tin plating tank to prevent tin whiskers, however, this practice may cause simultaneous deposition of silver and tin, and the process is difficult to accurately control, the ratio of silver and tin in the plating solution is unstable, which may also cause tin dissolution during chip reflow soldering.

[0006] As of now, there is no better alternative solution for the problem of protecting the exposed metal at the cutout of the side lead of the plastic package QFN packaging. It is necessary to provide a new method for protecting the exposed parts of the cutouts of the copper frame leads on the side of the plastic package QFN packaging. SUMMARY

[0007] To solve the above technical problems, the application provides a method for protecting exposed parts of lead cutouts of a plastic-sealed QFN package side copper frame.

[0008] The application adopts the following technical solutions:

[0009] The method for protecting exposed parts of lead cutouts of a plastic-sealed QFN package side copper frame comprises the following steps:

[0010] S1. Oil removal: the plastic-sealed QFN package containing exposed parts of lead cutouts is subjected to oil removal and water washing;

[0011] S2. Activation: after water washing, hydrochloric acid activation is performed, and after activation, second water washing is performed;

[0012] S3. Tinning: the plastic-sealed QFN package after second water washing is immersed in a tinning solution, and tinning is performed at 60-70℃ for 12-25 min;

[0013] S4. Cleaning: the plastic-sealed QFN package after tinning is subjected to third water washing, and then is transferred to an alcohol solution for dehydration and cleaning, and after cleaning, is air-dried to obtain the plastic-sealed QFN package with tinned exposed parts of lead cutouts.

[0014] Preferably, the tinning solution comprises a mixture of methylsulfonic acid and citric acid added at a mass ratio of (4-2):1, 50-200 g / L of stannous methanesulfonate, 0.5-5 g / L of OM-Ag additive, 10-100 g / L of tartaric acid, 10-50 g / L of hypophosphorous acid or sodium hypophosphite, 10-80 g / L of disodium ethylenediaminetetraacetate, 5-20 g / L of malic acid, 0.5-5 g / L of benzalacetone, and the rest is water.

[0015] Preferably, the oil removal is chemical oil removal, and the method comprises the following steps: first, the plastic-sealed QFN package containing exposed parts of lead cutouts is immersed in a chemical oil removal IV solution for 12-18 min; and then, the plastic-sealed QFN package is transferred to an electrolytic oil removal tank, and an electric current of 1-2.5 A / dm 2 is set to remove oil by electrolysis for 3-7 min.

[0016] Preferably, the chemical oil removal IV solution comprises 25-35 wt% of sodium oxide and 30-40 wt% of sodium carbonate; and the solution in the electrolytic oil removal tank comprises 40-60 g / L of sodium hydroxide, 0.5-6 g / L of surfactant, 10-30 g / L of sodium carbonate, and 15-25 g / L of trisodium phosphate.

[0017] Preferably, in the steps S1, S2 and S4, the water washing steps are the same, and the plastic package QFN package is sequentially immersed in three-stage water tanks for step-by-step water washing, and each stage of water washing time is 30-120s.

[0018] Preferably, the hydrochloric acid activation method is that the plastic package QFN package containing the exposed part of the lead cut is immersed in a hydrochloric acid solution with a mass fraction of 30-60% and a temperature of 30-60℃, and the activation time is 30-90s.

[0019] Preferably, the dehydration cleaning includes two-stage alcohol cleaning, each stage is immersed for 20-60s, and the volume fraction of the alcohol used for immersion is 50-70%.

[0020] Preferably, before oil removal, the plastic package QFN package containing the exposed part of the lead cut is subjected to a coating and mounting operation: according to the number, size and specific structure of the plastic package QFN package to be plated, a special tool, clamp or wire bundling method is selected to coat and mount the plastic package QFN package.

[0021] The application has the following advantages:

[0022] The application can uniformly plate a layer of tin around the side lead cut of the plastic package QFN package by adopting the method of chemical tin plating, which not only can protect the cut, but also can improve the soldering reliability during use of the element, achieving two goals at once.

[0023] The application adopts the method of chemical tin plating to tin the plastic package QFN package after cutting, and the exposed copper alloy at the cut of the copper frame after cutting is protected by the newly plated tin layer, which can effectively prevent the oxidation of the copper alloy at the cut, and the presence of tin makes the soldering assembly of the chip more convenient, which greatly improves the reliability of the soldering and use of the electronic element. The scheme of the application changes the unstable random problem in the original industry into a fixed scheme with quantifiable technical indexes, which greatly helps to improve the overall level of the industry, which is one of the core advantages of the application.

