SMT Red Glue: Why It’s Used & How to Optimize for Cost-Effective PCB Assembly

In SMT production, we sometimes encounter the following situation: before placement, the factory finds that there are relatively large or heavy SMD components on both sides of the PCB, so it is necessary to add a red glue process, which results in additional production costs. How can we avoid these extra costs? I will elaborate it on the following aspects: definition of the red glue process, common application scenarios, HQ Nextpcb’s red glue methods, negative impacts, and optimization suggestions.

1. Definition of the Red Glue Process

The red glue process refers to the practice in SMT production where red adhesive (usually an epoxy resin glue) is dispensed onto designated locations on the PCB for SMD placement. This is used to temporarily secure surface-mounted components (SMDs) and prevent them from falling off or shifting during subsequent soldering processes.

2. Common Application Scenarios

1）Large or heavy SMD components on both sides of the PCB
This is the most common scenario in HQ Nextpcb’s actual production. Such boards require twice of the reflow soldering processes (one for the top and one for the bottom). During the second reflow, the solder paste on the first side will re-melt, and since this side is facing down, large or heavy components may fall from the solder pads or float or shifting due to their own weight, leading to soldering defects.

Solution:
Red glue is applied to the sides of these components, as shown in the pictures below. With red glue, during the second reflow, the components are pre-fixed and will not fall off or shift due to the re-melting of the solder paste.

Common component types: large chip capacitors/inductors/crystals/transformers/SMD sockets, and other heavy SMD components.

2）Products with high vibration or impact resistance requirements
If the customer’s PCBA will undergo significant vibration or impact during assembly or use, they may require red glue to be applied to certain components for enhanced mechanical fixation, preventing loosening during use.
For example, in one of our previous PCBA projects, the customer used ultrasonic soldering for enclosure assembly, which exerted some vibration and impact. A fuse on the PCBA became loose, so the customer required red glue to be applied to the fuse during production to increase its secure attachment.

3）Products with poor flatness of PCB or components
If the PCB or the bottom of a component is not flat, the component may shift or float during reflow soldering. Pre-fixing the component with red glue helps improve the soldering yield.
For instance, we once had a product where an inductor was not very flat, and during reflow, the solder paste would flow under the component, lifting it when the solder solidified and causing floating soldering defects. Applying red glue to both sides of the component pre-fixes it and improves the yield.

These are the more common scenarios for using red glue in our actual production. Of course, there are other situations where the red glue process may be used, but those are not listed here.

3. Nextpcb’s Red Glue Methods

1) Manual dispensing
For small quantities, manual dispensing is usually chosen.

2) Semi-automatic dispensing machine
For larger quantities, a semi-automatic dispensing machine is used, which helps control glue consistency.

4. Negative Impacts

1) Increased complexity of the SMT process and longer lead times

A.Strict control over glue amount and position is required; otherwise, component shifting, cold solder joints, and other issues may arise, increasing production complexity.

B.Adding dispensing and curing steps increases the delivery timeline, especially for larger orders.

2) Increased production costs

A.Material cost: Red glue is a consumable, increasing material costs.

B.Labor cost: Extra steps such as dispensing and curing increase labor and management costs.

5. Optimization Suggestions

1) Optimize PCB design and layout of large/heavy components
Place large or heavy SMD components on the same side of the PCB whenever possible, avoiding large/heavy parts on both sides. Also, try to place them near the center or supported areas of the PCB to reduce the risk of falling due to gravity.

2) Optimize component selection

A.Avoid using components with poor bottom flatness.

B.Avoid using large or heavy SMD components.

C.If such components must be used, give preference to through-hole (THT) types.

FAQs About SMT Red Glue

Q: When is red glue required in SMT assembly?

A: Red glue is essential when double-sided boards have heavy components prone to displacement during reflow soldering. [Expand with 1-2 sentences]

Q: How does red glue increase SMT production costs?

A: It adds material expenses, labor for dispensing/curing, and prolongs lead times.

For more knowledge about PCB Assembly, you can try to download our complete PCBA Guide. 

NextPCB boasts a dedicated team of over 100 manufacturing engineers solely focused on addressing customers' PCB assembly (PCBA) production issues. We offer comprehensive services ranging from prototypes to mass production. Every PCBA order undergoes rigorous review by our expert manufacturing engineers before manufacturing commences. This approach significantly enhances customer efficiency and prevents over 99% of potential manufacturing problems. If you're seeking a reliable manufacturing partner for your products, NextPCB is definitely your optimal choice.