Paste Print Process Evaluation Throughput and Cycle (2)

Effectively evaluate the actual throughput of the press

In order to effectively evaluate the actual throughput of a printing press, the following variables must be considered:

● Cycle, and the process of measuring the board on the circuit board, positioning, sending to the printing height, returning to the conveying height, lowering the board. But does not include the actual printing action.

● Printing parameters, including: applied force, squeegee movement and speed parameters. These parameters are influenced by the board size, component density, component spacing, and solder paste composition (due to different rheological characteristics, large components generally mean different speeds).
Optimization of solder paste printing cycle requires the use of solder paste that can be printed quickly. The larger the circuit path size, the more important the effect of the actual printing action on the length of the cycle. If our solder paste can only print 2 inches per second, it takes 6 seconds to process a 12-inch circuit board with it. If you switch to a print of 8 inches per second, the printing time can be reduced to 1.5 seconds.

● Whether to use scraper or closed print head.
Closing the print head saves time when the solder paste is applied to the stencil. Even with the automatic solder paste coating system, the machine takes time to apply new solder paste to the stencil. Closed printheads show unique advantages when switching from one circuit board to another. Because all the solder paste has been installed in the closed print head. Only a small amount of solder paste can be scraped from the stencil before cleaning the stencil. And since the solder paste of the next product is already installed in the print head, the amount of solder waste is also small.

● Solder Paste: How to apply solder paste to a stencil when using a squeegee. The factors that affect it include the coating method (manual or automatic coating), as well as the hole density and the size of the PCB, which will determine the frequency of solder paste replenishment.

● Operational software "ease of use"
The software must be easy to use. The operation of all controllable functions must be easy to understand. The software interface must be as intuitive as possible to simplify the operation. This helps the assembly, conversion and normal operation of the machine and has a great influence on the long-term production of the system.

● Stencil cleaning frequency and method.
All solder paste printing processes require a certain amount of time to clean the stencil. The frequency of stencil wiping is a function of many variables, including stencil design, final surface treatment of printed circuit boards (Heat Air Leveling HASL, immersion silver, nickel/gold immersion, organic solderability protective layer OSP, etc.), printing process Board support, etc. Even the best-designed solder paste printing process must be template-cleaned, so we must evaluate how a machine performs this function. All modern solder paste printing machines provide template cleaning. It must be clear whether vacuum or solvents are needed to assist in cleaning when performing the template cleaning function.

● Slow separation distance and speed from the template to the board. All systems are different. Due to the increasing density, some PCB boards require slower separation speeds to improve the separation of the template from the deposited paste.

● Post-Print Inspection Most modern solder paste printers offer two-dimensional (2D) post-print inspection capabilities, and some can also provide three-dimensional (3D) post-print inspection capabilities for deposition of paste on critical equipment. All 2D and 3D post-examination systems work differently, so it is important to understand the value of additional work to understand the various variables and methods that can be measured, and how to use the resulting data.

● Assembly and conversion options, including related MTTA.

When changing from one product to another, a large number of solder paste printing device conversions are required. Many solder paste printing processes require several conversions in one day. It must be clear how much time your equipment takes to convert from one product to another. Which product conversion variables are particularly important for the optimal operation of the machine?

● Process Statistics Control Strategy (SPC)

As mentioned above, throughput is the number of qualified boards that are assembled within a given period of time. Process quality is critical to achieving the highest throughput, so it is important to understand process operations as “real-time” as possible. We cannot carry out remedial optimization work through discovered defects after the end of production operations. We must promote a "prospective" production that prevents the formation of a "reactive" production that is only used to detect defects.

The key to “proactive” production is a well-designed and successfully implemented Process Statistics Control (SPC) program. So, what features of the solder paste printing device contribute to the realization of the SPC program?

a. Operator Training and Discipline

A well-trained and disciplined honest operator is an important factor in ensuring the solder paste printing process performance. The content of operator training far exceeds basic machine operations. The operator must understand all the factors related to the performance of the solder paste printing process and know how they affect the entire process of the manufacturing operation, including the quality of the final product. Training is very important, and observance of process discipline is equally important. The operator must complete his work in the same way at any time and in any shift. Managers and support staff must help and “guide” the operators to ensure that they understand and follow the procedures. Electronic manufacturing is a process that requires teamwork. All people must actively participate and work best.

b. Process optimization (template design, optimization of operating parameters, etc.)

Process optimization is a tool that engineers and operators use to understand, determine, and quantify all of the performance parameters of a circuit board's actual printing process. They use this knowledge to improve the process. Many operating parameters must be quantified and optimized using statistical methods (such as experimental design DOE), including template speed, applied force, down stop, print stroke length, and so on. Other factors to consider include stencil design, printed circuit board design and final surface treatment processes, circuit board pillars, and solder paste selection. Printing processes are often identified as the main source of failure in the production line, but it must be remembered that the hardware itself is only part of the overall equation. If the correct process is not performed or improper raw materials are used, even the best hardware equipment will cause problems.

c. Equipment maintenance (PM)

A very important factor affecting throughput is the machine's boot time or downtime, which will be present anyway. If a machine needs a lot of overhaul work and it takes a lot of time to keep it running, then its cycle has no value. In order to achieve optimal machine maintenance, equipment owners must implement a rigorous preventive maintenance (PM) plan. A weekly/monthly/yearly maintenance schedule must be established to complete the required maintenance work.

Unfortunately, this is the most overlooked in our industry. Users are generally very busy and the equipment runs day after day. However, they overlooked a primary job, the PM program. The equipment should have a clearly documented preventive maintenance schedule and should be easily visible in the machine's normal PM area. The necessary preventive maintenance must be done without compromise. The device owner should monitor the performance data of his device throughout the run to determine the PM scheduler increase or decrease. After many times, we can know what adjustments need to be made to the supplier's recommended PM program to suit a particular process or a specific PCB board.

Optimize process throughput

Obtaining the cycle performance of the machine is a relatively simple matter for anyone, and all major suppliers will list cycle values ​​in the machine specifications. However, it is very difficult to find the information of throughput. Many variables controlling solder paste printing process throughput are not device controllable. Therefore, it is particularly important to work with suppliers to identify key attributes and optimize equipment parameters.

In order to optimize the throughput of the process, it must begin with solder paste printing equipment, which provides the ability to optimize all controllable variables. Then we must also work with equipment suppliers who have extensive practical experience and can assist customers in optimizing uncontrolled throughput variables for those devices. Of course, customers should not assume that a solder paste printing equipment supplier is an “expert” for the solder paste printing process. The equipment supplier may be an expert in equipment, but can he understand and help optimize the entire process? Before selecting a machine, customers should judge the supplier's understanding of the entire process. Remember that when choosing a supplier of equipment, you buy not only the equipment, but also all the company's resources, including services, spare parts, process development capabilities, and process optimization support.

The combination of all these variables determines the actual print throughput of a board design. Customers who are choosing to evaluate solder paste printing equipment must understand more than equipment cycle specifications, and be aware of all the throughput variables that equipment and equipment vendors' support organizations can assist in optimizing. The customer should place the value of the equipment more important than the equipment cost. The significance of equipment value and equipment cost is different, just like the difference between equipment cycle and process throughput. Therefore, in the long-term application, the efforts of the development process will return to the customer a more predictable and higher throughput process.

The key to “proactive” production is a well-designed and successful implementation of Process Statistics Control (SPC) procedures. Achieving good and effective SPC requires the need for operator training and discipline, process optimization, and equipment maintenance (PM).

Source: China Electronics Manufacturing Network