[0024] The chemical tin plating pretreatment process can be the same as the conventional electroplating pretreatment process, that is, the chemical oil removal, electrolytic oil removal, activation and other processes can be universal, which can bring the following advantages: 1) the equipment and chemical raw materials can be universal, reducing the operation difficulty; 2) reducing the management difficulty, effectively reducing the process cost and management cost; 3) improving the production efficiency, and not impacting the production of other non-similar products online; therefore, the application is especially suitable for large-scale production and popularization.

[0025] The application also improves the formula of the electroless tin plating solution. The conventional tin plating process is prone to tin whisker growth, mainly due to two core factors: 1) internal compressive stress: this is the original driving force for tin whisker growth, and the stress sources are diverse, such as the stress caused by the volume expansion when the intermetallic compound (such as Cu6Sn5 for copper material) is formed between the plating layer and the substrate, the residual stress introduced by the electroplating process itself, external mechanical stress or thermal stress, etc. 2) the properties of tin itself: tin is a soft, easily creep (slow deformation under stress) and diffusible metal; when there is internal compressive stress, tin atoms will seek a channel for stress release. If there is a thin oxide film or other weak point on the surface of the tin layer, tin atoms will preferentially "squeeze" out from there, forming single-crystal whiskers, i.e. tin whiskers.

[0026] Compared with the conventional tin plating process, the application uses a tetravalent tin deposition process, without the need for a tetravalent tin reducing agent, to achieve the purpose of electroless tin plating. The addition of the additive OM-Ag to the plating solution can control the deposition of silver ions, there is no risk of tin melting, and it has a good inhibitory effect on tin whiskers. Finally, multi-stage water washing is used to effectively clean the surface of the tin plating layer and effectively remove contaminants on the surface of the tin.

[0027] By using the method provided by the application, the thickness of the tin layer at the cutout of the lead of the plastic-sealed QFN package is about 0.7-1.0 μm, which can meet the protection requirements of the cutout bare metal, and is also conducive to the implementation of the subsequent welding process of the component. If there is a special requirement for the thickness of the tin layer of the product, the time of the product in the electroless tin plating solution can be adjusted according to the actual situation, and the general principle is about 1 μm / 20 min.

[0028] Compared with the manual tin dipping method commonly used in the industry at present, the application has the characteristics of stable and reliable process, uniform and consistent plating product, high production efficiency, etc., and also takes into account the advantages of low cost, and is particularly suitable for the production and supply of large quantities of plastic-sealed QFN packaged components. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 is a schematic diagram of the cut section of the plastic-sealed QFN package, and the blue area on the diagram represents the exposed part of the lead cutout.

[0030] Figure 2 is a schematic diagram of the plastic-sealed QFN package after electroless tin plating.

[0031] Figure 3 is a front view of the actual product after cutting the plastic-sealed QFN package.

[0032] Figure 4 is a front view of the actual product after electroless tin plating of the plastic-sealed QFN package.

[0033] Figure 5 is a side view of the actual product after cutting the plastic-sealed QFN package.

[0034] Figure 6 is the actual side of plastic package QFN after chemical tinning.

[0035] The meaning of the symbols in the figure is as follows:

[0036] 1-plastic package QFN 11-plastic base 12-heat sink area

[0037] 13-lead 14-lead cutout exposed site DETAILED DESCRIPTION

[0038] The technical solutions of the present application will be described in more detail below with reference to the accompanying drawings.

[0039] The plastic package QFN involved in the present application is a conventional plastic package QFN, and the plastic used is generally an epoxy resin composition such as CEL-9240HF, G700LALA, etc., and the frame lead and heat sink material is generally selected from C194 or equivalent high-strength, high-conductivity copper alloy, and the surface of the heat sink and the lead on the surface is coated with 5-25 μm of tin, and the peripheral side surface perpendicular to the heat sink surface and connected to the heat sink surface has a cut-out exposed metal lead. However, those skilled in the art know that various QFN packaging products that can achieve the tinning conditions of the present application are within the scope of protection of the present application.

[0040] A method for protecting the exposed lead cutout site of the side copper frame of a plastic package QFN, comprising the following steps:

[0041] S1. Oil removal: the plastic package QFN containing the lead cutout exposed site is subjected to oil removal and water washing;

[0042] S2. Activation: after water washing, hydrochloric acid activation is performed, and after activation, second water washing is performed;

[0043] S3. Tinning: the plastic package QFN after second water washing is immersed in a tinning solution, and tinning is performed at 60-70°C for 12-25 min; in the tinning solution, the exposed copper electrode potential is lower than tin, so that tin is deposited on copper;

[0044] S4. Cleaning: the plastic package QFN after tinning is subjected to third water washing, and then transferred to an alcohol solution for dehydration and cleaning, and after cleaning, air dried to obtain a plastic package QFN with tinned lead cutout exposed site.

