Key Advances in Void Reduction in the Reflow Process
Using Multi-Stage Controlled Vacuum
This paper explores new advances in the reflow soldering process including vacuum technology and warpage mitigation systems.
The first topic for discussion will be the implementation of a vacuum process directly in a conventional inline soldering system. The paper discusses the significant results that have been achieved in reducing voids to < 1%.
Another key area to be discussed is the maximizing of the unit per hour (UPH) of the system while still achieving the void rate reduction < 1%.
Another key capability that will be explored is the elimination of solder splatter during the process of vacuum purge down.
The second topic we will present is the mitigation of warpage on substrates or wafers. This is one of the key technical challenges facing the industry.
We will explore direct warpage mitigation as well as independent substrate/wafer clamping systems as well as inline and recirculating automation solutions.
Key Advances in Void Reduction with Vacuum Technology
One of the challenges facing the industry is the requirement to significantly reduce voids from the process.
The proven method of reducing voids has been through the implementation of vacuum chambers in conventional convection reflow machines.
Why Use A Vacuum In Reflow Soldering?
There are five ways to eliminate voids that will improve product performance.
1. Improve heat dissipation of components or solder joint structures (i.e., current density increases with voiding)
2. Improve long-term stability and reliability of solder joint against heat dissipation and vibration/shock
3. Improve chip performance in high-frequency applications
4. Maintain impedances within specification for components (e.g., power modules)
5. Mitigate or eliminate solder problems (e.g., bridging, solder splashes, etc.) at μBGAs
Advantages of Vacuum-Assisted Convection Reflow
The advantages of vacuum-assisted reflow are:
Vacuum-assisted reflow has been shown to reduce the voids in a solder joint by 99%
Vacuum pressure is reduced to 1-5 Torr during liquidous of the soldering process
Existing voids escape externally through the solder when a vacuum is applied.
◦ Trapped gas bubbles increase in size as pressure is reduced
◦ Larger bubbles are more likely to collide with other bubbles and ultimately collide with the edge of liquid solder to escape
◦ Larger bubbles are accelerated by stronger buoyancy forces making them more likely to escape
Exhibit 1: Comparison of Conventional Reflow and Vacuum-Assisted Reflow
The pressure inside trap gas per the Young- Laplace Equation
Pressure trapped inside a gas bubble changes according to Young-Laplace Equation
Pbubble = Pambient + 2g / r
(where “g” is surface tension and “r” is the radius of the bubble) The ideal gas law using Pbubble determines the actual bubble size .
Vacuum Applications
Convection reflow systems utilize a vacuum module that inserts directly in its reflow oven line
Reflow liquidous can be achieved before or after entering a vacuum module
◦ Vacuum module is inserted in a zone where reflow peak typically occurs
◦ Alternatively, reflow liquidous can be achieved inside the vacuum module through IR heating
Convection reflow with vacuum module is continuous and allows thermal profiles to be directly ported from non-vacuum reflow applications
Continuous operation facilitates low cost of ownership (COO) and high UPH
Exhibit 2: Conventional reflow with vacuum chamber module
Exhibit 3: Vacuum Chamber
Vacuum Soldering Overview
Vacuum-assisted reflow through the inclusion of a vacuum module in its reflow oven line is now standard technology
Vacuum-assisted reflow with convection heating utilizes continuous operation thermal profiles for low COO and high UPH.
Recent customer installation showed 10X reduction in voids, meeting specification of <1% total void area
Reflow time under a vacuum of 15 seconds was able to achieve <1% total void area specification
All pressures tested < 20 Torr met <1% total void area specification
Exhibit 4: Thermal Profile and Vacuum Profile
A major issue and concern with vacuum reflow is solder splatter. If the vacuum pressure is reduced too quickly solder balls can be “splashed” across the substrate. Heller has developed a multi-stage step-down algorithm that is computer controlled and can be customized for your process -- thus eliminating solder splatter on the product.
