Fine-pitch QFN (Quad Flat No-lead) device assembling device with grounding disc

Abstract

The utility model provides a low-defect-rate pre-welding device for a fine-pitch QFN (Quad Flat No-lead) device, which is characterized in that two fixing columns are arranged at the edges of two opposite sides of a base, and a placing groove corresponding to the overall dimension of the QFN device is formed in the center of the base; the steel sheet is square, alignment holes are formed in the positions, corresponding to the fixing columns, of the edges of the two opposite sides of the steel sheet, the center of the steel sheet is placed in the containing groove of the base according to the QFN pin pattern of the QFN device and the grounding disc pattern of 1: 1 in a laser holing mode, the QFN device is placed in the containing groove of the base with the pin face upwards, the alignment holes of the steel sheet and the fixing columns of the base are assembled, and the base 1 and the steel sheet are attached. The device and the method have the advantages that a mode of combining a pre-welding device and special steel mesh trepanning is adopted, a traditional SMT technology is improved, the novel fine-pitch QFN device low-defect-rate assembling and connecting device with the grounding bonding pad and the technological method are provided, welding is completed through the device, welding defects during QFN assembling and connecting can be effectively reduced, the quality of the QFN device is improved, and the quality of the QFN device is improved. And the qualified rate of products is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of printed board assembly, in particular to a device mounting and connecting device for surface-mounted devices with ground pads on the bottom and no exposed pins. BACKGROUND

[0002] QFN (Quad Flat No-leads) belongs to the square flat no-lead package form, which is generally square or rectangular. The design of the large-area exposed pads in the center of the bottom makes it have excellent heat dissipation performance. Due to the good thermal performance, small size and light weight of QFN package, it is more and more widely used in electronic products. Unlike the widely used BGA device, QFN package device has only metal terminals or pads on the bottom of the package body without solder balls. During soldering, the printed tin paste is used to directly replace the solder balls or solder columns, which effectively reduces the distance between the chip and the printed circuit board, shortens the lead-out path, transmits electrical signals faster, and has better electrical performance.

[0003] However, due to the low soldering height, small pin pitch, and easy oxidation of the surface soldering end of the QFN device, some problems are brought to the device assembly process such as tin paste printing, patching, reflow soldering, etc. in the prior art: the oxidation of the device soldering end easily affects the wettability of the soldering end, the soldering tin cannot form good infiltration with the soldering end, leading to less soldering tin and virtual soldering defects after soldering; the small pin pitch of the device easily causes tin paste printing to connect tin, leading to bridge connection defects after soldering; the device has no pins and low mounting height, making it difficult for the flux gas generated during the soldering process to escape from the solder, and the large-area heat dissipation pad is easy to form a high cavity rate defect.

[0004] Therefore, it is necessary to improve the existing SMT process method of QFN devices to meet the QFN device assembly and inspection requirements.

[0005] The existing assembly process of fine-pitch (usually less than 0.5mm) QFN devices first prints tin paste on the printed board pads through a printing machine, then places the QFN device on the corresponding position of the printed board through a patching machine, and finally completes the soldering through reflow soldering.

[0006] The existing fine-pitch QFN device assembly technology has three defects:

[0007] (1) During the printing process of fine-pitch pins, adjacent pins are easy to cause tin paste bridge connection, leading to soldering defects;

[0008] (2) The oxidation of the device pins easily leads to virtual soldering;

[0009] (3) The oxidation of the device ground pads easily leads to high cavity rate. SUMMARY

[0010] To overcome the shortcomings of the prior art, the present invention provides a fine-pitch QFN device assembly device with a grounding plate.

[0011] This invention utilizes a pre-soldering device to pre-print a layer of solder paste with a thickness of 0.1mm to 0.15mm onto the leads and ground pad of QFN devices and then reflow soldering it. The flux in the solder paste removes the oxide layer on the leads and ground pad. Then, based on the dimensions of the QFN device's printed circuit board pads, a stencil with a grid pattern and a lead-size inward / outward extension method is used to print 0.1mm to 0.13mm thick solder paste onto the entire printed circuit board. This process prevents bridging after soldering. The increased amount of solder paste and the grid pattern facilitate flux escape, reducing the void ratio of the ground pad. The pre-soldering device, stencil opening technology, and process methods enable the assembly of fine-pitch QFN devices.

