Lead-containing and lead-free mixed solder potting chip welding method

Abstract

The invention relates to the technical field of welding, and discloses a lead-containing and lead-free mixed solder potting chip welding method, which comprises a heat insulation tool design and manufacturing step, a furnace temperature determination step, a reflow soldering step of other devices, a solder paste printing step, an automatic mounting step, a reflow soldering step, an X-ray inspection step and a circuit test step. The special welding process and the corresponding heat insulation tool are designed for the lead-containing and lead-free mixed solder potting chip, high-quality welding of the lead-containing and lead-free mixed solder potting chip is achieved, and the lead-containing and lead-free mixed solder potting chip has the advantages of being good in welding stability, high in production efficiency, high in standardization degree and the like. According to the invention, the welding process of the lead-containing and lead-free mixed solder potting chip is separated from the welding process of other surface-mounted devices on the printed circuit board, so that the welding quality of the lead-containing and lead-free mixed solder potting chip is ensured without influencing the welding quality of other devices, and repeated process verification for each product is avoided; the test period is shortened, and the production quality and stability are improved.

Description

Technical Field

[0001] The present invention relates to the field of welding technology, in particular to a welding method for encapsulating a chip with mixed lead and lead-free solder. Background Art

[0002] With the continuous improvement of chip integration levels and increasing requirements for chip reliability, the application of mixed lead- and lead-free solder potted chips is increasing. This type of chip uses a double-layer BGA package, that is, the chip has an internal layer of solder balls for bonding the chip's upper circuitry to the chip substrate, and an external layer of solder balls for bonding the chip substrate to the external printed circuit board. Typically, the upper layer of the chip substrate uses lead-free solder balls, which are encapsulated with potting compound to enhance vibration resistance. The lower layer of the chip substrate uses lead solder balls.

[0003] This type of mixed lead-free solder potted chip is extremely sensitive to the temperature during soldering because the solder balls on the upper layer of the chip substrate are potted. During soldering, the potting compound will expand due to heat. If the upper solder balls are melted at this time, the molten solder will easily flow along the gaps in the potting compound, causing a short circuit. Therefore, the mixed lead-free solder potted chip is extremely sensitive to the temperature during soldering. Although the upper layer of this type of chip uses lead-free balls with a higher melting point, and the lower layer uses lead solder balls with a lower melting point, when the conventional setting temperature is used and the rest of the components of the printed circuit board are reflow soldered together, the temperature difference between the upper and lower layers of the chip cannot ensure that the lead solder balls on the lower layer reach the soldering temperature while the lead-free balls on the upper layer do not melt. There is still a high probability that the chip will short-circuit, resulting in chip scrapping and product rework. Therefore, achieving high-quality soldering of mixed lead-free solder potted chips and improving the soldering stability and efficiency of such chips are problems that technicians in this field urgently need to solve. Summary of the Invention

[0004] In view of this, the present invention provides a method for soldering a lead-free mixed solder potting chip to solve the problems of poor soldering stability and low yield rate in the prior art of soldering a lead-free mixed solder potting chip.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A method for soldering a chip encapsulated with mixed lead-free and lead-containing solder, comprising the following steps:

[0007] Thermal insulation tooling design and manufacturing steps: Design the corresponding thermal insulation tooling according to the size of the lead-free mixed solder potting chip to be soldered, and manufacture the thermal insulation tooling according to the design documents;

[0008] Steps for determining the furnace temperature: For the lead-free mixed solder potting chip to be soldered, use a sample chip to simulate soldering on a sample board, measure the chip temperature curve, and determine the appropriate furnace temperature;

[0009] Reflow soldering steps for other components: reflow soldering the other components except the lead-free mixed solder potting chip;

[0010] Solder paste printing steps: Use a steel mesh to print solder paste separately on the printed circuit board where the chip is to be soldered with mixed lead-free solder potting;

[0011] Automatic placement steps: Use the automatic placement machine to program and individually place the lead-free and lead-free mixed solder potting chips to be soldered;

[0012] Reflow soldering steps: Use insulation tooling to cover the lead-free mixed solder potting chip to be soldered, and send the printed circuit board assembly along the conveyor belt into the reflow soldering furnace to complete the reflow soldering.

