IPC-7801 Reflow Oven Process Control Standard
Working Draft 2-27-2014
(Text highlighted in GREEN is for the use of typesetting only and will be removed before publication)
1 Scope This standard is intended to provides process control for solder reflow ovens by baseline and periodic verifications of oven profiles using a standard methodology. Equipment calibration and maintenance guidelines are provided.
This standard is intended to verify the operating parameters of the reflow oven. This standard is not intended for the assembly product profile/recipe. For detailed information on development or verification of a product profile/recipe see IPC-7530.
This standard does not provide guidance for vapor phase processes.
For detailed information on development or verification of a product profile/recipe see IPC-7530.
1.2 Purpose Intended for users of reflow equipment to baseline performance and periodically verify and demonstrate acceptable oven profile performance repeatability.
2 Applicable Documents
The following documents of the issue in effect on the invitation for bid form a part of this specification to the extent specified herein.
2.1 Joint Industry Standards1
J-STD-033 Handling, Packing, Shipping and Use of Moisture/Reflow Sensitive Surface Mount Devices
J-STD-075 Classification of Non-IC Electronic Components for Assembly Processes
2.2 IPC2
IPC-1601 Printed Board Storage and Handling Guidelines
IPC-7530 Guidelines for Temperature Profiling for Mass Soldering Processes (Reflow & Wave)
IPC-9194 Implementation of Statistical Process Control (SPC) Applied to Printed Board Assembly Manufacture Guideline
IPC-T-50 Terms and Definitions for Interconnecting and Packaging Electronic Circuits
2.3 ANSI3
ISA MC-96.1 – Temperature Measurement Thermocouples
(Footnotes)
1. www.ipc.org
2. www.ipc.org
3. www.ansi.org
3 Terms and Definitions Other than those terms listed below, the definitions of terms used in this standard are in accordance with IPC-T-50.
Convection Reflow Soldering A solder reflow process where the primary means of heat transfer is by the recirculating flow of heated air or nitrogen in a Reflow Oven.
Delta T Temperature variation across an assembly, specifically the difference between the highest and lowest temperatures on an assembly when measured at Peak Temperature in the profile (Figure 3-1). A low Delta T is desirable.
Figure 3-1 Identification of temperature delta at peak reflow temperature (NEW)
Delta T = Difference between highest measured temperature and lowest measured temperature.
Golden Board A reusable test vehicle with thermal characteristics similar to a production assembly, used to measure Reflow Oven performance and repeatability, and establish reflow Recipes. Sometimes called a “witness board”, “standard test vehicle”, “thermal profiling board”, or “reference board”.
Liquidus The temperature at which a solder alloy is completely melted.
Peak Temperature The maximum temperature experienced at any point on an assembly during reflow soldering. The highest point on a Reflow Profile.
Preheat A preliminary phase of the soldering process during which the product is heated at a predetermined rate from ambient temperature to a desired elevated temperature. Preheat reduces thermal shock and ensures uniform heating of the assembly up to Liquidus.
Profiling System A data logger or measuring instrument for logging temperature and time data from thermocouples. Also “Profiler”.
Ramp A controlled and uniform increase or decrease in temperature, represented as a constant slope on the Reflow Profile. The magnitude of the slope is known as the Ramp Rate.
Recipe A defined set of process parameters programmed into a Reflow Oven (Figure 3-2). It includes a specific conveyor speed and temperatures within each Reflow Zone, and possibly the flow rates of air or nitrogen. The recipe typically varies with the thermal mass and other heat transfer characteristics of the assembly being soldered.
Figure 3-2 Example of reflow oven recipe set points (NEW)
SP = Set Point
PV = Present Value (Measured Value)
Reflow Oven A solder reflow system, typically using mostly convection heating in an air or nitrogen environment. Conveyorized systems incorporate multiple reflow zones in series. Batch ovens offer lower throughput and are less common.
Reflow Profile A graphical representation of temperature for a single or multiple locations on an assembly, plotted against time, during the reflow process. It may be a recommended “baseline” or Target Profile, or reflect actual measurements. Also known as "Oven Profile".(Figure 3-3 & 3-4)
Figure 3-3 Zones is a typical reflow profile (IPC07530-3-2)
Figure 3-4 Typical graph representation of a thermal profile (NEW)
Reflow Zone A local environment within the Reflow Oven where a specific temperature is maintained in accordance with a Recipe. Heating zones are arranged in order ofr increasing temperature along the direction of conveyor travel. Cooling zones control the rate of solder solidification and cooldown of the assembly.
