Key Points for Quality Control of Needle Row Bus Surface Mounting Process (SMT)

Introduction

As the connector category with the largest amount of board-to-board connections, the SMT (Surface Mount Technology) welding quality directly affects the electrical performance and mechanical reliability of the product.Common SMT defects such as false soldering, tinning, stele erection, offset, etc., lead to poor contact if light, and short-circuit burning if heavy.In this paper, we sort out the core points of SMT quality control of the needle row mother from the four dimensions of process, material, design, and testing.

I. Characteristics and Challenges of Needle and Female SMT

1.1 Specificity of the needle female SMT

Compared with resistive capacitance, IC and other SMD components, the SMT process of the needle row mother has its unique challenges:

1.2 Common SMT Defect Types

The most common types of defects in pin header SMTs:

II. Steel mesh design and solder paste printing

2.1 Design principles of steel mesh openings

The opening design of the steel mesh (Stencil) directly determines the amount of solder paste deposited and is the first gateway to SMT quality.

Key Design Points:

Pitch Reference:

2.2 Solder paste selection

Note:

• The solder paste needs to be reheated for more than 4 hours after being removed from the refrigerator
• Stir thoroughly before printing
• Control environment: temperature 23 ± 3 ℃, humidity 50 ± 10%
• Reflux within 4 hours of printing

2.3 Printing Process Control

Process parameters	Recommended Scope	Description
Printing speed	20~50mm/s	Too fast can lead to tin deficiency
Scraper pressure	5~10N/25mm	Tin thin due to excessive pressure
Release speed	0.5~2.0mm/s	Slow release to ensure molding
Cleaning frequency of steel mesh	Once every 8-10 tablets	Bottom surface cleaning anti-tin

III. Control of the mounting process

3.1 Requirements for mounting accuracy

The pin header belongs to large-size SMD components, and the mounting accuracy requirements are relatively low, but the alignment requirements of the positioning column and pin are strict.

Parameter	Recommendation requirements	Description
X/Y Offset	≤0.05mm	pin center deviation from pad center
Rotation Angle	≤0.5°	Long row needle rotation deviation has a large impact
Mounting pressure	Gently press to pin contact pad	Overpressure Assembly Compression Soldering Paste
Nozzle Selection	Special elongated suction nozzle	Ensure stable suction

3.2 Role and Requirements of Positioning Columns

The female end of the needle row usually has a positioning column (Post/Peg) at both ends, whose functions include:

1. SMT mount positioning: Insert PCB positioning holes for precise alignment

2. post-weld reinforcement: Increases mechanical strength and anti-plugging force

3. Prevent offset: Prevents component drift during reflow soldering

Positioning column design points:

• The diameter of the positioning column is 0.1~0.15mm smaller than the PCB hole diameter
• The length of the positioning column is 0.5~1.0mm
• The positioning column pad opening should be appropriately reduced to avoid too many solder lifting elements

3.3 Attachment precautions for long rows of needles

For Long Row Pins/Females > 20pin:

1. Suction nozzle design: Use multiple suction nozzles or strip suction nozzles to ensure uniform stress

2. Mounting pressure: Pressure should be evenly distributed to avoid tilting in the middle or at both ends

3. Flatness control: Long rows of needles are easy to deform, and incoming materials are checked for coplanarity

4. PCB support: When mounting, add support under the PCB to prevent plate bending from affecting the mounting

IV. Control of reflow soldering process

4.1 Temperature curve setting

Reflow soldering is the core part of the SMT process, and the rationality of the temperature curve directly determines the welding quality.

Pin header reflow curve recommended:

Temperature zone	Temperature range	Time	Heating rate
Preheating zone	Room Temperature ~ 150°C	60~90s	1~2℃/s
Constant Temperature Zone	150~180℃	60~90s	—
Return zone	180 ~ Peak	30~50s	1~2℃/s
Peak Temperature	245 ~ 255 ℃ (lead-free)	10~20s	—
Cooling zone	Peak ~ Room Temperature	—	2~4℃/s

Note:

• Peak temperature is determined according to the plastic material of the connector
• PA6T/PA9T/LCP materials can withstand peaks of 260°C
• The upper temperature limit of PBT material is about 230 ℃, and lead process or low-temperature solder is required
• Temperature difference control: temperature difference of the same plate ≤ 5 ℃

4.2 Common reflux defects and countermeasures

Defects	Cause Analysis	Resolve Countermeasures
Monument Erecting/Head Raising	Uneven amount of solder at both ends, uneven temperature	Optimize steel mesh opening, adjust temperature curve
False soldering/less tin	Insufficient amount of solder paste, insufficient temperature	Increase mesh thickness/opening, increase peak temperature
Tin Lian/Bridge	Too much solder paste, too high temperature	Reduce mesh openings, optimize reflux curves
Tin Beads	Heating too fast, flux splashes	Reduce preheating rate, optimize steel mesh
Plastic deformation	Temperature too high, time too long	Reduce peak temperature and shorten high temperature time
Component Drift	Asynchronous melting at both ends	Optimize temperature uniformity, check mounting

4.3 Nitrogen return

For products with fine spacing (≤ 1.27 mm) or high quality requirements, nitrogen return is recommended:

Advantages of nitrogen return:

• Reduce oxidation and improve wettability
• Reduce tin beads and tin lian
• Improve weld consistency
• Reduce Peak Temperature Requirements

Disadvantages:

• Increased nitrogen costs
• The appearance of the solder joint is brighter, and the difficulty of visual inspection is increased.