[0013] Furthermore, in the design and manufacturing steps of the thermal insulation tooling, the length, width and height of the inner cavity of the thermal insulation tooling are 2 mm larger than the chip size, the wall thickness of the thermal insulation tooling is 1 mm, and the four sides of the thermal insulation tooling are open at the lower position.

[0014] Furthermore, in the step of designing and manufacturing the thermal insulation tooling, the thermal insulation tooling is made of anti-static synthetic stone material, and multiple layers of thermal insulation tin foil are pasted on the top outer surface of the thermal insulation tooling.

[0015] Furthermore, in the furnace temperature determination step,

[0016] A sample chip encapsulated with mixed lead-free and lead-containing solder was soldered onto a sample board. Meanwhile, the wires of a furnace temperature test thermocouple were soldered and fixed to the upper and lower layers of the sample. The sample was then covered with a heat-insulating tool and conveyed along a conveyor belt into a reflow oven to simulate reflow soldering. The oven temperature curve for the mixed lead-free and lead-containing solder encapsulated chip soldering was obtained.

[0017] If the furnace temperature curve is not suitable, adjust the furnace temperature setting.

[0018] Furthermore, in the reflow soldering step of the remaining components, a suitable furnace temperature is set according to the size, thickness, material of the printed circuit board and the packaging characteristics of the remaining components except the lead-free mixed solder potting chip, and the reflow soldering of the remaining components is completed, and the lead-free mixed solder potting chip is separated from the welding of the remaining components.

[0019] Furthermore, in the solder paste printing step, solder paste testing equipment is used to test the printing quality of the solder paste after printing is completed.

[0020] Furthermore, in the automatic placement step, the chip placement quality is inspected after the placement is completed.

[0021] Furthermore, in the reflow soldering step, the furnace temperature of each temperature zone of the reflow soldering of the lead-free and lead-mixed solder potting chip is set according to the adjusted furnace temperature curve.

[0022] Furthermore, after the reflow soldering step, the method further includes: an X-ray inspection step: using an X-ray machine to visually inspect the soldering quality of the lead-free and lead-containing mixed solder-filled chips.

[0023] Furthermore, in the X-ray inspection step, the inspection positions include the internal solder joints on the upper layer of the chip and the external solder joints on the lower layer of the chip.

[0024] Furthermore, after the reflow soldering step, the method further includes: a circuit testing step: using a multimeter to measure impedance at corresponding points of the printed circuit board assembly to confirm whether there is an internal short circuit after the lead-free mixed solder encapsulated chip is soldered.

[0025] It can be seen from the above technical solution that, compared with the prior art, the present invention provides a method for welding lead-free mixed solder potting chips, and proposes a complete reflow soldering process method and thermal insulation tooling design method for this type of chip. The method has strong versatility and can be applied to the welding of all types of lead-free mixed solder potting chips, and is suitable for actual engineering use. The method separates the welding process of the lead-free mixed solder potting chips from the welding process of other surface-mount devices on the printed circuit board, thereby ensuring the welding quality of the lead-free mixed solder potting chips without affecting the welding quality of other devices, avoiding repeated process verification for each product, greatly shortening the test cycle, and improving production quality and stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 A flow chart of a chip soldering method for potting with mixed lead-free and lead-free solder according to a specific embodiment of the present invention;

[0027] Figure 2 A schematic diagram of the heat-insulating tooling structure provided in a specific embodiment of the present invention;

[0028] Figure 3 A schematic diagram of a furnace temperature curve provided for a specific embodiment of the present invention. DETAILED DESCRIPTION

[0029] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0030] Reference Figure 1 As shown, the present invention proposes a method for soldering a chip with mixed lead-free and lead-free solder potting, comprising the following steps:

[0031] Thermal insulation tooling design and manufacturing steps: Design the corresponding thermal insulation tooling according to the size of the lead-free mixed solder potting chip to be soldered, refer to Figure 2 As shown, the thermal insulation tooling is made of synthetic stone material, with multiple layers of thermal insulation tin foil attached to the top of the tooling. The thermal insulation tooling is processed and manufactured according to the design documents.