Soak A portion of the Reflow Profile where the oven temperature is held nearly constant to allow temperatures to stabilize or become uniform across the assembly being soldered, minimizing Delta T.
Spike A portion of the Reflow Profile where the oven temperature increases rapidly past the Liquidus point and then cools off quickly. This Ppermits wetting and flow of solder during reflow while minimizing time at Peak Temperature.
Thermocouple (T/C) A temperature sensor consisting of two dissimilar metals in physical contact. The electrical potential (voltage) between the contact point and a reference point in the circuit is proportional to the temperature difference between those two points. This small voltage is measured by a test instrument, which typically converts it to a temperature.
User The entity individual responsible for controlling and performing reflow soldering.
4 Thermal Profiles – SnPb and Pb-Free
4.1 Recommended Reflow Profile Specifications The recommended baseline reflow profile is defined by the solder paste supplier/manufacturer for the flux component and the alloy used. The oven supplier does not define the recommended reflow profile specification. The specification recommendation should be cross referenced with component specifications and board specifications to ensure the the lowest maximum temperature limitations on all materials used isare not exceeded.
Meeting the requirements of an acceptable reflow profile will produce consistently acceptable solder joint quality and prevent as well as undamaged to components and PCBsprinted boards for a typical printed board assembly. (MB,HP)
The solder paste manufacturer has determined that achieving the recommended reflow profile will yield the overall optimum solder joint quality for a typical printed board assembly. Variations/deviations from this recommended baseline reflow profile may be required for printed board assemblies which have a large variation of thermal mass (eg. very large thermal mass components and very small thermal mass components on the same assembly).
The baseline recommended profile is typically available within the suppliers solder paste data sheet provided with the solder paste. The data sheet is typically available on the solder paste supplier’s website.
For guidance on setting up a specific recipe, see IPC-7530.
4.2 Example of a Typical Tin-Lead Reflow Profile Specifications A general ramp/soak/spike profile for eutectic an SnPb alloy may look like Figure 4-1.The elements of a ramp/soak/spike profile are:
· Preheat (Ramp and Soak) – 2 – 4 min
· /sSoak time = 60-90 seconds
· Liquid PointLiquidus = 183˚C
· Recommended peak temperature = 210-225˚C
· Time above liquidliquidus = 30-90 seconds
· Recommendations from solder paste manufacturer will vary based on formula and manufacturer. Consult your solder paste manufacturer’s data sheet for specific time/temperature recommendations.
·
· Caution: The profile should be cross referenced with component manufacturer recommendations to ensure the maximum temperature limitations on all materials are not exceeded.
Figure 4-1 Typical SnPb ramp/soak/spike reflow profile (NEW)
4.3 Example of a Typical Pb-free (SAC305) Reflow Profile Specifications A general ramp/spike profile may look like Figure 4-2. The elements of a ramp/spike profile are:
· Preheat/soak time = 60-18090 seconds
· Liquid PointLiquidus = 217-220˚C
· Recommended peak temperature = 235-255˚C
· Time above liquid liquidus = 40-7560-90 seconds
(sometimes 40-60 seconds)
· Recommendations from solder paste manufacturer will vary based on formula and manufacturer. Consult your solder paste manufacturer’s data sheet for specific time/temperature recommendations.
·
· Caution: The profile should be cross referenced with component manufacturer recommendations to ensure the maximum temperature limitations on all materials are not exceeded.
Figure 4-2 Typical SnPb Pb-Free ramp/soak/spike reflow profile (NEW)
Users should monitor time above the component manufacturer’s maximum temperature threshold to insure that sensitive components are not damaged. See J-STD-033 and J-STD-075 for thermal damage considerations for moisture sensitive components, and IPC-1601 for printed boards.
4.4 Pb-free (SAC305) reflow profile temperatures Pb-free reflow profiles go to higher temperatures than those for SnPb solder Pb-free reflow profile temperatures are much higher than for SnPb solder (Figure 4-32), and the user mustshall ensure all materials used in the printed board assembly (i.e. the components and printed board) can withstand these temperatures.