[0012] To avoid various defects generated during the QFN welding process, this invention provides a low-defect-rate assembly device and process method for fine-pitch QFN devices, which can obtain welded products with low QFN welding defect rates. At the same time, the process method is simple and easy to operate.

[0013] The technical solution adopted by this invention to solve its technical problem is:

[0014] A pre-welding device for low defect rate of fine-pitch QFN devices includes a base 1, fixing posts 2, QFN placement groove 3, steel sheet 4, alignment holes 5, and QFN pin patterns 6. The base 1 is square, with two fixing posts 2 on the edges of two opposite sides of the base 1. The center of the base 1 has a placement groove 3 corresponding to the outer dimensions of the QFN device. The steel sheet 4 is square, with alignment holes 5 on the edges of two opposite sides corresponding to the positions of the fixing posts 2. The center of the steel sheet 4 is laser-drilled according to the QFN device pin and grounding plate pattern 1:1 QFN pin pattern 6. The QFN device is placed in the placement groove 3 of the base 1 with the pin face upward. The alignment holes 5 of the steel sheet 4 are assembled with the fixing posts 2 of the base 1, so that the base 1 and the steel sheet 4 fit together.

[0015] The height of the QFN device after it is placed in slot 3 does not exceed the height of the upper surface of the base 1.

[0016] The fixing post 2 is circular, and the alignment hole 5 is a circular hole that matches the size of the fixing post 2.

[0017] The two opposite sides of the placement slot 3 are provided with long notches to facilitate the removal of the QFN device.

[0018] Both the base 1 and the fixing column 2 are made of carbon fiber composite material.

[0019] Both sides of the steel sheet 4 are coated with a nano-coating.

[0020] The beneficial effect of the present application is that the traditional SMT process is improved by adopting the pre-welding device combined with the special steel mesh opening, a new low defect rate assembly device for QFN device with fine pitch and ground pad is provided, the welding defects during the assembly of QFN can be effectively reduced by using the device to complete the welding, and the qualified rate of products is improved. In addition, the process method is simple and has wide operability. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 for fine pitch and ground pad QFN device;

[0022] Figure 2 for process flow chart;

[0023] Figure 3 for pre-welding device lower tool structure diagram;

[0024] Figure 4 for pre-welding device lower tool side view;

[0025] Figure 5 for pre-welding device upper tool structure diagram;

[0026] Figure 6 for QFN device fine pitch pin and ground pad steel mesh opening pattern;

[0027] Figure 7 for QFN device actual reflow temperature curve;

[0028] Figure 8 for X-ray diagram of test piece after using new QFN assembly process;

[0029] Figure 9 for X-ray diagram of test piece with multiple defects prepared by conventional QFN assembly process.

[0030] Wherein, 1-base, 2-fixed column, 3-QFN placing groove, 4-steel sheet, 5-alignment hole, 6-QFN pin pattern. DETAILED DESCRIPTION

[0031] The present application is further illustrated below in conjunction with the drawings and examples.

[0032] A pre-welding device for low defect rate of fine-pitch QFN devices includes a base 1, fixing posts 2, QFN placement groove 3, steel sheet 4, alignment holes 5, and QFN pin patterns 6. The base 1 is square, with two fixing posts 2 on the edges of two opposite sides of the base 1. The center of the base 1 has a placement groove 3 corresponding to the outer dimensions of the QFN device. The steel sheet 4 is square, with alignment holes 5 on the edges of two opposite sides corresponding to the positions of the fixing posts 2. The center of the steel sheet 4 is laser-drilled according to the QFN device pin and grounding plate pattern 1:1 QFN pin pattern 6. The QFN device is placed in the placement groove 3 of the base 1 with the pin face upward. The alignment holes 5 of the steel sheet 4 are assembled with the fixing posts 2 of the base 1, so that the base 1 and the steel sheet 4 fit together.