[0032] Oven temperature determination steps: For the first soldering of lead-free mixed solder potting chips on new products, use samples to simulate soldering on the sample board, measure the oven temperature chip temperature curve, and refer to Figure 3 As shown in the figure, the two temperature curves are the temperature curves of the upper layer and the lower layer of the chip, respectively, and the applicable furnace temperature is obtained.

[0033] Reflow soldering steps for other components: reflow soldering the components except the lead-free mixed solder potting chip, and do not solder the lead-free mixed solder potting chip for the time being.

[0034] Solder paste printing steps: Use a steel mesh to print solder paste separately on the position where the lead-free mixed solder potting chip is to be soldered on the printed circuit board.

[0035] Automatic placement steps: Use an automatic placement machine to program and individually place the lead-free and lead-free mixed solder potted chips to be soldered, and use 2D AOI equipment to inspect the chip placement quality.

[0036] Reflow soldering steps: Use insulation tooling to cover the lead-free mixed solder potting chip to be soldered, and send the printed circuit board assembly along the conveyor belt into the reflow soldering furnace to complete the reflow soldering.

[0037] X-ray inspection steps: Use an X-ray machine to visually inspect the soldering quality of the lead-free and lead-free mixed solder potted chips.

[0038] Circuit test steps: Use a multimeter to measure the impedance at the corresponding points of the printed circuit board assembly to confirm that there is no internal short circuit after soldering the lead-free mixed solder potting chip.

[0039] In a specific embodiment of the present invention, the welding method for the JBC01G chip includes the following steps:

[0040] S1: Design the appropriate thermal insulation tooling based on the dimensions of the JBC01G chip to be soldered. The JBC01G chip uses a double-layer BGA package, with an internal layer of solder balls for attaching the chip's upper circuitry to the chip substrate, and an external layer of solder balls for attaching the chip substrate to the printed circuit board. The upper layer of the chip substrate uses lead-free balls and is potted with potting compound, while the lower layer uses Pb90Sn10 high-lead balls. The chip's dimensions are 25mm x 25mm x 5.3mm.

[0041] The length, width and height of the inner cavity of the thermal insulation tooling are all 2mm larger than the chip size. If the inner cavity of the thermal insulation tooling is too small, the chip after mounting may be bumped when the thermal insulation tooling is added, causing the chip position to shift. If the inner cavity of the thermal insulation tooling is too large, the thermal insulation tooling may not be added due to interference from other devices around the chip. The wall thickness of the thermal insulation tooling is 1mm. If the wall thickness of the thermal insulation tooling is too thick, the lower layer of the chip may not be soldered at a high temperature and may not meet the reflow requirements. If the wall thickness of the thermal insulation tooling is too thin, the thermal insulation effect may be insufficient, the solder balls inside the chip may melt, and a short circuit may occur along the gap of the potting glue. Therefore, refer to Figure 2 As shown, the inner dimensions of the thermal insulation tooling are 27mm x 27mm x 7.3mm, and the outer dimensions are 29mm x 29mm x 9.3mm. The four sides of the thermal insulation tooling are opened at the lower position, with a length of 25mm and a height of 2mm, to ensure that the hot air under the chip can flow normally and the solder joints under the chip reach the welding temperature.

[0042] The thermal insulation tooling is manufactured according to the determined dimensions. The thermal insulation tooling is made of synthetic stone material. However, the thermal insulation capacity of the synthetic stone material alone is not enough to create a sufficient temperature difference between the upper and lower layers of the chip. In order to ensure that the internal solder joints of the upper layer of the chip do not melt and the external solder joints of the lower layer of the chip can reach the appropriate welding temperature, 20 layers of thermal insulation tin foil are attached to the top outer surface of the thermal insulation tooling to ensure that there is a sufficient temperature difference between the upper and bottom layers of the device, ensuring that the upper solder balls do not melt while the bottom of the chip reaches the welding temperature.

[0043] S2: A JBC01G chip sample was potted with a mixed lead-free and lead-free solder paste to simulate soldering on a production prototype. The thermocouple wires for furnace temperature testing were soldered to the upper and lower layers of the sample to monitor the temperature of the upper and lower solder joints, respectively. The thermocouple wires were first soldered to the upper and lower layers of the chip, then secured with red glue.