V. Incoming Material Quality Control

5.1 Incoming Inspection Items

The quality of the incoming material of the needle row mother directly affects the SMT yield, and the key inspection items include:

Inspection items	Inspection method	Eligibility Criteria
Coplanarity	Three-dimensional/coplanarity tester	≤0.10mm
Pin Spacing	Image Measuring Instrument	±0.03mm
Pin Straightness	Microscope	≤0.03mm
Positioning column size	Micrometer/Imager	±0.05mm
Plating thickness	XRF Thickness Gauge	Conforms to specification
Solderability	Wetting test	More than 95% infiltration
Packaging method	Visual	Strapping/pipe fitting without deformation

5.2 Common incoming issues

Question	Impact	Treatment
Pin deformation/bending	Mounting offset, false soldering	Full inspection or return
coplanarity outliers	Some pins are not tinned	Returns or screening
Poor coating	Poor solderability, false soldering	Returns and exchanges
Plastic deformation	Defective mounting, floating height	Returns and exchanges
Incorrect knitting tape positioning	Suction offset	Adjust the braid or change material

VI. Influencing Factors of PCB Design

6.1 Pad design

PCB pad design is the basis of SMT quality, pin pad design points:

Parameter	Testimonials	Description
Pad Width	pin width + 0.2~0.4mm	0.1~0.2mm on each side
Pad Length	Mounting direction length 0.3~0.5mm	Increase weld area
Solder resistance windowing	Pad +0.1mm	0.05 mm larger on one side
Positioning aperture	Positioning column +0.1~0.15mm	Ensure smooth insertion
Positioning hole welding ring	Hole +0.4~0.6mm	Increase weld strength

6.2 PCB layout considerations

1. Symmetrical design: The size and heat capacity of the pads at both ends of the row of pins are as symmetrical as possible to prevent the erection of stele

2. Uniform heat dissipation: Avoid needle pin ends near large copper surfaces or heating elements

3. Solder dam: Pin small spacing row of pins, solder blocking dam between pads to prevent tinning

4. Craft side: Reserve process edges near long rows of needles for easy mounting and inspection

VII. Testing and Quality Verification

7.1 SMT process test points

Process	Test content	Detection method	Sampling ratio
Post-Print	Solder paste thickness, offset	SPI (Solder Paste Inspection)	100%
After mounting	Position offset, missing parts	AOI (Automatic Optical Inspection)	100%
After reflux	Bogus soldering, tin lining, offset	AOI + Visual Inspection	100% + sampling
All done	Electrical Performance Testing	ICT/FCT	On-demand

7.2 Key Quality Indicators

Metrics	Typical industry level	Referral Goal
SMT First Piece Pass Rate	95%~98%	≥99%
Defective Welding Rate (DPMO)	500~2000	≤500
False soldering rate	0.1%~0.5%	≤0.1%
Lian Tin Rate	0.1%~0.3%	≤0.05%

VIII. FAQ Troubleshooting Guidelines

8.1 Virtual soldering troubleshooting process

1. Confirmation Phenomenon: Is it an individual pin or a whole row?Is it fixed position or random?

2. Check incoming materials: Pin coplanarity, coating, solderability

3. Check print: Sufficient amount of solder paste, with or without offset

4. Check the mounting: Mounting pressure, position accuracy

5. Check reflux: Temperature curve, temperature uniformity

6. Check PCB: Pad design, surface treatment, oxidation conditions

8.2 Tin removal process

1. Confirmation Phenomenon: Lian Tin position, spacing, quantity

2. Inspection of steel mesh: size of opening, shape of opening, wear condition

3. Check print: Printing pressure, demolding speed, steel mesh cleaning

4. Check solder paste: particle size, viscosity, expiration

5. Check reflux: Peak temperature, reflux time, heating rate

6. Check PCB: Pad spacing, solder mask quality

Conclusion

The SMT quality control of the needle row mother is a systematic project, involving the quality of incoming materials, steel mesh design, printing process, mounting accuracy, reflux curve, PCB design and other links.Only by systematically controlling the whole process can the welding quality be ensured to be stable and reliable.

Shenzhen Ruixin Shengye Electronic Technology Co., Ltd. ‘s needle row mother products are strictly SMT process verification, providing detailed pad design guidelines and reflux temperature curve recommendations.Our technical support team can assist customers in process optimization and quality improvement to ensure that products achieve high yield and high reliability on the customer’s production line.