[0033] The height of the QFN device after it is placed in slot 3 does not exceed the height of the upper surface of the base 1.

[0034] The fixing post 2 is circular, and the alignment hole 5 is a circular hole that matches the size of the fixing post 2.

[0035] The two opposite sides of the placement slot 3 are provided with long notches to facilitate the removal of the QFN device.

[0036] Both the base 1 and the fixing column 2 are made of carbon fiber composite material.

[0037] Both sides of the steel sheet 4 are coated with a nano-coating.

[0038] The assembly method steps of the low defect rate assembly device for fine-pitch QFN devices are as follows:

[0039] Step 1: Place the QFN device in the QFN placement slot 3 of the base 1 with the exposed pins and grounding plate facing upwards, and install it together with the fixing post 2 of the base 1 through the alignment hole 5 of the steel plate 4.

[0040] Step 2: Print solder paste on the exposed pins and grounding pad of the QFN device loaded into the pre-soldering device through the steel sheet 4. After printing, remove the steel sheet 4 from the base 1, take out the QFN device and place it in a high-temperature resistant tray, and place it in a hot air reflow oven for soldering.

[0041] Step 3: Place the printed circuit board to be soldered into the solder paste printer, load the whole board printing stencil, and laser-cut and print the stencil according to the "grid bridging" and "inward shrinkage and outward extension" pattern;

[0042] Step 4: Place the printed circuit board with solder paste on it into the pick-and-place machine and mount the QFN devices.

[0043] Step 5: Place the printed circuit board with QFN devices mounted into a hot air reflow oven for soldering.

[0044] In step 1, the steel sheet 4 has a nano-coating on both sides. The center of the steel sheet 4 is laser-drilled according to the QFN device pin and grounding plate pattern 1:1 size QFN pin pattern 6. The thickness of the steel sheet 4 is 0.1mm to 0.15mm. The placement groove 3 is processed in the center of the base 1 according to the shape of the QFN device. Symmetrical notches are processed on both sides of the placement groove 3.

[0045] In step 2, the peak temperature of reflow soldering is 210℃~220℃, and the reflow time is 30~60s.

[0046] In step 3, the stencil opening of the QFN device grounding pad adopts a "grid bridging" method, characterized by dividing the exposed grounding pad into several identical areas with a bridging width of 0.12mm to 0.15mm; the stencil opening of the QFN device pin pad adopts an "inward shrinkage and outward extension" method, with the pin width shrinking inward by 10% to 50% mm and the pin length extending outward by 10% to 50% mm for opening, preventing solder paste bridging by shrinking the pin opening and compensating for solder paste volume by extending the pin opening; the stencil thickness is 0.08mm to 0.13mm, and the printing pressure is 2.0Kg to 3.2Kg.

[0047] In step 4, the placement height of the pick-and-place machine is 1-6 mm.

[0048] In step 5, the printed circuit board is heated using a preset reflow soldering profile, so that the solder paste on the device pads is kept above 183°C for 60 to 120 seconds, and the peak temperature of the solder paste on the QFN device pads reaches 210°C to 220°C.

[0049] This invention provides a low-defect-rate assembly device and method for fine-pitch QFN devices with grounding pads, wherein the QFN device is as follows: Figure 1 As shown, the outer dimensions are 5mm × 5mm, the pin dimensions are 0.53mm × 0.2mm, and the spacing is 0.45mm; the recommended pad size for the QFN pin printed circuit board is 1.18mm × 0.23mm. The manufacturing process is as follows: Figure 2 As shown, it includes the following steps:

[0050] (a) Assembling QFN devices

[0051] (1) Place the QFN device in Figure 3 The QFN is placed in slot 3 of the pre-welding device lower fixture shown, and then the pre-welding fixture base 1 is placed inside the printing press. Figure 5The pre-welding device shown in the figure is installed in one body with the workpiece steel sheet 4 through the fixing column 2 and the alignment hole 5; the pin opening size of the QFN pin pattern 6 is 0.53mm*0.2mm, the thickness is 0.12mm, tin paste is printed on the device pin and the grounding disc, the tin paste particle size selected is 3, and the size is 25μm~45μm; then the QFN device after printing the tin paste is placed in the hot air reflow soldering furnace to perform welding, the reflow soldering peak temperature is 220℃, and the reflow time is 48s.