Exhibit 5: Multi-step Purge Down to Eliminate Solder Splatter
A user-friendly GUI makes this easy to set up. The user-friendly software package has many “knobs” to control the purge down process as well as timing for each phase of the thermal excursion. Including:
Conveyor speed into the chamber
Conveyor speed on exiting the chamber
Residence time in the chamber
Temperature in the chamber
Refill time
Refill rate
Exhibit 6—User-Friendly Software with Multiple Control Points
Results: DBC at Heat Sink
Exhibit 7—No Vacuum vs. Vacuum on Heat Sink (<1% voids)
Results: Automotive QFN
Exhibit 8—No Vacuum vs. Vacuum on QFN (<1% voids)
Comparison of Conventional Convection Reflow With Vapor Phase Soldering
One area we would like to discuss is a comparison of conventional convection reflow with vapor phase soldering.
Vapor phase soldering has been successfully applied in many applications.
Higher UPH and lower COO are the key advantages of convection reflow in comparison vapor phase soldering.
The design of the conventional convection reflow systems incorporates a belt transportation system utilizing multiple process lanes. This configuration maximizes total UPH in high volume applications. Customer applications have proven that they can achieve void rates < 1% with configurations of 5 production lines.
Depending on the customer application, the cost of chemicals to support a vapor phase system can be very high.
This high level of system output in terms of UPH coupled with the lower operational costs of conventional reflow results in a much lower cost of ownership per unit.
Dual Lane Conveyor Systems for Higher UPH
And for applications with ultra-high UPH, a dual lane conveyor is available. The dual-lane conveyor can provide double the UPH of the standard system -- thus satisfying the requirements for many consumer-related products.
Exhibit 9—Dual Lane Conveyor System
Substrate and Wafer Warpage Mitigation
Warping of components and substrates is a serious problem facing the industry. Often it induces fatal interconnection defects and initiates a weak connection between the silicon die and substrate/wafer substrate and wafer package.
Defects that occur due to this warpage include:
Misalignment of components
Non-wetting
Contact area decreases causing interconnection faults
Vertical warping and curling of structures in the surface micromachining leads to degradation of the functionality of devices
Types of Systems
A wide range of warpage mitigation solutions have been developed based upon customer feedback
These solutions are utilized to mitigate substrate and wafer warpage during the convection reflow process
The systems can include either stand-alone independent fixtures or fully integrated solutions
Automation solutions include single and multiple lane configurations as well as pallet recycling systems
The two types of systems are outlined below
1. Independent Fixture Systems
Vacuum chuck system operates by independently loading the substrate/wafer chuck with a vacuum charge
The vacuum charge will last > 30 minutes
The vacuum chuck can be manually or automatically loaded onto the conveyor system
Heller offers full automation with automatic chuck recycling
2. Direct Suction Systems
Direct vacuum system operates by utilizing a mesh belt conveyor system to provide vacuum suction from beneath the substrate/wafer
The system can be used in multiple lane configurations
There is no additional automation required
Summary
Vacuum reflow soldering has been shown to have a significant impact on void levels in various applications. Including:
Improved heat dissipation of components or solder joint structures (i.e., current density increases with voiding)
Improved long-term stability and reliability of solder joint against vibration/shock
Improved device performance in high-frequency applications
Maintaining impedances within specification for power modules
Mitigating or eliminating solder problems such as bridging, solder splashes on various components (e.g., μBGA)
The Heller Vacuum-Assisted Reflow Ovens have been successfully implemented at numerous customers in both the semiconductor and SMT asembly industries. The system is highly configurable to eliminate solder splatter and provide void rates of <1%. Its user-friendly software makes set up easy and fast.
The current Heller 3rd Generation Vacuum Reflow Oven Line offers reliable, industry- leading solutions for the most challenging voiding issues
Heller is committed to continuous improvement and will work closely with customers to exceed all requirements and specifications
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