In SMT PCB production, solder paste printing is a critical step. Since the solder paste is used to directly form the soldering joint, the quality of solder paste printing affects the performance and reliability of the surface mount assembly. Quality solder paste printing guarantees a quality solder joint and final product. Statistics demonstrate that 60% to 90% of soldering defects are related to solder paste printing defects. So it is very important to understand what causes defects in solder paste printing.
ItemFactorsAnalysis1Solder PastePowder formationThe irregular shape of solder powder will easily clog stencil apertures. This will cause a big slump after printing. It can also cause solder ball and short bridge defects after reflow.
A spherical shape is best, especially for fine-pitch QFP printing.Particle SizeIf the particle size is too small, the results will be poor paste adhesion. It will have a high oxygen content and cause a solder ball after reflow.
The particle size should be controlled to about 25 ~ 45 μm in order to meet the requirements for fine-pitch QFP soldering, If the partical size desired is 25 to 30 μm, it should applied with less than 20 μm solder paste for an ultra fine-pitch IC.FluxFlux contains a thixotropic agent, which allows the solder paste to have pseudoplastic flow characteristics. Since the viscosity decreases when the paste passes through the stencil apertures, the paste can be applied to the PCB pads rapidly. When the external force stops, the viscosity will recover to ensure no deformation occurs.
The flux in the solder paste should be controlled to between 8 and 15 percent. A lower flux content will result in excess amount of solder paste applied. Conversely, a high flux content will result in an insufficient amount of solder applied.2StencilThicknessA stencil that is too thick will cause a solder bridge short.
A stencil that is too thin will cause an insufficient solder to be applied.Aperture sizeWhen the stencil aperature size is too big, a solder bridge short can occur.
When the stencil aperature size is too small, and insufficient solder paste will be applied.Aperture shapeIt is best to use a circular-shaped stencil aperture design. Its size should be slightly smaller than the PCB pad size, preventing a bridging defect during reflow.3Printing parametersBlade Angle Speed & PressureThe blade angle affects the vertical force applied on the solder paste. If the angle is too small, the solder paste will not be squeezed into the stencil apertures. The best blade angle should be set around 45 to 60 degrees.
A higher the printing speed means that less time will be spent in applying the solder paste through the stencil aperture surface. A higher printing speed will cause insufficient solder to be applied.
The speed should be controlled to around 20 ~ 40 mm/s.
When the blade pressure is too small, it will prevent the solder paste from being cleanly applied to the stencil.
When the blade pressure is too high, it will result in more paste leakage. The blade pressure is typically set at about 5N ~ 15N / 25mm.4Printing process controlPCB moistureIf the PCB moisture is too high, the water under the solder paste will quickly evaporate, causing the solder to splash and creating solder balls.
Dry the PCB if it was fabricated over 6 months ago. The recommended drying temperature is 125 degrees for 4 hours.Paste storageIf the solder paste is applied without a temperature recovery period, the water vapour in the surrounding environment will condense and penetrate the solder paste; this will cause the solder to splash.
Solder paste should be stored in a refrigerator at 0 to 5 degrees.Two to fours hours before use, place the paste in a normal temperature environment.
1 program composition
1) NC PROGRAM: Z-axis components are placed at X, Y positions at θ angle
2) PCB PROGRAM: substrate length, width, thickness and PIN spacing
3) ARRAY PROGRAM: Z specified components
4) PART LIBRARY: Component Information
5) MARK LIBRARY: tag information
2 NC PROGRAM
1) File name: P— [0-9, A-Z, +, -,.]