[0045] The plating solution in step S3 includes 50-100 g / L of a mixture of methanesulfonic acid and citric acid added in a mass ratio of (4-2): 1, 50-200 g / L of stannous methanesulfonate, 0.5-5 g / L of OM-Ag additive, 10-100 g / L of tartaric acid, 10-50 g / L of hypophosphorous acid or sodium hypophosphite, 10-80 g / L of disodium ethylenediaminetetraacetate, 5-20 g / L of malic acid, 0.5-5 g / L of benzalacetone, and the balance being water.

[0046] More specifically, before operation, first, according to the quantity of the packaged product or the specific structure size, a special tooling, clamp with complex single precision higher or a simple and effective way of bundling wires is selected to coat and mount the plastic package QFN package after cutting legs.

[0047] After mounting, the product to be coated and the hanger are immersed in a chemical degreasing solution IV for 12-18 min, the chemical degreasing solution IV including 25-35 wt% of sodium oxide and 30-40 wt% of sodium carbonate; then the product to be coated is electrolytically degreased in an electrolytic degreasing tank, the electrolytic degreasing solution including 40-60 g / L of sodium hydroxide, 0.5-6 g / L of surfactant, 10-30 g / L of sodium carbonate, and 15-25 g / L of trisodium phosphate; the temperature of the electrolytic degreasing solution is set to 60-80°C, the time is set to 3-7 min, and the current density is set to 1-2.5 A / dm 2 .

[0048] After degreasing, the hanger together with the product to be coated is quickly taken out and immersed in a water tank for 30 s-120 s. Then the hanger is taken out from the first water tank and quickly immersed in a second water tank for second-stage water washing, the soaking time being the same as that of the first-stage water washing. Third-stage water washing is performed in the same way.

[0049] After water washing, the hanger together with the product to be coated is activated with hydrochloric acid, the concentration of the hydrochloric acid being 30%-60%, the temperature being 30-60°C, and the activation time being 30 s-90 s, and then the hanger together with the product to be coated is subjected to three-stage water washing in the same way.

[0050] Finally, the hanger together with the product to be coated is put into a chemical tin plating solution for tin plating. The plating solution is ORMECON CSN 7001 chemical tin plating solution, the temperature is set to 60-70°C, and the time is 12-25 min.

[0051] After the plating is completed, the product is taken out and washed with water in three stages. Finally, the product and the hanger are immersed in an alcohol solution with a volume fraction of 50-70% for dehydration cleaning. The dehydration cleaning is performed in two stages. The first stage is immersed for 20-60 seconds, and then the second stage is immersed for 20-60 seconds in the second alcohol tank. The volume fraction of the alcohol used for immersion is 50-70%. After the immersion, the product is dried in a cold air environment. At this time, the tin plating is completed, and the lead cutout part of the plastic package QFN package is plated with tin.

[0052] It should be emphasized that the tin plating solution used in the present application can realize controllable deposition of silver ions. The additive OM-Ag is not simply added to the plating solution, but through a complex technology of complexation and slow release, the reduction and deposition rate of silver ions (Ag + ) is controlled to maintain an ideal proportion and rhythm with the deposition of tetravalent tin ions (Sn4 + ). This technology can form Ag3Sn intermetallic compound particles on one hand: during co-deposition, silver (Ag) and tin (Sn) will immediately react to form very fine, dispersed and uniformly distributed Ag3Sn intermetallic compound particles, which have high hardness and stable properties; on the other hand, it can form a "pinning effect": the dispersed, fine Ag3Sn particles, like countless tiny "anchor points" or "roadblocks", are embedded in the grain boundaries and crystal lattices of tin. When internal stress in the coating occurs due to various reasons and tries to release through diffusion of tin atoms and grain boundary sliding, these hard Ag3Sn particles will strongly pin the grain boundaries and dislocations, greatly hindering the movement and rearrangement of the grains. At this time, the stress in the tin layer cannot be released through the formation of a single, large channel (i.e. tin whiskers), but is dispersed, offset and relaxed throughout the coating by these particles. Without concentrated stress, tin whiskers lose the motivation to grow. Therefore, the special tin plating solution of the present application does not simply "cover" or "delay" the tin layer stress (i.e. tin whiskers), but from the perspective of materials metallurgy, by changing the essential microstructure and stress state of the coating, it achieves efficient and fundamental suppression of tin whiskers.

[0053] It should also be noted that the pretreatment processes involved in the present application, such as chemical degreasing, electrolytic degreasing, washing, activation, etc., are basically the same as conventional nickel plating and gold plating processes. Therefore, in the case of achieving the purpose of degreasing or washing, activation, the present application does not make special strict provisions for the specific operation.

