The production process of DRAM memory modules is a complex, multi-stage process that requires qualified personnel, specialized technical facilities as well as dedicated software and test methodology. The entire production process can be divided into two main processes: assembly and testing, and additional processes such as material logistics and others. Both main processes require prior design and planning so that the implementation goes smoothly and the product meets the quality expectations.

I. Assembly process

SMT assembly takes place on one of the most modern assembly lines in Europe. Our company uses Panasonic machines, which provide the highest level of quality while maintaining very high efficiency.

Before production starts, in parallel to ordering electronic components, it is necessary to order a stencil for a solder paste printer. Due to the required high precision of printing, the stencils are made from sheets of stainless steel with a laser cut and thickness matched to the product. In some cases, electropolishing is applied, which allows to add additional properties during the paste printing operation.

The stencils prepared this way are adjusted to the standard of the VectorGuard system which is used in the company. For the production of each stencil, it is crucial to prepare technical data, such as calculating the volume of paste applied through the stencil, defining the shape of the paste after printing, possible use of aperture reduction or the so-called “Overprint”.

  1. The first stage of assembly is solder paste printing carried out by a Panasonic SPG stencil printer (screen printer). Its task is to precisely apply the solder paste to the printed circuit board through a specially prepared template dedicated to the product. The accuracy of the print determines whether the final product from the SMT line will be properly manufactured and will not have defects such as short circuits between component terminals, etc.
  2. The printed circuit board with the applied paste is transported automatically to the SPI machine called “Solder paste inspection”, made by PARMI SIGMA-X, where they are subjected to an automatic inspection process using precise lasers. The inspection takes a few seconds, when the machine checks the accuracy and correctness of applying the solder paste to the PCB, which, after verification, is transferred to the assembly of components.
  1. The assembly of SMD components, both passive and active, takes place in Panasonic NPM-D2. NPM-D2 machines offer very high efficiency while using very advanced methods of controlling SMD components before mounting on a PCB. During the assembly of passive components, parameters such as length, width and height of the component are measured, and for components with leads in the form of pins or balls, the number of leads, their size, possible dimensional deviations, etc. are additionally checked. To verify these parameters, 2D and 3D cameras are used, which perform a split-second inspection for all components carried by the assembly head.

  1. Panasonic NPM-D2 machines have a modern modular structure, which allows for increased efficiency by adding additional modules to the production line.
  1. Special Panasonic DGS software is dedicated to operate the production line, in which programs are prepared in accordance with the BOM (Bill of material; material structure of the product), component parameters are defined, and information about the efficiency and quality of the process is collected.
  1. After assembling the whole package, it goes to the visual inspection station, where a trained operator checks the correctness of the surface assembly.
  1. The next step is soldering in a reflow oven. The reflow oven is designed to heat the packet to the soldering temperature and then cool it down to the ambient temperature. In order for the solder joints to have the appropriate properties and be free from defects, it is necessary to precisely set the temperature curve (profile) and rigorous monitoring of these parameters. The heating profile is obtained by comparing the process parameters recommended by the paste supplier with the parameters resulting from the package characteristics (type of SMD components used) and the required operating cycle. Prepared in this way, the initial profile is tested with the use of electronic components and a special control and measurement device – ECD SUPER MOLE 2. In the case of various electronics packages, setting the appropriate profile requires a lot of knowledge and experience of the engineer. The solder quality is checked under a microscope and in an X-ray inspection machine.
  2. The next stage of production is the “Automatic optical inspection” AOI machine. The panels are subjected to automated control using a laser through the highly efficient AOI PARMI Xceed machine. The machine checks the correctness of soldering and the quality of the solder joints, and also dimensions, part numbers, position etc. of components are checked. Machine is also capable of mechanical damage of the PCB surface verification.
  1. For the product manufactured with the use of BGA (Ball Grid Array) memory components, additional control in the process is required in order to check the correctness of connections. BGA components have terminals under the circuit, which are invisible to the naked eye after the soldering process, therefore the use of an X-ray machine takes place. The machine used in WESA generates radiation with the use of appropriate equipment powered by electricity, thanks to which full control over the duration and power of radiation is ensured. Based on the obtained photos, the trained staff is able to determine where in the SMT line any problems have occurred and how to prevent them.