[0044] According to relevant standards, the peak temperature for tin-lead alloy reflow soldering is generally between 210°C and 230°C. The JBC01G chip has lead-containing solder balls (tin-lead solder balls) on the bottom layer. To minimize the temperature of the upper solder balls, the furnace temperature should be set to keep the lower solder balls as close to 210°C as possible, but not lower than 210°C. The furnace temperatures for the 10 temperature zones are: 140°C, 160°C, 160°C, 170°C, 170°C, 180°C, 190°C, 235°C, 245°C, and 225°C, with a conveyor speed of 94 cm / min.

[0045] Use heat-insulating tooling to cover the sample, and send the sample along the conveyor belt into the reflow oven to simulate reflow soldering and obtain the oven temperature curve of lead-free mixed solder potting chip soldering. Figure 3 As shown, Figure 3The "Upper 1" curve in the middle represents the temperature curve for the chip's upper layer, and the "Lower 1" curve represents the temperature curve for the chip's lower layer. The maximum temperature of the chip's upper layer is 193.77°C, while the melting point of lead-free solder is 217°C. This temperature is lower than the melting point of the lead-free solder balls. Even with fluctuations in furnace temperature and measurement errors, the lead-free solder balls within the chip are prevented from melting. The maximum temperature of the chip's lower layer is 212.55°C, meeting the soldering temperature requirement of 210°C to 230°C. This confirms that the temperature settings for each zone of the reflow oven are appropriate.

[0046] S3: According to the size, thickness, material of the printed circuit board and the packaging characteristics of the remaining devices except the lead-free mixed solder potting chip, set the appropriate furnace temperature to complete the reflow soldering of all other devices, and separate the lead-free mixed solder potting chip from the welding of other devices.

[0047] Because other main chips on the PCB required high-temperature soldering, the JBC01G chip could not be reflow-soldered with the other surface-mount components. Based on the PCB size, thickness, and material, as well as the packaging characteristics of the components other than the JBC01G, a 10-zone oven with temperatures set at 150°C, 170°C, 180°C, 190°C, 200°C, 205°C, 205°C, 248°C, 253°C, and 242°C was used, with a conveyor speed of 94 cm / min. This allowed reflow soldering of all components except the JBC01G.

[0048] S4: Since other components on the printed circuit board have been soldered, it is not possible to use the entire board stencil printing. Therefore, the JBC01G chip printing stencil is used to print solder paste at the corresponding position on the printed circuit board. After printing is completed, the solder paste printing quality is tested using solder paste testing equipment.

[0049] S5: To ensure placement accuracy, use a placement machine to automatically place the JBC01G chip. Create a program to place the JBC01G chip, call the placement program, and automatically place the chip to be soldered. After placement, use 2D AOI equipment to inspect the placement quality of the chip to ensure it is not offset or skewed.

[0050] S6: Based on the determined furnace temperature curve, set the furnace temperature and conveyor speed for the reflow soldering of mixed lead- and lead-free solder potted chips in each temperature zone. Specifically, set the 10 temperature zones to 140°C, 160°C, 160°C, 170°C, 170°C, 180°C, 190°C, 235°C, 245°C, and 225°C, with a conveyor speed of 94 cm / min. After the furnace temperature reaches the set temperature, use insulation to cover the JBC01G chip to be soldered. Then, feed the printed circuit board assembly along the conveyor into the reflow oven to complete the reflow soldering.

[0051] S7: Due to the high sensitivity of the JBC01G chip to temperature, for batch-produced products, 10% of the first products are sampled and the soldering quality is visually inspected using an X-ray machine. The inspection locations include the internal solder joints on the upper layer of the chip and the external solder joints on the lower layer of the chip, with a focus on whether there are any short circuits in the internal solder joints on the upper layer of the chip.

