Key Process Control Points for PCBA Dispensing

Material Management and Pre-treatment

Proper Storage of Adhesives

Adhesives used in PCBA dispensing must be stored under specific conditions to maintain their performance. For instance, some adhesives require refrigeration at temperatures between 0°C and 10°C and should be stored vertically to prevent damage from stacking. Before use, these adhesives need to be thawed at room temperature for at least 4 hours. If bubbles, dryness, or expiration are observed after thawing, the adhesive should not be used. Once opened, the adhesive should be resealed and refrigerated again, and then thawed for 30 minutes before reuse, with a cumulative room-temperature storage time not exceeding 12 days.

Cleaning and Inspection of PCBAs

Before dispensing, the PCBA must be thoroughly cleaned to remove contaminants such as flux residues, soldering tin slag, and dust. This can be done using anhydrous alcohol and a stiff brush, followed by drying with a烘干机 (dryer) or hot air gun. After drying, a detailed inspection should be carried out to check for obvious cold solder joints, short circuits, and residual soldering tin slag. Only PCBAs that pass the inspection can proceed to the next step.

Dispensing Equipment and Parameter Control

Selection of Dispensing Equipment

Automatic dispensing equipment is generally required for PCBA dispensing to ensure precision and consistency. The equipment should be capable of controlling the dispensing amount through air pressure and time, with an accuracy of up to 0.001g. Different types of dispensing valves, such as time-pressure valves, screw pumps, or jet valves, can be selected based on the adhesive's viscosity and dispensing requirements. For example, screw pumps are suitable for high-precision and consistent dispensing as they are less affected by adhesive viscosity.

Parameter Optimization

The dispensing amount is a critical parameter that directly affects the quality of the dispensing process. The diameter of the adhesive dot should be half of the pad spacing, and after surface-mount technology (SMT) placement, the diameter should expand to 1.5 times the original size to avoid浸染 (contamination) of the pads. The back pressure should be adjusted appropriately; excessive pressure can lead to adhesive overflow, while insufficient pressure can cause intermittent dispensing or漏点 (missing dots). Additionally, the viscosity of the adhesive is affected by temperature, with a 5°C change in ambient temperature potentially causing a 50% fluctuation in the dispensing amount. Therefore, a constant temperature environment should be maintained during dispensing.

Dispensing Position and Quality Requirements

Precise Dispensing Positioning

The dispensing position must be accurately controlled to avoid contaminating non-dispensing areas such as pads, gold fingers, test points, and optical components. For example, adhesives should not be applied to plugin holes, pads, sockets, network ports, chips, surface-mount components, or component vias. Special attention should be paid to ensuring that the adhesive does not exceed the edge of the PCB or form sharp protrusions.

Quality Standards for Dispensing

The adhesive should completely cover the toes and heels of the pins without any residue in non-dispensing areas. Issues such as wire drawing, accumulation, or lumps are not allowed, and the surface should be clean and free of impurities. For different types of components, specific dispensing requirements should be followed. For example, for卧式 (horizontal) components, the dispensing height should be at least 25% of the component's body diameter, and the adhesive length should be at least 50% of the body length. For立式 (vertical) components, the dispensing height should be at least 30% of the component's height, and the width should be at least 50% of the width.

Curing Process Control

Selection of Curing Equipment and Parameters

Curing equipment such as回流炉 (reflow ovens) or ovens can be used for adhesive curing, and they should be equipped with fans to ensure uniform temperature distribution, with an allowable error of ±5°C. The curing temperature and time should be set according to the characteristics of the adhesive. For example, when using an oven for curing, the temperature should be set based on the adhesive's specifications, and the保温时间 (holding time) should meet the requirements (excluding the time for heating up and cooling down). When using a reflow oven, the curing curve for underfill adhesives can be referenced to ensure uniform heating.

Curing Quality Inspection

After curing, the PCBA should be inspected to ensure that the adhesive has cured completely. Insufficient curing can lead to a decrease in the adhesive's performance, affecting the reliability of the product. The surface of the adhesive should be firm, and there should be no soft or sticky areas. Additionally, the functionality of the PCBA should be tested to ensure that the dispensing and curing processes have not affected its normal operation.