II. DRAM module testing methodology

  1. The first stage of checking DRAM memory modules after the SMT assembly process is the OPEN / SHORT test, during which the correctness of assembly is checked. The SPD (memory configuration) is uploaded to the modules. The test is performed with the use of specialized testers manufactured by CST – one of only a few manufacturers of this type of equipment in the world. These advanced devices, in critical situations, make possible to measure electrical parameters and eliminate errors in the process.
  1. DRAM memory modules that have successfully passed the first stage of qualification are subjected to another test, called the application test, in which the proprietary test program, Wilk Elektronik, is used. Like a typical operating system, the test program manages the memory and tasks in which the read / write process and verification of reference data takes place. These are built in such a way to simulate the intensive work of modules in the computer, such as image or sound processing or the work of multithreaded applications. In a situation when the program detects a model data error, an error is signaled. Thanks to the algorithms specially built at Wilk Elektronik, the program allows you to locate a damaged memory chip. It should be emphasized that the construction of test algorithms requires updating in order to ensure maximum diagnostic efficiency for each new memory chip introduced into the production process. Similarly, the expansion of test stands with new models of PC motherboards requires an update of the test program to add new algorithms to indicate the location of the fault. These algorithms are created by a unique method developed at Wilk Elektronik. Changes and updates to the software are developed and implemented by the Wilk Elektronik Laboratory department
  1. The operation of the test program is controlled via the LAN. The program downloads the test settings from the server and compares it with the tested memory module. For example, if the memory speed or capacity is different from the one set by the test stand configuration, an error is signaled. It is an additional stage of control that confirms the correctness of the uploaded configuration. At this stage, the program saves the test result in the server logs when it indicates damage, also with the list of tests that detected the damage. In this way, statistics are created that allow for the improvement of test procedures and the quality of memories produced by Wilk Elektronik. With the quality improvement process, there are procedures to follow, which on a quarterly basis the statistics of complaints are analyzed and the catalog of test procedures is updated. Thus, the process includes the Deming cycle, which includes planning, implementation, checking, and improving test procedures.
  1. Product identification is ensured by a unique test number stored by the test program in the EEPROM memory and the serial number on the label. Thanks to this, it is possible to link the memory module with testing process flow and the production order, and then with the BOM and suppliers. The overriding goal of this procedure is to ensure the traceability of products required by a wide range of contractors.
  1. The final stage of the testing methodology adopted by Wilk Elektronik is the evaluation of the results obtained from the testing process. This is done in the quality control department, which decides about releasing the product for sale. In this regard, the catalog of test procedures, in which the failure rate (and the so-called “Re-test thresholds”, upon exceeding which the test procedure is retested) is taken into account. This procedure is related to the so-called “Bathtub curve”, acc. which damage occurring in the first period of use can be triggered by tests in the application. Following the curve definition, the decreasing waste in successive repeated test cycles indicates that the product has reached the “life” period, in which damage should arise only as a result of improper operation or damage of a random nature.
  1. A supplement to the diagnostic capabilities of memories on test stands is their validation in a climatic chamber. Validation in the chamber allows you to confirm the correct design and construction of memory, selection of components, assembly process and ensures confirmation of the possibility of working in the specified parameters. Validation in a climatic chamber is carried out according to customer specification or the worldwide standard of the IEC60068 family. Validation in a climatic chamber can take place at a temperature from -70ºC up to 180ºC and humidity up to 95% at 95ºC. The validation methods developed by Wilk Elektronik made it possible to replace the explosive glycol at negative temperatures with water, which is successfully used at a temperature of -45°C. Moreover, the methods of sealing the climatic chamber, i.e. protection against moisture penetration, were developed by Wilk Elektronik for the needs of the process. The validation process is adapted to the current needs of customers, i.e. it requires equipment preparation procedures and the flow of the process. The same applies to the software used to verify the correct operation in climatic conditions. The procedures adopted at Wilk Elektronik allow the use of equipment supplied by the customer.

III. Working environment

In order to ensure appropriate health and safety conditions, correct operation of machines, high-quality product, the production process must be subject to appropriate conditions, including:

  • Appropriate ESD clothing for employees:
    • footwear
    • apron
    • gloves
    • wristbands
  • ESD protection with designated EPA zones
  • ESD floor
  • ESD protection measurements by workers prior to commencing work
    • Cleaning the stands with a dedicated ESD cleaning fluid
    • Elimination of antistatic discharge emitters from the EPA zones
    • Grounding of stations
    • Special mats or tops for workstations
  • Air conditioning of the hall to maintain a constant temperature – temperature and humidity are constantly measured
  • Local air conditioning – the use of a dedicated Panasonic air conditioner for a Panasonic SPG stencil printer that maintains constant working conditions inside the machine
  • Ensuring maximum cleanliness – reducing pollution and eliminating dust with the use of a machine for washing the floor and local vacuuming
  • Maintaining machines and devices in full efficiency and cleanliness – the TPM methodology was used to ensure the implementation of preventive works.