Figure 4-32 Comparision of SnPb vs. Pb-free reflow profiles (NEW)
5 Golden Board Design for Process Verifications
5.1 Optimal Golden bBoard - Uusing the actual product is always the best Golden board. The Aactual productprinted circuit assembly gives feedback on the actual process and should provide more accurate data for troubleshooting or process improvement. However, it is not always possible to use actual product; customers may not supply extra assemblies and/or cost may be prohibitive. In these cases, a proxy for theor Golden board may be useful.
5.2 Other Options for the Golden board The Golden Board design should be a board that can be produced easily and quickly for low cost.producible easily and quickly in quantity with low cost, and easily replaced. After multiple uses, the board can break down or delaminate, components sustain damage, and intermetallic compound (IMC) thickening changes thermal properties. The Golden Board can provide representative data on the thermal excursion of product through the oven. When used consistently, the purpose of the Golden Board is to show consistency and repeatability information to confirm that the way the process was operating yesterday is the way it is still running today. The Golden Board can be any size or shape that best represents the product. Golden Boards do not have to be complicated or complex. In fact, since their main function is to show repeatability, simpler is often better because simpler is easier to make in quantity and maintain consistency.
5.3 Golden board Materials The laminate construction should be the same as that used in the product or approximate the thermal properties.
Most printed board laminates will experience physical changes including delamination after repeated reflow cycles. To extend use of the Golden Board, alternative materials can be considered.
5.3.1 T/C Location Thermocouples can be placed in a variety of configurations including cross belt (transverse to the line of travel), along the line of travel, or a combination. (Figure 5-1)
Figure 5-1 Thermocouple placements (NEW)
Locating the T/Cs on the Golden Board is critical for acquiring accurate and useable data from any Profiling System. Typical locations on the Golden Board include leading and trailing edges, left, center, and right.
It is common to use 3-5 measurement points (Figure 5-2). More connection points can be made depending on the capability of the profiler being used.
Figure 5-2 Common thermocouple configurations (NEW)
5.3.2 T/C Attachment Methods – Bolt on T/C The Bbolt on t/c method is very simple to do and repeat using a screw and a nut. This method is inexpensive, robust, and can survive many passes through the oven. Care mustshall be taken not to touch the T/C wires to the outside of the screw head.
Some view the thermal mass of a screw head as not representative of an electronic component and as an “insulator”. Still others consider the screw as repeatable and consistent from location to location, and using them on a Golden Board can provide a repeatable thermal mass and rate of heat transfer.
A locking mechanism, thread locker, or adhesive should be used to hold the nut in place. See Figure 5-3.
Figure 5-3 Bolt on thermocouple method (NEW)
5.3.3 T/C Attachment Methods – Eyelet T/C The eyelet method is also simple and repeatable, using a screw and nut to secure the eyelet. This method is a slightly more expensive but is extremely robust. See Figure 5-4.
Figure 5-4 Bolt on eyelet T/C attachment method (NEW)
5.3.4 Golden Board T/C Assembly methods UV and non-UV cure adhesives are relatively inexpensive and quick to apply. Attachment may be less robust, but a small adhesive dot has less thermal mass and insulating properties than a bolt or eyelet, and will give a more accurate temperature. The T/C should make intimate contact with the Golden Board. See Figure 5-5.
Figure 5-5 Epoxy T/C attachment method (NEW)
6 Profiling Equipment Requirements
6.1 Types of Profiling Equipment
6.1.1 “Built-in” Profilers Profilers which are integrated as part of the oven system.
These systems typically use long, trailing T/C wires to acquire real time data (AKA the “fishing method”). They can be included on most ovens at little additional cost, but often have limited features. See Figure 6-1.
Figure 6-1 “Built-in” thermocouple attachment sites on a reflow oven (NEW)
6.1.2 Remote Profilers Remote profilers offered by third party suppliers use short T/C wires with the profiler box trailing the product as it passes through the oven. The box can be a singular item or can be an integral part of a larger reflow pallet. These systems are more expensive but have many additional features and are generally easier to use. Remote profilers can log data to be downloaded after the test run, or provide real-time RF data transmission. See Figure 6-2.