[0052] (2) the printed board to be welded is placed in the tin paste printing machine, the tin paste is printed on the surface of the printed board through the whole plate steel mesh, the steel mesh thickness is 0.1mm, the "cross frame bridge" width is 0.12mm, the fine pitch pin opening size is 1.77mm*0.11mm, the tin paste particle size selected is 3, the size is 25μm~45μm, and the printing pressure is 2.5Kg. Figure 6

[0053] (3) the printed board printed with the tin paste is placed in the chip mounter to perform chip mounting, and the QFN device mounting height is set to 1mm.

[0054] (4) the printed board after completing the chip mounting is placed in the hot air reflow furnace, the reflow soldering curve is used to heat the printed board assembly, the tin paste on the device pad is maintained for 60~120s at 183℃ or above, the peak temperature of the tin paste on the QFN device pad reaches 210℃~220℃, and the preferred temperature is 220℃. Figure 7 The actual reflow temperature curve for welding the QFN device by using the present application.

[0055] (II) detection

[0056] In order to highlight the effect of the present application, a welding comparison sample is also prepared according to the conventional process.

[0057] Figure 8 The X-ray diagram after the new fine pitch QFN device with grounding disc assembly process is adopted, Figure 9 The X-ray diagram after the conventional fine pitch QFN device with grounding disc assembly process has multiple defects. As can be seen, after the new process is adopted, the welding sample welding point is intact without defects. In summary, the present application effectively solves the defects of the fine pitch QFN device with grounding pad welding.​

Claims

1. A fine-pitch QFN device assembly device with ground plate, comprising a base (1), a fixed column (2), a QFN placement slot (3), a steel sheet (4), a positioning hole (5) and a QFN pin pattern (6), characterized in that: The fine-pitch QFN device has a low defect rate and a pre-welding device, the base (1) is square, two fixed columns (2) are arranged at the edges of the opposite two sides of the base (1), and a placement slot (3) corresponding to the size of the QFN device is arranged at the center of the base (1); the steel sheet (4) is square, positioning holes (5) are arranged at the positions corresponding to the fixed columns (2) at the edges of the opposite two sides of the steel sheet (4), and a QFN pin pattern (6) is laser-punched at the center of the steel sheet (4) according to the pin and ground plate pattern of the QFN device; the QFN device is placed in the placement slot (3) of the base (1) with the pin facing upward, and the positioning holes (5) of the steel sheet (4) are assembled with the fixed columns (2) of the base (1), so that the base (1) and the steel sheet (4) are attached.

2. The fine-pitch QFN device assembly device with ground plate according to claim 1, characterized in that: The height of the QFN device behind the placement slot (3) does not exceed the height of the upper surface of the base (1).

3. The fine-pitch QFN device assembly device with ground plate according to claim 1, characterized in that: The fixed column (2) is circular, and the positioning hole (5) is a circular hole matched with the size of the fixed column (2).

4. The fine-pitch QFN device assembly device with ground plate according to claim 1, characterized in that: The placement slot (3) is provided with long notches on the two opposite sides of the two opposite sides, which facilitates the removal of the QFN device.

5. The fine-pitch QFN device assembly device with ground plate according to claim 1, characterized in that: The base (1) and the fixed column (2) are made of carbon fiber composite material.

6. The fine-pitch QFN device assembly device with ground plate according to claim 1, characterized in that: Both sides of the steel sheet (4) are coated with a nano coating.

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