[0054] In addition, the plastic package QFN package coating rack mode can be selected according to the actual situation of the size structure of the element, the batch size, etc., to determine the quality and quantity of the completed coating process. When the product quantity is small or the rapid sample is made, the coating hanging mode can adopt the traditional wire bundling mode, but there may be a problem of thin plating layer in individual areas due to the shielding of the metal wire. When the product quantity is large and the plating layer requirement is high, the hanging mode selects special tool electroplating, and the design of the special tool is based on the product size to meet the requirements of nickel plating coating. This part belongs to the conventional operation process in the art, and the present application does not make special limitations.

[0055] The tin layer thickness of the plastic package QFN package side lead cutout part provided by the method is about 0.7-1.0 μm, which can meet the protection requirements of the cutout bare metal, and is also beneficial to the implementation of the element post-welding process. If there is a special tin layer thickness requirement for the product, the time of the product in the tin plating solution can be adjusted according to the actual situation, and the general principle is about 1 μm / 20 min.

[0056] The tin-plated chip prepared by the method is transported or stored by vacuum packaging, and the vacuum packaging can be stored or transported in an environment with a temperature of 5-35℃ and a humidity of 30-70%.

[0057] The above is only a preferred embodiment of the present application, and is not used to limit the present application; although the present application has been described in detail with reference to the foregoing embodiments, it should be understood by those skilled in the art that any modification, equivalent replacement and improvement within the spirit and principle of the present application should be included in the protection scope of the present application.

Claims

1. A method for protecting the exposed area of a lead cut of a side copper frame of a plastic package QFN package, characterized in that, The method comprises the following steps: S1. Oil removal: the plastic package QFN package containing the exposed lead cutout is subjected to oil removal and water washing; S2. Activation: after water washing, hydrochloric acid activation is performed, and after activation, the second water washing is performed; S3. Tinning: the plastic package QFN package after the second water washing is immersed in a tinning solution, and tinning is performed at 60-70°C for 12-25 min; S4. Cleaning: the plastic package QFN package after tinning is subjected to the third water washing, and then is transferred to an alcohol solution for dehydration cleaning, and after cleaning, air drying is performed to obtain the plastic package QFN package with the exposed lead cutout tinned; The tinning solution comprises 50-100 g / L of a mixture of methyl sulfonic acid and citric acid added according to a mass ratio (4-2):1, 50-200 g / L of stannous methanesulfonate, 0.5-5 g / L of OM-Ag additive, 10-100 g / L of tartaric acid, 10-50 g / L of hypophosphorous acid or sodium hypophosphite, 10-80 g / L of disodium ethylenediaminetetraacetate, 5-20 g / L of malic acid, 0.5-5 g / L of benzalacetone, and the balance is water.

2. The method of claim 1, wherein the method further comprises: applying a protective layer on the exposed portion of the copper frame after the cutting of the copper frame. The oil removal is chemical oil removal, and the method is that the plastic package QFN package containing the exposed part of the lead cut is first immersed in a chemical oil removal IV solution for 12-18 min; then the plastic package QFN package is transferred to an electrolytic oil removal tank, an electric current of 1-2.5 A / dm 2 is set, and electrolysis is performed for 3-7 min to remove oil. ​ 3. The method of claim 1, wherein the method further comprises: applying a protective layer on the exposed portion of the copper frame after the cutting of the copper frame. 3 The chemical oil removal solution IV comprises 25-35 wt% of sodium oxide and 30-40 wt% of sodium carbonate; and the solution in the electrolytic oil removal tank comprises 40-60 g / L of sodium hydroxide, 0.5-6 g / L of surfactant, 10-30 g / L of sodium carbonate, and 15-25 g / L of trisodium phosphate.

4. The method of claim 1, wherein the method further comprises: applying a protective layer on the exposed portion of the copper frame after the cutting of the copper frame. 4 In the steps S1, S2 and S4, the water washing steps are the same, which are three-stage water washing, and the plastic package QFN package is sequentially immersed in three-stage water tanks for step-by-step water washing, and each stage of water washing is performed for 30-120 s.

5. The method of claim 1, wherein the method further comprises: applying a protective layer on the exposed portion of the copper frame after the cutting of the copper frame. 5 The hydrochloric acid activation method is that the plastic package QFN package containing the exposed lead cutout is immersed in a hydrochloric acid solution with a mass fraction of 30-60% and a temperature of 30-60°C, and the activation time is 30-90 s.

6. The method of claim 1, wherein the method further comprises: The dehydration cleaning comprises two-stage alcohol cleaning, each stage is immersed for 20-60 s, and the volume fraction of alcohol used for immersion is 50-70%.

7. The method of claim 1, wherein the method further comprises: applying a protective layer on the exposed portion of the copper frame after the cutting of the copper frame. 7 Before oil removal, the plastic package QFN package containing the exposed lead cutout is also subjected to coating and mounting operation: according to the number, size and specific structure of the plastic package QFN package to be plated, a special tool, clamp or wire bundling mode is selected to coat and mount the plastic package QFN package.

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