2) X, Y coordinates
3) Z No: ZA+ZB, K TYPE and Q TYPE, in single and double FEEDER at 8mm width
The difference between K TYPE and Q TYPE: * PIN, no PIN
* PITCH: K-21.5mm Q-20mm
* ORG: FULL is 1, K-Z1, Q-Z2 when HALF
FEEDER: HALF must be used when mixing single and double
Z No: Single input K TYPE: Product number, Q TYPE: Even
4) θ angle: θ1, θ2 two, θ3 origin return
Θ1:0° 90° 180° 270°
Θ2: [set angle – θ1] + correction angle Reverse time is –
5) S&R: STEP REPEAT, PATTERN REPEAT
On time is +
6) NO MOUNTING: 0-normal patch, 1-non-patch
7) SKIP BLOCK: 0 – unconditional execution, 1 to 9 – conditional jump, 7 – unconditional jump
8) MARK: 0 – no MARK, 1 – individual MARK, 2-PCB MARK, 3-PATTERN MARK
9) LAND TEACHING: 0-NO, 1-LAND TEACHING [Recommended for the second leg of each side]
10) BAD MARK: 0-NO, 1-BAD MARK [SENSOR] 2-BAD MARK [PCB CAMERA]
11) PROGRAM OFFSET: X=, Y=, move the first point of the patch to the camera center
The machine automatically finds PROGRAM OFFSET
12) Z ORG is normally 1 and Z No can be set
* NC PROGRAM sequence
S&R->BAD MARK->MARK/PROGRAM->ROGRAM OFFSET->MARK
3 ARRAY PROGRAM
1) File name: P— [0-9, A-Z, +, -,.]
2) Z No: Fixed cannot be changed
3) SHAPE CODE: shape coding [machine]
4) PARTS NAME: component name [person]
5) VACUUM OFFSET: NOZZLE↑ +, NOZZLE↓ – [-3～3mm]
6) MASTER Z No.: master, slave Z axis
4 PCB PROGRAM
1) File name: P— [0-9, A-Z, +, -,.]
2) X: PCB length
3) Y: PCB width
4) T: PCB thickness [NO USED]
5) Whether the PIN is used: 0-not used, 1-automatic adjustment
6) Hole spacing: X-10
7) Conveyor speed: 1[H]~8[L] speed, X, Y table speed when fully automatic control
5 MARK LIBRARY
1) SHAPE CODE: shape coding, —
X, Y: MARK size
PCB material: 0-copper foil, 1-solder
TYPE: 0-shade, 1-binarization
6 PARTS LIBRARY
1) Shape coding: SHAPE CODE [—, 0-9, A-Z, +, -,.]
2) Component type CLASS: 1 to 99 [1 to 19 transmission recognition, 20 to 99 reflection recognition]
Reflection recognition: blue light is absorbed on the orange reflector, and the surface of the component is reflected [Figure A]
By recognizing: the white light of the halogen lamp shines on the orange reflector, and the reflector reflects the light.
Depending on the component, the edge of the component is reflected to the camera [Figure B]
* High reflection recognition accuracy, high passability through recognition, LED off when used
* For identification θ: CHIP angular deviation > 35° NG, QFP angular deviation > 25° NG
TYPE: For component color, normal condition is 1 [black best]
3) SHUTTER [Shutter]: 0-on, 1-in [Generally open]
Closed left and right to ensure component identification [Figure D]
4) Component dimensions SIZE: up, down, left, right
Hand-drawn tape to see the reverse side of the component is the same as the camera [Figure C]
5) Component thickness THICKNESS: T-component body thickness
6) Thickness tolerance TOL: T<1 is 20% T≥1 is 15%
7) HEAD SPEED: 1[H]~8[L] X, Y TABLE SPEED: 1[H]~8[L]
8) NOZZLE SELECT: 1~5
9) CAMERA: 0-S, 1-L
10) Component feed direction FEED DIRECTION: 0 to 7, 45° interval
11) Packing method: 0-PAPER [including 32mmPEELING] 1-EMBOSS 2-BULK
12) PUSHPIN: 0-NO USE 1-USE only for 8mm bandwidth
13) Number of feeds FEED COUNT: 1~4 spacing 12mm
14) Auxiliary feed: NO USE
15) Component error correction RECOVERY: 0-NO, 1-YES, 2- large parts are sucking
16) CHIP STAND: 0-NO, 1-YES
[Components stand up, thickness sensor is detected, LINE SENSOR application]
17) VACUUM OFFSET: absorbing, for components [-3mm~+3mm]
18) LEAD OUT SIZE: Up/Down Left/Right
19) LEAD PITCH: leg spacing
20) LEAD PITCH TOL: tube leg tolerance
21) LEAD COUNT: Up/Down Left/Right Legs
22) Electrode part ELECTROD: The length direction of the element is the length direction of the electrode [Fig. E]
The width direction of the component is the width direction of the electrode
23) CUT LEAD: cut tube legs SIDE: 1~4, there are cut legs on the side
COUNT: Cut off: POSTION: Position [Figure F]
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