[0052] S8: Since the gaps in the potting compound inside the JBC01G chip are small, sometimes it is difficult to identify the short circuit caused by solder along the gaps in the potting compound even with X-rays. Therefore, every time a JBC01G chip is inspected by X-ray, the impedance is measured at the corresponding points on the printed circuit board using a multimeter in combination with the printed circuit board design layout. Three test points are selected. If the impedance of the three test points is normal, it can be confirmed that there is no short circuit inside the chip and the chip welding is qualified.

[0053] The above description of the disclosed embodiments is presented in an incremental manner to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein but is intended to be embodied in the widest possible manner consistent with the principles and novel features disclosed herein.

Claims

1. A method for soldering a chip with mixed lead and lead-free solder, characterized in that: The steps include: Thermal insulation tooling design and manufacturing steps: Design the corresponding thermal insulation tooling according to the size of the lead-free mixed solder potting chip to be soldered, and manufacture the thermal insulation tooling according to the design documents; Steps for determining the furnace temperature: For the lead-free mixed solder potting chip to be soldered, use a sample chip to simulate soldering on a sample board, measure the chip temperature curve, and determine the appropriate furnace temperature; Reflow soldering steps for other components: reflow soldering the other components except the lead-free mixed solder potting chip; Solder paste printing steps: Use a steel mesh to print solder paste separately on the printed circuit board where the chip is to be soldered with mixed lead-free solder potting; Automatic placement steps: Use the automatic placement machine to program and individually place the lead-free and lead-free mixed solder potting chips to be soldered; Reflow soldering steps: Use insulation tooling to cover the lead-free mixed solder potting chip to be soldered, and send the printed circuit board assembly along the conveyor belt into the reflow soldering furnace to complete the reflow soldering.

2. The method according to claim 1, wherein In the design and manufacturing steps of the thermal insulation tooling, the length, width and height of the inner cavity of the thermal insulation tooling are 2 mm larger than the chip size, the wall thickness of the thermal insulation tooling is 1 mm, and the four sides of the thermal insulation tooling are open at the lower position.

3. The method according to claim 1, wherein In the steps of designing and manufacturing the thermal insulation tooling, the thermal insulation tooling is made of anti-static synthetic stone material, and multiple layers of thermal insulation tin foil are pasted on the outer surface of the top of the thermal insulation tooling.

4. The method according to claim 1, wherein In the furnace temperature determination step, a lead-free mixed solder potting chip sample is soldered to a sample board, and the wires of the furnace temperature test thermocouple are respectively soldered and fixed to the upper and lower layers of the sample board. Then, the sample board is covered with a heat-insulating tool, and the sample board is sent into a reflow oven along a conveyor belt to simulate reflow soldering, thereby obtaining a furnace temperature curve for soldering the lead-free mixed solder potting chip board. If the furnace temperature curve is not suitable, adjust the furnace temperature setting.

5. The method according to claim 1, wherein In the reflow soldering step of the remaining components, the appropriate furnace temperature is set according to the size, thickness, material of the printed circuit board and the packaging characteristics of the remaining components except the lead-free mixed solder potting chip, and the reflow soldering of the remaining components is completed, and the welding of the lead-free mixed solder potting chip and the remaining components is separated.

6. The method according to claim 1, wherein In the solder paste printing step, solder paste testing equipment is used to test the printing quality of the solder paste after printing is completed.

7. The method according to claim 1, wherein In the automatic placement step, the chip placement quality is inspected after the placement is completed.

8. The method according to claim 1, wherein In the reflow soldering step, the temperature of each temperature zone of the reflow soldering of the lead-free and lead-mixed solder potting chip is set according to the adjusted furnace temperature curve.

9. The method according to claim 1, wherein After the reflow soldering step, the following steps are also included: X-ray inspection step: using an X-ray machine to visually inspect the soldering quality of the lead-free and lead-mixed solder-encapsulated chips.

10. The method according to claim 9, wherein In the X-ray inspection step, the inspection positions include the internal solder joints on the upper layer of the chip and the external solder joints on the lower layer of the chip.

11. The method according to claim 1, wherein After the reflow soldering step, the following steps are also included: Circuit testing step: Use a multimeter to measure the impedance at the corresponding points of the printed circuit board assembly to confirm whether there is an internal short circuit after the lead-free mixed solder potting chip is soldered.

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