PCBA Tech - SMT patch processing POP assembly process introduction

POP chip stacking technology is a new high-density assembly form developed by modern electronic information products to improve logical operation functions and storage space. This article mainly analyzes and summarizes the problems and countermeasures in the realization of POP assembly process from the perspective of equipment technology. It focuses on the optimization method and scope of the process parameters of the main processes in the POP assembly process, and discusses the issues that should be paid attention to in the process control. These are the keys to ensuring the success rate of POP chip stacking.

1 Overview

POP (Package on Package) is a device chip stacking technology, which is a new way of miniaturization and high-density assembly of devices developed to improve logic operation functions and storage space.

POP technology is widely used in high-end terminal products. At present, the 0.4 mm pitch BGA POP technology has mass production capacity.

At present, the main difficulties of 0.4 mm pitch BGA POP assembly process are:

• 0.4 mm pitch lower BGA printed solder paste and reflow are easy to bridge;

• The placement accuracy of the lower two layers is very demanding and easy to shift;

• It is difficult to control the amount of flux dipping for the upper chip.

2 Solder paste printing

2.1 Influencing factors

The printing link is a systematic project where PCB, stencil, solder paste and equipment work together in a certain environment according to a certain method, with many variables and complex interaction mechanisms. Summarize its main influencing factors,

The quality of solder paste printing is affected by factors such as hardware, process parameters, environment, and process control. For the PCB and stencil design, solder paste selection, process control and other issues that exist in the reliable printing of fine-pitch components, there are detailed analysis and discussion in many documents, and I will not repeat them here.

2.2 Support method

Common support thimble includes "hard" thimble and "soft" thimble

The solder paste printing of fine-pitch components needs to ensure that there is no gap between the PCB and the stencil, and the PCB and the stencil are in a flat and non-deformed state during the entire printing process. It is usually mistaken that the higher the top of the thimble, the tighter the bonding between the PCB and the stencil, which helps to improve the printing quality. However, if the top of the thimble is too high, the PCB and the stencil will have a certain amount of pre-deformation, as shown in Figure 4. On the one hand, the alignment of the stencil opening and the pad may be offset, which may cause the printed solder paste to shift; on the other hand, the stencil and the PCB will be separated during the movement of the squeegee, causing the amount of solder paste obtained in different areas to be uneven, or even The amount of solder paste is insufficient; at the same time, when the stencil is separated during the printing process, the separation speed and separation distance parameters lose their meaning and are easy to sharpen.

The introduction of the printing support fixture can effectively ensure the flatness of the PCB and the SMT stencil and the close integration between the two, and the improvement of printing problems such as 0.4 mm/0.35 mm pitch, soft/thin plate deformation and other printing problems is obvious.

2.3 Scraper

In the printing process, the solder paste should form a good rolling effect. Due to rolling, the solder paste in the forefront area of the squeegee will be partially filled into the stencil hole, the flux in it will pre-wet the stencil hole wall, which is conducive to the further filling and demoulding of the subsequent solder paste, so as to obtain the ideal amount of solder paste And shape. The original "solder paste rolling column" diameter is about 15mm, when it is reduced to one-half of the original, new solder paste needs to be added. The "solder paste rolling column" should be uniform and smooth.

In order to achieve a good rolling effect, in addition to ensuring the proper viscosity and volume of the solder paste, various equipment suppliers have sought to improve the structure and working principle of the scraper. For example, DEK's vibrating scraper, ProFlow, Minami's rotating scraper, etc.

2.4 Stencil cleaning frequency

On the premise of ensuring that the bottom of the steel mesh is clean, the cleaning frequency of the steel mesh should be reduced as much as possible. There are burrs of various sizes on the wall of the steel mesh processed by the cutting method, which hinder the filling and demolding of the solder paste. During normal printing, the wall of the steel mesh will be wetted by the flux, and various forces will reach a balanced state. During the cleaning process, the flux wetting film is destroyed due to the action of alcohol and vacuum, and the burrs are re-exposed. The new balance can only be re-established after several printing cycles. This is the reason why the stencils that are cleaned in production will be less tin when they are printed for the first time.

Too frequent cleaning may also cause the solvent to be mixed into the solder paste and affect the viscosity of the solder paste; solvent volatilization affects the optimum working temperature of the solder paste and stencil, and disrupts the system balance.

3 Patch

Compared with conventional components, the most critical issue of the POP placement process is the realization and control of the flux (solder paste) dipping action and the guarantee of the BGA placement accuracy.

3.1 Patch mode

FUJI NXT/AIM equipment has 3 POP placement modes:

⑴ Absorb the device - image recognition - dip in the flux - patch;

⑵ Absorb the device - image recognition - dip in the flux - image recognition - patch;

⑶ Pick up the device - dip in the flux - image recognition - patch.

Two kinds of fluxes were selected for comparison test during the introduction stage, namely: blue flux and colorless flux.

• After the device is dipped in blue flux, image recognition cannot be performed, so only the first patch mode can be used;

• The white soldering flux will not affect the image recognition, so the second or third patch mode can be used.

It is recommended to use white flux and the second patch mode.

3.2 Dip the flux (solder paste)

FUJI Flux Unit device can realize automatic supply of flux (solder paste) and automatic thickness control.

The new process introduction verification results prove that the effect of using flux is better than using solder paste. The thickness of the flux is adjusted according to the pitch value of the solder ball of the lower chip, and the wetting should be greater than 50% of the height of the solder ball. The wetted surface requires a level, generally 0.4 mm pitch PoP is adjusted to (0.19～0.20)mm, and 0.5 mm pitch is adjusted to (0.19～0.23)mm. The thickness of the flux is adjusted by the gauge that comes with the device.

3.3 Image recognition

There are two digital cameras in the FUJI placement machine, one reads the PCB Mark information to calculate the Mark center to locate the PCB. The other reads the key information of the device pin (solder ball), body and other key information to calculate the center position of the device, so as to mount the device to the corresponding position in the program.